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flattracker 08-19-2020 11:50 PM

The Conversion of my 86 Crown Supercoach
This is the documenting of the Conversion of my Crown Tandem Axle School Bus to a Motor Home.

About me:
I am a retired Software Engineer with a thirty one year career in the aerospace industry. I also have experience in hardware and software test. Before that I served eight years in the U.S. Air force, working on Electronic Countermeasures. This is my second bus conversion.

About the bus:
It is a 1986 Crown, 39 feet in length, tandem axles, originally equipped with a Cummins Big Cam I 300 engine, an Alison HT740 automatic transmission, Rockwell differentials with a 4.10 gear ratio, and rear air ride suspension. It was last in service in King City, Ca.

TJones 08-20-2020 12:10 AM

Welcome! Sounds like a nice bus. I love the classic look of Crowns. Looking forward to your build.


Native 08-20-2020 02:35 AM

Oh goody ... a crown conversion build thread I can read daily ... from the ground up.
Welcome to the site.

flattracker 08-21-2020 02:09 AM

The Conversion of my 86 Crown Supercoach
I have been approaching my conversion project with an engineering point of view, so some of what I document will be somewhat detailed.

An old racing buddy of mine found the bus in Salinas Ca. in the hands of a private party and told me about it. I purchased the bus for $3000. My friend serviced it for me and removed the seats as my insurance company would not cover the bus until I produced a picture of the interior with no seats. With insurance coverage secured, with title in hand I purchased a trip permit at the Oregon DMV. My brother and I drove about 500 miles to Salinas, installed a queen size bed in the rear of the bus, a bathroom vanity with a counter top attached, and a three burner cook top with 5 gallon propane supply. Now configured as a motor home per California and Oregon requirements, I drove it home., took a trip to DMV and passed the inspection by DMV to verify it met Oregon requirements. I now had an Oregon legal motorhome.
I had made arrangements to keep the bus in a storage yard at Bly, Ore. so I now had the start of my project.

The design process:
I made a list of the features I wanted in this motor home, basically a list of requirements for the new motor home, some detailed some general (the engineer part of me). The requirements list changed when components I wanted became available.
Making a wish list of what you want in your conversion allows you to price out what you wish to put in it. It also will make clearer to you how much work you will have to do to complete your project. Since I have already converted a Crown to a motor home before I knew what I was getting into and kind of how to do it.

In my next post I will provide my requirements list.

flattracker 08-22-2020 01:27 AM

The conversion of my 86 Crown Supercoach
2 Attachment(s)
Tonight's post provides some pictures of my Crown inside and out as well as my requirements that describe how the conversion will be constructed and what items will be included.

My approach to how and what goes into my conversion is designed to suit my wife and I (and our cats) and others would find different priorities. That being said certainly one could change their own design to suit themselves better (more beds, different seating, kitchen size, etc).

Keep in mind that designing a motor home conversion is a task and to complete that task, one has to define that task first (much like problem solving), so listed below is a set of major requirements for the finished project.

Our project has the following general requirements:

1) Sleeps only a couple (two) in one bed, which must be comfortable.
2) I must have a full bathroom, toilet, shower, sink, with doors for privacy.
3) It must have a full kitchen, Fridge, gas range with oven, two tub sink, and counter space.
4) It must have a front room with seating, dining table, TV and stereo system.
5) It must have closet space for two, storage for dishes, food, utensils, cleaning products, etc.
6) It must have adequate heating and cooling.
7) It must be constructed so that any part of heating, cooling, plumbing, electrical can be repaired or replaced by one person (me).
8) It must be able to be used in Winter without failure from freezing.
9) It must have solar, generator, and outside power capabilities.
10) It must have a washer/dryer (because the wife wants it to).
11) The design must follow the K.I.S.S principle.
12) It must be made so that it won’t squeak or rattle or vibrate going down the road.
13) No fresh water plumbing underneath the floor.
14) It will use Propane for cooking and heating.
15) It will be insulated to increase efficiency for heating and cooling.
16) It must be made SAFE to use for driver and passengers.
17) The Wife must be consulted for layout and appearance issue and reasonable compromise reached for these things.
18) It must be easy to use and operate.
19) No structural changes to the bus. (don’t add or move entry doors, cut or modify frame)

A lot of the above items are common sense.

