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Jonathancrystal08 06-01-2020 05:31 PM

1974 GMC 4905a
We are checking out a 1974 GMC 4905a. 40ft, diesel pusher (no odometer) but Detroit diesel engine was replaced in 2006 -- was living in NE with school district. I read the responses from Crown Guy when someone else asked about purchasing an old MCI bus. I understand the $$ issue, bigger bus, more expensive to fix, to operate, to own. It's a 4 speed manual, owner says it drives well, no rust, all windows intact and in pretty good shape. Inside has been gutted except for a few seats, floor has been replaced with plywood. Has already been titled as bus conversion just need to get in bathroom and kitchen to change to motor home. Here is the link to the listing.

We plan on living in it full time and will be traveling but not like every two weeks. Maybe every few months while visiting, working, etc. Is it better to get a school bus or is a coach a better choice? We have 4 kids and will be doing a solar/off-grid set up. We have a decent budget and are still young so will have income coming in, not living on a retirement budget here, lol.

Just would love to hear some thoughts on the older GMC coaches. We will be driving out to see it and I know there are a ton of things to look for (from the previous post as Crown Guy said too many to list) and we have asked a ton of questions of the owner, which they have graciously provided answers to. The owner had it serviced last year, no issues except of the A/C wiring. Was told would have to be redone to get it working. How big of an issue is that?? And we were planning on doing a mini-split so is it even worth spending money to fix?

Just a few questions that I hope someone may have some insight on.


Crown_Guy 06-03-2020 01:38 AM

I don't want to be a negative influence but I need to bring you up to speed on what it's like today to own and fix any of the old magnificent GMC coaches. I LOVE these and I learned to drive them back in the day when they ruled the roads, 50 years ago now. In their time they were without pear in their engineering and reliability. Superior in all ways for fleet operations and why they were in such widespread use.

MCI was a distant second and Canadian competitor with only MC2's, MC5, and early MC7 40ft offerings. Compared to the GMC's they were still too complicated with many belts, and other systems prone to failure. The GMC's were SIMPLE with NO belts and everything gear driven off the engine accessory drive or crankshaft PTO for the A/C.

At the time they were in service they were well supported by GMC and parts were readily available and everybody knew how to work on them. Many of the parts were compatible across various model lines. The City and Urban transit models like TDH, SDH, TDM etc. used a lot of the same body and engine/transmission, suspension parts and even glass and body parts could be found and used in some of the highway various PD highway coaches, 4104, 4106, 4107/8 4903/5. This kept the costs of parts down while at the same time making them available anywhere in the country from many distributors. They were very reliable, easy to fix, and parts where available making them real money makers for the operators using them.

Standardized designs made them easy for Drivers to know what to expect and Mechanics to troubleshoot and repair. The epitome of superior design and execution.

Typical fleet operations had the drivers hopping from one bus to another sometimes several times a day and certainly from day to day. The familiar and standard controls and functions made it easy with many advantages to the drivers and company's.

That was then, this is now.

As much as I love these GMC's, times have changed, much for the worse, where it now is almost impossible to find the simplest and commonest of parts and most must be found in scrap yards and taken off another bus. It MUST be kept in mind that EVERYTHING on the GMC's was/is proprietary and specific to the GMC product lines and their corporate engineering culture and norms. Totally and completely specific to whatever model you happen to have, with a few cross vehicle bits of commonality, they are still only within the GMC vehicle universe.

Another thing to keep firmly in mind is that the Parlor Car (PD Parlor car Diesel) highway coaches were designed and aimed at the FLAT LAND high speed highway market and they were certainly NOT good at all when off paved and as flat a possible roads. I can tell stories about how dangerous they could be when trying to take them into whatever silly ass camp I was sent to with the things. They are geared with a 1st gear that can barely get them to start on level ground when fully loaded, where steep driveways, entry roads, or any number of other normal real world obstacles will cause you no ends of trouble and most likely can leave you stuck and stranded in need of a tow truck to get you out.

Oh, and the solenoid actuated Reverse gear is a built-in disaster waiting to happen, which is why we NEVER stopped or parked a GMC in a position that we couldn't drive forward to escape. If we didn't do that eventually the reverse solenoid would fail and we wouldn't be able to get the damn thing into reverse and back out of the situation. In truth it could be done with a helper at the rear to kick the Reverse lever on the transmission in sync with the driver pulling it into reverse gear, two people needed to do this. I suppose you could train one of your kids....

Dirt roads, with tons of dust kicked up by the A/C condenser fans, any and most kinds of off paved road access roads, camps, RV parks are all made to make your life very exciting.