Some of the following design details I have derived from the above requirements:

Power system:

1) Solar Power – The solar system shall use flexible solar panels mounted to the roof. The solar array shall be capable of generating 1 Kw on a bright day. I am using 100 watt panels. The power generated by the array will be passed through a charge controller to a battery bank located in the trunk of the bus (Most Crowns have a large trunk located at the rear of the bus).
2) The battery bank shall be capable of storing 1000 KwHr of energy.
3) There shall be a 65 Amp battery charging system that will, in addition to the solar array, charge the batteries when the bus is using shore or generator power.
4) The DC power from the batteries (12VDC) shall be available to a 3Kw true sine wave inverter, and to various 12VDC circuits.
5) All 12 VDC circuits shall be protected through circuit breakers in a central panel.
6) All 120 VAC circuits shall be protected through AC circuit breakers in a central panel
7) The 65 Amp battery charging system will only operate when external or generator power is being used.
8) The washer/dryer will only operate when using external or generator power.
9) The AC power source switching (External/Generator/Internal) shall be mutually exclusive, meaning only one at a time is available.
10) When connecting to external power, the system shall be capable of accepting 240 VAC and distributing such power to 120 VAC circuits.
11) When using Generator power the system will accept 240 VAC and distribute such power to 120 VAC circuits through standard electrical boxes with circuit breakers.
12) When using internal power, the system will accept 120 VAC from the inverter and distribute such power to 120 VAC circuits.
13) The 12 VDC circuits will have power available regardless of which power source is selected.
14) The power system shall have voltage/current/frequency monitoring at all times, in all power source modes.
15) The power system shall be capable of complete shutdown by use of switching.
16) The Crown bus electrical system shall be isolated from the motor home electrical system, except when cross-connected through a battery switch for emergency use to start the bus engine if the Crown bus batteries become unable to start the bus.
17) A second 250 Amp alternator shall be installed on and rotated by the bus engine. This alternator shall be connected to provide additional charging of the trunk mounted battery bank.
18) The second alternator shall be capable of cross connection to the bus batteries for emergency charging if the bus batteries become excessively discharged.
19) The generator shall be an Onan 6.5 DKD diesel generator, capable of 6.5Kw at 240 VAC.
20) The generator shall have a 20 gallon fuel tank mounted forward of the engine compartment.
21) There shall be a semi-automated fuel transfer system to transfer fuel from the main engine fuel system to the generator fuel tank. The fuel transfer system will stop fuel transfer when the generator fuel tank gets nearly full or when the main fuel tank get down to 25% full.
22) The generator fuel tank shall have a manual fuel filler capable of being fueled using a standard fuel pump as found at a gas station or truck stop.
22) There will be a 20 gallon fuel tank mounted in the trunk, capable of transferring fuel to the main fuel tank through the main tank fuel filler. This transfer shall be manually operated.


1) The kitchen shall have a Suburban 3 burner range/oven operated using Propane.
2) The kitchen shall have a 10 Cubic foot refrigerator made by xxxx, operating from 120 VAC.
3) The kitchen shall have a two tub stainless steel sink xxx X xxx X xxx inches. The faucet shall supply hot/cold water through one spout and a sprayer.
4) The counter tops shall be acacia wood butcher block 1 ˝ “ thick and 25 inches deep.
5) Under the sink/counter shall be a 40 gallon fresh water storage tank. This tank shall be filled from a standard outside RV water connection through a manually operated valve.
6) The outside water supply shall be passed through a pressure regulator and a filter.
7) When connected to outside water, the water shall be distributed through the bus using outside water pressure.
8) When water source is internal the water in the storage tank shall be distributed by a 3 GPM electrical pump operated on 12 VDC.
9) The kitchen walls will be covered with sheet aluminum for ease of cleaning and fire safety.