Another thing to consider is that the GMC 4903/5 40ft has an exceptionally long wheelbase between the axles making it even more exciting to get in and out of some driveways. This was done to improve the ride and give more luggage bay space. The 40ft 490x's were famous for getting high centered and I dare say that anyone who drove them very much managed to do it when they least expected it, we all did. Happened to the best of us. Very much the nature of the beasts. They absolutely require a level of concentration and attention to the driving duties, the outside terrain, and road conditions to anticipate, and avoid very bad situations, that they were not trusted to all drivers. They were probably the most demanding coaches to drive well, while at the same time the most rewarding and comfortable to drive for the guy that really "GOT" them. They are not good at all in tight quarters or even the shortest of steep, peaked, or a tight turning access drive/road.

Another thing to pass on is the little tidbit that the 4107/8 4903/5 V8 powered GMCs are without a doubt the absolutely hardest to master, and learn to shift, coach of anything ever built. When cold the multi-plate wet clutch is almost impossible to shift until the engine, trans, and oil warm up so the rpms don't drop so instantaneously like they do when cold. Even when they were in regular use they were known and accepted to be extremely difficult to shift when cold. We used to sit in the driveway and listen to see if the driver would make the shift to second when leaving the yard. Most didn't and have to grind and find the gear much to the amusement of those of us listening for it. In today's world of all automatic transmissions I doubt if anybody would, or could, even be able to learn when cold, on one of these if they had no experience. They often reduced extremely experienced drivers to fits of foul language and mighty musculature fights with them trying to get them do what we wanted, but they fought back valiantly. It was a mighty struggle to be sure, while cold anyway. Once they warmed up they got much better and were nice to drive, but they were ALWAYS very unforgiving and if you screwed up, got tired, or relaxed your concentration they would slap you up-side the head to remind you to get your head back in the game and drive them correctly or they'd mess with you big time.

Very serious coaches to drive and only the most experienced and capable drivers really got the most out of them and made them sit up and bark like a dog. Most guys didn't, and just drove them sloppily and beat them up, while breaking them in the process. I saw this many times through the years. There were some fleets I worked at that nobody would drive the 4107's they had since they had been driven so hard the shift towers were LOOSE in the floor and it could hardly be shifted. I did and preferred them to the other POS's in the rest of the fleet. They were a challenge to drive but not too bad as long as you knew what the shift linkage was going through, and could compensate, which I did.

Without knowing your background or the condition of the shift linkage, even if it's in good and maintained condition, they are a truly difficult bus to drive and requires a certain high skill level to get the most out of. The normal shifting has to be very precise in getting it into gear or it will just yell at you and grind until you back off and try again, very humbling and kept us always on our toes. Then there's the wonderful clutch pedal and it's "Bear Trap" over-centered design where it got progressively harder and stiffer to hold as you raised it to engage the clutch. Very difficult and weird to get the hang of initially. Once it was all the way to the floor it didn't take any effort to hold it there, hence the over-center description. Just another little thing we had to learn and accepted as the way the world worked.

There's a story on the BCM site by a guy with a 4108 4spd where he was coming out of the Crazy Horse Monument in SD up the mile long or so access road down into it from the highway. When he reached the highway stop sign he was unable to get the bus moving with the little bit of grade he was still sitting on. He was forced to back down most of that mile to a flat spot and get it moving again. He learned the hard way the most important lesson we all knew about driving a GMC on other than totally flat ground. Never let it stop on ANY grade and always keep it moving, or it may not start up again. I would have seen it coming and planned/timed the entry into the highway between traffic in such a way that I simply didn't stop at all and made the turn onto the highway in one motion. I did it all the time when needed and it was part of the operational limits of driving them.

Back then before automatic transmissions all drivers had to learn their profession and we all talked among ourselves about how best to do things. The skills were absolutely required and we just took in stride because that's all we knew. Today it's all different and I doubt many could even learn to drive a GMC Buffalo, as they were called, I didn't like the term but it was in common usage.

I would have to say that for the use as a conversion with the intention of going into unknown and possibly very difficult camp grounds, parks, hotels even, doesn't matter, the fact is they are very restricted to where they can safely navigate off flat paved roads. Beware.

I liked all GMC's and still consider them the best overall highway coaches designed and built. One indicator is how many are still on the road and in what condition they are in. Take a look next time at a nice example of a 60 year old 4104, or a nice 40 year old 4108 that still runs good and compare it to a 20 year old Prevost, or Van Hool and see how well the GMC has aged. The others are all beat up with all that cracked fiberglass and plastic throughout but not the GMC. MCI's are only a little better than the others. Even they don't age as gracefully as a well kept GMC does.

If you buy this or any other GMC in comparable condition please keep in mind it will most likely cost you a lot more than you can possibly imagine now. And there isn't any market to speak of for selling it again so expect to flush whatever you paid for it.

You may even be forced to buy a second bus to try and gain some useful ability to go anywhere and that will likely be a school bus of some kind. Cheaper and somewhat easier to acquire and gain some use from. I've seen this before and understand how easy it is to get in over your head. We all do that. It even makes sense to have two different buses for different trips. A school bus for rough-road camps and destinations and a coach for those long highway trips on the interstates where the school bus would be a pain. I plan to get an MCI myself in order to have the option for either bus whenever the whim strikes me. They all have their distinct appeals and strong points.

Whatever you decide, go into it with your eyes open and be prepared for all the unexpected "Learning Experiences" as I call them. They can really open your eyes and drain your bank, that's the same for any bus, some are just better at it than others.

You need to plan on finding, and good luck with that, and acquiring a full set of manuals for it. Separate Maintenance manual, Wiring, and parts book were published for each model. If the wiring is needing attention you will quickly descend into the pits of hell without a manual to know at least where to start from. Guaranteed that through the years there have been unending "field expedient" repairs that will cause all kinds of trouble without knowing whats going on. Simply "getting that taken care of" can and probably will present you with your First Learning Experience. There are several components in the A/C system as designed that will provide many hours of fun and games. For instance the A/C, switch in front with driver, must not be turned on except at a low engine idle speed due to the fact that the Compressor is engaged through a separate clutch assembly on the PTO from the crankshaft near the fan. If done at high speed it will burn out the clutch and screw up the compressor. There are electrical interlocks to prevent this and if they don't work as they should, expensive things can get damaged. There may be later designs with a different compressor used and a pulley mounted clutch involved, which may be a little more forgiving, but in general we always made sure to start the A/C at low idle only.

Taking out the original A/C system system may be a good idea, BUT, think it through before cutting and chopping. There may be serious reasons for retaining some of the air handling spaces and equipment. Normal coach heat is integrated into the overall air conditioning and air handling system. Some of the ducts and underfloor spaces and motor/fans/filters could be useful while driving down the road. You may even be able to incorporate your new split system onto existing duct-work without impacting the interior space. Take it slow and gather information from others before getting carried away. The original coach heating/window defrost/air conditioning system, is very well engineered and integrated together, so you should tread lightly when removing stuff that you may later regret doing. Don't forget the wiring either, when you make these level of changes, all sorts of Unexpected, and Unintended little surprises will be uncovered. Others have already experienced many of these particular "Learning Experiences"....if you know what I mean.

Also keep in mind that these are 24V Neg ground vehicles with 12V headlights, so you can find them on the road. You must get a set of manuals if you expect to do anything to it. All Manuals are Exceedingly hard to find today.

One other very important thing to keep in mind is that they are all built as a single stressed uni-body style, aircraft type construction, where every single piece of the body is stressed and carries a certain amount of the overall loads. What this means is that the cutting or modification of any of the body sections for any purposes may in fact degrade the vehicle integrity and it's ability to withstand normal driving stresses.

The engine/transmission cradle assembly is bolted directly to the body, there is no frame structure underneath, and the axles are bolted to the body as well, so the strength of the body is of paramount importance.

As long as you don't cut or chop into the existing structure, it can have some holes drilled, even though that gives me pause. But be careful in how you add solar panels or make holes in bulkheads to accommodate new equipment, or cut holes in the exterior body for vents etc. These are balanced and stressed for the body structure remaining intact and not damaged by corrosion or other problems. Any roof raises make my skin crawl and as far as I'm concerned the vehicle is then compromised completely and unsafe.

The water can be very cold, and deep. Good luck.

TomA 06-03-2020 02:46 AM

All of the above in spades, but also...

These coaches are notoriously difficult to sell, particularly with the manual transmission. Most are for sale for years before the owner wakes up to the reality they are worth a pittance, cost a fortune, and just aren't as useful as the coaches that replaced them long ago. Sort of like a wooden boat.

Accordingly, this one is priced about double what anybody familiar with a buffalo and wants one will pay. Those guys already have one or two and a parts bus to begin with. They are aging out of the game, too. New blood for an expensive, heavy, nearly undrivable low headroom high maintenance bus that, to be fair, is gorgeous and gets more attention on the road than anything newer? Not many takers.

Lastly, don't take that long wheelbase lightly. Nothing on the road is longer, and when in service these buses needed planned routes and couldn't service some terminals normally because they just couldn't make it around the block...

The other thing most people don't think about is that if anything happens to you and you need someone else to drive it, even for a short distance, exactly noone will really be qualified and anybody who tries may damage it, or worse. There was an old flyable DeHavilland Comet airliner stranded in Chicago for many years until it was finally disassembled for a museum. Why? Because there wasn't a pilot left in the world with a current rating on it...

A Buffalo is like a steam car. You have to know everything about it and do lots of that all the time. Parts are a challenge. Very few people know enough to help you with it. It doesn't do hardly anything as well as anything newer that's more conventional, but man, when it shines and you rumble into view with it, everybody knows you're there and wants to see. You pay for it, and if that isn't satisfying to you, get something else.

If you like the attention, get a PD4106 instead. Its the best bus of the breed. The 4104 is nearly as good, just slower.

Crown_Guy 06-03-2020 04:03 AM

Sorry for the add-on but I missed that bit about the floor being replaced with new plywood. That could be a potential Red Flag actually since the original floor was a very high strength wood, maybe even Oak which I had on a GMC transit bus years ago. It was as hard and strong as Iron. It was about 1.75" thick or so and I'm sure GMC continued with this level of floor construction through the years.

All floors on GMC coaches are a very integral part of the overall uni-body box construction, along with the roof and lower baggage bay floor. These Elements are critical and they must be stressed properly and work together to carry the loads.

Anyone who ignorantly simply "put in a new plywood" floor has probably caused structural damage to the overall integrity and it will show up as excessive body flexing and maybe even weird highway driving and handling where it might bounce or wander a little as it drives. The original body stiffness and solidity would definitely be gone and replaced with an unpredictable measure of movements that could not be good.

Only someone with previous experiences with these as designed would ever even notice. I have serious problems with anyone doing any kind of big cuts to or holes in any GMC due to the nature of the uni-body construction. This is one reason also why they are limited in their usefulness for converting to an RV. You must not cut or chop on them very much at all.

Crowns all have an exceedingly expensive and superior quality Marine Grade 1.75" plywood floor which isn't even a load carrying member. They just used it because they knew it was the best and would last the decades they were built to take. It added stiffness to be sure to the body and all but Crowns all have super heavy frames and aircraft style side-wall support stringers. The floor just helps out and takes the abuse from the passenger loads.

And as stated above the GMC's really aren't worth anything at all today. I'd like to get one when I win the lottery but I'd only want it as a collectible, fully seated and restored as a driveable Historical Display Vehicle. For my own enjoyment and reliving my past glory days and fond memories. Something which, also as said above, only I could probably ever drive....Boy but those were the days.

TomA 06-03-2020 06:33 AM


Originally Posted by Crown_Guy (Post 390306)
...All floors on GMC coaches are a very integral part of the overall uni-body box construction, along with the roof and lower baggage bay floor. These Elements are critical and they must be stressed properly and work together to carry the loads.

Indeed. The loading and stresses on those monocoques was so critical that even GM didn't always get it right.

Once they got into service, the famous PD-4501 Scenicruisers quickly developed stress cracks in the aluminum ribs above the rear axles. 100% of them were modified at the factory with additional diagonal bracing and a big heavier patch to the outer skin, which you can still see today on any of them, unless one has (foolishly) been reskinned there without it...

Those old GM buses are not only charming, but very reliable, not much harder to drive than any bus (especially a 4104/6 with the Allison 730 automatic) and have a substantial and committed following of owners to keep them on the road. Just don't buy one because you want a good cheap bus to convert, and you saw this and thought it might suit.

Don't be scared off if you really want one, though. Very good examples in nice shape are available. I can't think of any other vehicle you could buy that's already 50-75 years old with millions of miles on it, works as well now as the day it was made, looks spectacular and will not only last longer than anything else you could get, or you, but probably your grandchildren as well...

Ronnie 06-03-2020 08:02 AM

I want to add that as far as shifting gears, many have real trouble doing it. But I have been surprised that some take right to it and do great.

I maintain a small fleet of industrail locomtives, including one built in 1939 that has a 4 speed manual, non syncro, sliding gear transmission. Most locomotives are diesel electric, not this old beast. One of my younger students who I was teaching to be a locomotive engineer was doing well in his training, and he really wanted to try the old one. I figured there is no way this youngster would be able, but I let him try. Darn if he did not take to it like a duck to water. Really does very well with it.

So maybe you will take right to it and do great, or not. Try it before the old wise ones scare you away.

Now I am not saying this bus is either a great one or not. I prefer a skoolie. Just imagine trying to launch a boat with the GMC.... and since I am often towing my boat it just would not fit my needs.

cadillackid 06-03-2020 08:16 AM

beautiful bus!! ive seen some awesome conversions with them.. but yeah they are beasts.. crown guy has it right, I do love these. . (and most other classic busses)..

the A/C is the least of the worries.. ive worked on the A/C on these and other classics.. while ive had good luck getting them to work, its bothing like a modern A/C.. if you are going to hyper insulate the inside of that bus with spray foam and the like.. you may be able to cool it with a couple mini splits.. might be able to keep from sweating going down the road but I surely wouldnt have a hoodie on..

the bus A/C itself is an interesting engineering marvel of gears, clutches, solenoids, and mini driveshafts!

even with a new engine, the price seems out of league even for a classic.. just my opinion there


P.S. if the bus could be had for half that price.. id probably buy it, learn it, embrace it, build it, and enjoy it.. thats just me.. im always a glutten for punishment and up for the challenge of learning..

ha! am in the process of learning Detroit as we speak so that I can get

JackE 06-03-2020 11:09 AM

I love the look of these old GMC's. I know anything is possible, but not always practical. Would it be overly difficult to plant an 8.3 with a 3060in this? If the cost were right on both busses and the transplant wasn't horrible, it would be a fun project.

Ronnie 06-03-2020 12:28 PM


Originally Posted by JackE (Post 390328)
I love the look of these old GMC's. I know anything is possible, but not always practical. Would it be overly difficult to plant an 8.3 with a 3060in this? If the cost were right on both busses and the transplant wasn't horrible, it would be a fun project.

the engine being crossways has a rather unusual drive about 75 degree or so of an angle. Kind of hard to see in the pictures but I have seen them in person. Also note that the Detroit is setting up on an angle, the other half of the "V" is straight up. so it is a v-8 rolled on it's side.

TomA 06-03-2020 01:08 PM


Originally Posted by JackE (Post 390328)
I love the look of these old GMC's. I know anything is possible, but not always practical. Would it be overly difficult to plant an 8.3 with a 3060in this?

IMPOSSIBLE in a stock bus, which uses a transverse engine and unique V-drive transmission. This design tucks the motor right up against the rear axle, and allows for another row of seats (revenue) making the bus more commercially successful.

No other engine/transmission combination will fit in a GM parlor coach or fishbowl, unless you re-engineer and lengthen the bus aft of the drive axle to make room for the engine to sit north-south, with a standard transmission behind the drive axle. That's a lot of work.

Its been done, and one such bus (maybe the only one, ever) was featured by Bus Grease Monkey last year, but nobody would do that today. The '40s-'70s parlor and New Look GM coaches are all V-drive, except the 4501 Scenicruisers, but I haven't seen anything newer transplanted into one of them, either. That leaves you the choice of Detroit Diesel 6-71 or 8V-71 two stroke power, and either a 4 speed Spicer manual or 3 speed Allison automatic transmission. Period. Some later chassis can accommodate the 92 series engines, and I suppose a Series 60 (or 50) might work in a Fishbowl but that's just a guess.

The point is, these buses were designed around their drive trains, and almost nothing else will fit into them.

JackE 06-03-2020 01:30 PM

A lot more projecting than I want for retirement days. I'll just admire them from afar. Although, I wouldn't mind having a go at learning to drive one. I learned on an old International cab-over with a Detroit something or other in it back in the '80's. I think it had a 10 speed trans in it.

cadillackid 06-03-2020 02:04 PM


Originally Posted by TomA (Post 390349)
IMPOSSIBLE in a stock bus, which uses a transverse engine and unique V-drive transmission. This design tucks the motor right up against the rear axle, and allows for another row of seats (revenue) making the bus more commercially successful.

No other engine/transmission combination will fit in a GM parlor coach or fishbowl, unless you re-engineer and lengthen the bus aft of the drive axle to make room for the engine to sit north-south, with a standard transmission behind the drive axle. That's a lot of work.

Its been done, and one such bus (maybe the only one, ever) was featured by Bus Grease Monkey last year, but nobody would do that today. The '40s-'70s parlor and New Look GM coaches are all V-drive, except the 4501 Scenicruisers, but I haven't seen anything newer transplanted into one of them, either. That leaves you the choice of Detroit Diesel 6-71 or 8V-71 two stroke power, and either a 4 speed Spicer manual or 3 speed Allison automatic transmission. Period. Some later chassis can accommodate the 92 series engines, and I suppose a Series 60 (or 50) might work in a Fishbowl but that's just a guess.

The point is, these buses were designed around their drive trains, and almost nothing else will fit into them.

The drivetrain is one of the main reasons to own one of these.
From what I understand all the scenic ruins era were originally built with dual 4-71s and at some point were retrofitted with 8V71s so technically a scenerio cruiser could get a different power plant. Thereís not much in the V8 world other than an 8V71 Iíd want though.

What donít people like about the drivetrain in this 4905? The trans?

A well maintained Detroit is gonna live a long time ! Granted the hydraulic fan drive and A/C are royal pain or can be.. this one they have already ruined the A/C anyway by stripping the interior

Jonathancrystal08 06-03-2020 02:25 PM

Thank you all for all the replies and information. I have been reading all these replies with my husband and to cadillackid -- my husband is a glutton for punishment too and he loves getting into to new things. He has always been the one that keeps going until it's dead not He is also a quick learner and will try anything once or twice if he still can't get it. He never quits on anything. and therein lies the problem lol.

Crown guy - We really appreciate the information. I am not a manual driver and although I think my husband would probably figure it out...he'd be the only one who could drive it....I don't know if I could -- i have a hard time with the regular manuals on his VW TDI and this VW Bug that he had.

We have definitely learned a lot here and I for one am really glad that we got on here and started asking questions before taking the leap. I was worried about the price. Wasn't really sure if it was worth it or if it was going to cost us more in the end. I mean we have some finances right now, but when we finally get moving after all the work is done, we are going to be doing it with limited income and don't want a huge headache and money pit to have to deal with.

An additional thought is to getting it moving as Crown guy had talked about. We were thinking on towing a toad with this, but from all the info, this would not seem to work on that. Additionally, the places we have been in the past and plan to go in the future, this baby would probably not be able to make it and we would be getting stuck often.

And I have to say, the current owner does not know much about this bus. She bought it a few months ago, it has changed hands about 5 times since it was sold by the school district 3 years ago. That had us a little concerned also because we were wondering why it was sold so often....maybe a money pit that they wanted to get rid of. And it's unfortunate she can't even drive it herself.

We have learned a lot more info with the information you guys have provided and that helps to ask the right questions. We truly appreciate it and will continue on our journey of finding the right bus/coach for our family. Luckily, we are not in a hurry and have time to really search and I appreciate that there is this forum to help us with that.

Thank you guys!!!!!

Crown_Guy 06-04-2020 04:22 AM

Towing anything with a GMC is problematic I would caution. The engine and transmission are carried as a single unit by a cradle sub-assembly. Designed to be quickly removed as a complete power package and replaced with another rebuilt ready to go sub-assembly. This was one of the nicer aspects of the GM coaches and was a singular feature of all GM's in City, Suburban, or Highway/Parlor Car service.

Fleet operators could remove and replace the entire engine cradle in about 4 hours and get the bus right back on the road and in revenue service. Very solid and reliable design and interesting because the the whole thing was held to the bus by only 4 attachment point bolts. Two are right under the rear window and the cradle hangs from these two, and the other are at the lower front attaching to the body right at the bottom of the body behind the rear axle on the engine bulkhead.

Even as it facilitates quick change maintenance operations, it also means that the rear bumper, which is attached to this sub-cradle assembly, would be carrying all the loads for any towed anything attached to the bumper. This is NOT a designed in stress load pattern I'd ever want to add to the cradle. There just isn't any really good or proper way to add a tow capability to a GM without jeopardizing the longevity of the engine cradle assembly. Those 4 attachment bolts would be the points of maximum stress buildup and become the failures when anything let go.

I've seen, and tried to drive one, GM 4106, where the cradle was so loose it moved from side to side (left to right) right where it hung. As I tried to pull away from a curb on a slight hill, the bus didn't move, with the clutch fully out, it just sat there and thrashed around with the engine cradle jumping all around at the rear. I was empty at the time and the bus should have easily started up the hill, but all traction motion was being absorbed and translated into this weird rear-end shimmy. Once I figured out WTF was happening, I pushed the clutch in and parked it where it sat and called the shop to come get it. I'm sure this was one of the symptoms and precursors for the famous GM engine falling out of the bus syndrome which happened from time to time on raggedy ass maintained fleets, which as it happened this was one of those. Really, really, bad maintenance.

I know some guys do tow cars with their GM coaches, but I would hope they have engineered the setup to bolster, if possible, the attachment system, as well as making close inspection of the whole engine cradle and attachments a high priority as they operate the bus with the very non-standard loads and stresses. I would advise against towing any vehicle, also since the rear bumper itself is only bolted on the cradle assembly almost as an after thought with 4 to 6 bolts to some steel straps on the cradle holding it away from the cradle, not really very structural in nature or anchored to any solid bus body points at all. Just not designed for towing.

Regarding the bus where it sits right now. First concern should be the batteries and the condition they're in. The very worst thing that happens to batteries is to sit and not be driven. A typical set of brand new batteries will be turned to junk if they sit for 6-8 months without being used or charged. That's a cool $500 Bill min. Learning Experience for a set of two 8D 1300 CCAHr batteries.

Even if the engine starts and it sounds good to you, do you know what a DD 2-stroke sounds like when they're good??? Not likely, no insult intended merely an observation.

Then assuming the air system builds pressure and the air ride suspension raises up as it should, which will take 5-10 minutes at low idle. When air pressure gets to about 60psi flip on the fast idle and if that works the air will build faster and make sure it builds all the way to at least 115 psi or as high as it will go. It's important that you let it build all the way to the air governor cutoff pressure so you can see it works correctly and what it actually is set at. Assuming the air valves for the windshield wipers are leaking as the pressure comes up twist them back and forth and leave them in the counterclockwise "off" position. They always leak air until the pressure builds up and will get better with constant driving of the bus.

Did I mention that yet? you need to drive it about 50 miles a month religiously or all kinds of things will keep cropping up and causing problems. They must be driven.

The air pressure must be s high as possible when it comes time to attempt to release the parking DD3 brakes. Even if the bus starts and sounds good the DD3 brakes can make the whole thing hopeless as a moving vehicle. Push the round black knob at the right of the driver seat down and apply a FULL pedal to the floor service brake application and hold it about 3 seconds. The button should stay down (in) and when the brake pedal is released the bus should be able to be moved, the brakes are released. If they don't release stand up, exit the bus, get in your car and drive away.

If the DD3 parking brake works and the brakes are released then you can TRY to get the thing into 1st gear. Notice I said TRY. Usually when cold the clutch drags so much even when on the floor that you can't FORCE it into gear and it just grinds and won't let you do it. Many times you must stop the engine, put it in 1st gear, then restart the engine. If it won't start look for a "neutral override" switch or button and hold that down when cranking the starter, which should now work. Remember that the clutch is down and the bus is in gear now so be careful. You can now try letting the clutch up gently and see how it engages, watch out though it'll be trying to throw you out of the seat as it comes off the floor and gets immensely harder to hold as it rises up. You'll no doubt stall the engine several times as you get the hang of this.

Welcome to the FUN. From here on out it's just a matter of how much you might REALLY want to ever learn to drive the beast. That's entirely up to you.

Even if this all works OK there's another uniquely GM item which is the nature of the steering gear and how it gets the wheel's motion to the front wheels. Many GM's have some many millions of miles on them that the steering gets extremely sloppy where you might have as much as 1/4 to 1/3 or more of wheel slop making it very difficult to keep it straight and in a lane. This was fairly normal and at the time was an accepted quirk to be compensated for and if it got bad enough could be repaired and parts replaced. But today the parts are not readily available so beware of this.

There are many more gotchas and I don't really want to scare you, (too) much.

I'd pass on this one if I were you. There are so many more, newer, and better coaches, if you insist, for about the same price, which will be much better for you.

Easier to drive, maintain (as coaches go), parts and Manuals availability, experienced mechanics who'll know how to work on them, the list is long. You don't need this kind of brick in the face pain for a first bus experience.

You might want to consider a nice run of the mill well regarded school bus to start off with and learn your "Lessons" on a much less rabidly vicious and quirky vehicle. Believe me that's enough trouble you don't really need but you'll have anyway.

Since I'm totally partial to Crowns which is obvious by my posts, which I recommend you find and read, I'd say that you couldn't go wrong with a well taken care of Crown in good running condition and properly maintained. A Crown has one big advantage that almost all the major running gear components are off the shelf major heavy duty highway truck components still in common use and readily available today. So getting stuck on the road makes it easier, not perfectly worry free mind you, but much better to find parts and service if needed. You can't even say this about all the various mass produced school bus makes. They are usually built on medium duty truck chassis and getting those parts may or may not be to tough, but the school bus body application makes some parts rather more difficult. Plus they have model year styling issues where specific parts are model year specific.

Crowns don't do that and throughout most of their production years they were very much the same styling and body wise and used the same parts.

Crowns are NOT, by any stretch, the cheapest way to go, and you'll pay a premium for what they represent, as they are the absolute best engineered and manufactured coach certainly of any school bus, and they hold up extremely well when compared to true highway coaches like the GMC, MCI etc. They can outlast your lifetime and many will be around for many decades to come, plus they are so well done that I like to say that a Crown will always get you home, and if properly planned, maintained, and you carry some key parts with you, they won't strand you on the road in the middle of the night somewhere. Plus good ones are getting harder to find so the prices are rising as supply dwindles.

That should be worth considering since you plan on taking the family all around. They are superior road buses and a true joy to drive. They can be had with stick shifts or Allison automatics so anybody can drive it. They can be costly to fix sometimes, depending on the problem, just like any heavy duty vehicle but nothing like any coach would ever be. When a Crown is fixed they tend to stay fixed due to the simple systems and over engineering. Usually you don't have to worry about that problem again while you own the Crown since you can't drive it enough to reach the level of service miles they are designed to deliver.

If you should be interested in either a Coach or Crown, contact me directly and I'll look around for you to see what's available. By the way you haven't said where you're located. I can try to get you in touch with operators who would like to sell their vehicles.

bus-bro 06-04-2020 09:10 PM

Well Crown_Guy you even scared me off that bus. Still, my dream bus is a GM pd 4104, with a turbo'd 671, automatic transmission, power steering, and spring brakes. I think there are some of those out there where that work was done by a true and heroic bus nut. I'd even love a stock pd 4104. It could park next to my Bluebird.:rolleyes:

Crown_Guy 06-04-2020 10:37 PM


Originally Posted by bus-bro (Post 390552)
Well Crown_Guy you even scared me off that bus. Still, my dream bus is a GM pd 4104, with a turbo'd 671, automatic transmission, power steering, and spring brakes. I think there are some of those out there where that work was done by a true and heroic bus nut. I'd even love a stock pd 4104. It could park next to my Bluebird.:rolleyes:

Good. Any GMC is something only an experienced buyer with vast $$$ resources and mechanic and parts sources should ever consider getting.

All that really isn't possible on a 4104. No room physically for the turbo itself and re-routing the exhaust would be extremely difficult since the engine is right up against the bulkhead and the muffler is in that restricted space too. There just isn't enough room around the engine to bring the exhaust out to the rear/visible side to power a turbo, and then leave out through a muffler especially if it had to go back again to where it's mounted behind the engine. No physical room at all.

The whole engine compartment space is JUST barely able to contain the 6-71 as it is. A turbo would require that the door be altered to allow room for it to be mounted.

It just isn't possible to add any kind of auto transmission either. The only one that could be added would be the awful one used in transit buses and that had one speed. That's right ONE gear which locked up at around 25mph on the low geared city buses. Before lockup it was in torque converter and wasting energy and building heat. Not anything you'd want on a highway bus. The other VT30 was sized for the 8V-71 engine and even that was not very much better, with 3 gears. These have been successfully swapped into some V8 powered coaches.

Even if you did manage to increase the power from the engine, the other point to remember is that the new torque output would most likely tear the transmission V-Drive and it's bearings up, along with the transmission not being able to handle all that extra torque. The final drive V-configuration components are made with a lot of aluminum housings and not built to handle that level of torque.

Power steering was a luxury extra cost option on most all 4104's and not readily available, I suspect due to the nature of it's being rather new technology and certainly it would add maintenance cost to fleet operators so they shied away from it at first.

I learned to drive on 4104's and love them to death but the company I worked for had only 3 with power steering. The rest were hard core "Armstrong" power steering. It's all GMC engineering so if it's not already onboard it probably isn't easily added as an after market upgrade.

Spring brakes. As originally built the 4104 still had, and relied, on an old fashion manual Stage Coach style parking brake lever next to the driver left side. I think they were still used until the end of production. Even the early 4106's had the same parking brake lever. There just wasn't the technology to provide air(DD3), or spring powered parking/emergency brakes systems yet.

A 4106 would provide better power without all the hassles and the final drive and differential are beefed up to handle the 8V-71 engine. As noted before by another poster the 4106 is a better all around candidate with most all of the 4104's qualities.

All these fantasy wishes for how to improve on any particular bus you might like to acquire, whatever it might be, are all essentially the same when it comes to the reality that you'll only ever find one of the very limited examples of whatever comes available and be happy with what it has onboard already. Any other modifications to or imagined improvements to said dream bus will ultimately and usually ends up in defeat and after pouring $$$ into a project, losing heart, and leaving it unfinished. The woods, literally, are full of these sad stories all over the country.

I happen to know of a nice 4104 that's already converted but would need some effort by a dedicated new owner to clean it up a little and make it really a good example to own and drive. As it happens I actually drove this exact 4104 when it was in service and I worked at that same company I mentioned about. I found it in a storage yard while driving around and contacted the current owner. He had me drive it to where it is now, which is also where I keep my Crowns. So I know it still drives OK. I see it whenever I'm there. He doesn't seem to be using it at all and I think he would be willing to sell it if anybody were to be interested. I have no interest in this at all and would do it to help him out and whoever might want to buy it. It's very clean and straight and I can give details of it's service history to anyone interested in it. Contact me direct if interested, I gave my email above.

bus-bro 06-04-2020 11:38 PM

Don't tempt me with a pd 4104! Don't tempt me! :whistling:

Crown_Guy 06-04-2020 11:43 PM

I can be evil like that. I can't help myself when I do that. 4104, 4104, 4104...41...

Kubla 06-04-2020 11:57 PM

I was informed tonight that ZF made a transmission to replace the V drive allisons in city buses, did not get any details though

Jonathancrystal08 06-05-2020 05:12 PM

Thanks for the info Crown_guy, I emailed you. Any help you can provide would be awesome. We are not in a hurry, and are willing to wait for the right deal.

TomA 06-05-2020 06:55 PM


Originally Posted by Kubla (Post 390578)
I was informed tonight that ZF made a transmission to replace the V drive allisons in city buses, did not get any details though

That's the Ecomat, which is a great transmission, but its huge and heavier than an Allison VT730. Also fully electronic, and expects a fair amount of data from the engine to work. ZF have a number of different tailshaft angles and configs for it, and if you used a lighter engine, like a Cummins 5.9, it would THEORETICALLY work. but that's a whole lot of engineering to hang that from the roof of a GM coach (yes, that is how the engines are mounted,) and make it work at all let alone be a good set of compromises. The factory package from the 1950s is already a good, proven and durable set of compromises, so the bar you're trying to get over is very high. Worst of all, you're starting with about $50k worth of stuff (built on different continents in different centuries) on a major undertaking that nobody knows anything about.

To get from the back of a napkin sketch to driving it down the road would take YEARS and the likelihood of failure is very high. That's something to do if you already have a serviceable bus you like and use, and you want to prove something to yourself or the rest of the world. As Crown Guy wisely says, if you want to use it, just buy a nice example with the best available equipment and go from there.

Kubla 06-05-2020 07:13 PM

I know all about putting electronic controlled transmissions in buses that had a mechanical one, spent much time reading Christopher's thread on putting one in his redbyrd, and I have an allison 2000 for my bus, paid more for the wiring harness than the trans

TomA 06-05-2020 07:19 PM

Crystal, why don't you consider taking it at right angles and going for something like this:

Nice, clean, low mileage bus with a gem of a drivetrain, no emissions and tons of space. Keep the roof and the windows as they are, and start your conversion. This one is priced high, but it is a pristine example. In my experience, paying too much for the best available vehicle usually works out to be a bargain...

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