1) The toilet and bathroom sink shall be enclosed in a “water closet”, installed on the right side of the bus. The “water closet” walls shall be covered with FRP.
2) The Shower shall be installed on the left side of the bus and shall be 30” x 30”. The shower will have plywood walls covered by FRP.
3) The water supply for the shower shall be passed overhead in the transition between the bathroom and utility cabinets/closet to the rear.


1) The bedroom shall have a custom built queen sized bed mounted to the floor. The pedestal under the bed shall have two drawers installed for storage.
2) The bedroom shall have access through the emergency door for egress if needed.
3) The back wall of the bus shall be enclosed to provide storage cabinets, shelves and electrical power.

Front room:

1) The front room shall have three sets of reclining “tour bus” seats. Two of the sets will be mounted facing each other, with a dining table mounted between to provide a booth for dining. The booth will be mounted on the left side of the bus. The third seat will be mounted to the right side wall of the bus in a forward position. The third seat will be a single modified tour bus seat with cup holder, phone holder, and USB charging connections

2) The front room will have a 55” 4K TV mounted on the right wall across from the booth. The TV will be mounted to a lift mechanism to raise the TV up for viewing and lower it when not in use.

3) The front room will have a stereo system with:
a) Two 350 watt per channel amplifiers.
b) Peavey graphic equalizer
c) Preamp/FM/Tuner
d) Power control panel
e) 300 watt subwoofer amplifier
f) 15” subwoofer speaker/cabinet
g) Two Carvin speakers (2’ x 3’ cabinets)
4) The stereo system will be connected to a Raspberry Pi 4 single board computer using the Libre Elec /KODI software to play movies/music and to the TV through HDMI interface for playing movies.


1) A 40,000 btu suburban propane burning ducted forced air furnace shall be installed underneath the gas range/oven. Ducting will provide forced air heat to the front room, kitchen, and the bathroom. The furnace will operate using 12 VDC and be thermostatically controlled. The intake air to and exhaust from the furnace will be through the left wall of the bus.
2) A 16,000 btu suburban propane burning unducted forced air furnace shall be installed on the left side of the bus in the bedroom. The furnace will operate on 12 VDC and be thermostatically controlled. The intake air to and exhaust from the furnace will be through the left wall of the bus.
3) The original factory installed heating system in the front of the bus will be retained.


1) There will be two 13,000 btu coleman roof mounted air conditioners installed, one over the front room and one over the bedroom. They will operate on 120 VAC.

Wall/Cabinet Framing:

1) All walls and cabinets and counters, tank mounts will be constructed using UniStrut.
2) Where needed the UniStrut will be welded using MIG.
3) Rivnuts will be incorporated where attaching framing to bus ceiling where needed.

Generator mounting:

The generator will be mounted in the spare tire well of the bus, as well as its fuel tank. The generator fuel tank can be filled using fuel transfer from the main tank or through manual filling through conventional car type filler inside generator access panel.

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Native 08-22-2020 03:28 AM

It is refreshing to see a build thread that actually has a plan.


Originally Posted by flattracker (Post 401856)
2) The battery bank shall be capable of storing 1000 KwHr of energy.

Did you really mean 1000 KwHr? That is 1 MwHr of storage.

flattracker 08-22-2020 05:11 AM

It was misstated on the battery capacity. I believe I meant 1000 amp hours. thanks for catching it

TJones 08-22-2020 09:13 AM

Great detailed plan. Specified like a true engineer.:thumb: I have a BS in mechanical engineering that I ended up using to be a ski instructor and owning a landscape company.:biggrin:


flattracker 08-22-2020 10:47 PM

The Conversion of my 86 Crown Supercoach
With my new to me 1986 Crown bus, a place to keep it and permission to work on the conversion I was ready for the project.
I didn't have any of the items to install in the bus yet, and while I had experience with converting my 1981 Crown, It was the thirty five foot two axle version, and is equipped with the Detroit 671 diesel with an Alison automatic transmission. The new Crown being equipped with the tandem axles and the Cummins Big Cam III 300 presented a different "canvas" for me. While the 81 Crown was reliable, it is on the slow side on hills.

I began the design process that resulted in the requirements and specifications listed in the previous post. Once I determined what I wanted in the conversion, I began purchase of materials and components to install in the bus. Most items I acquired through eBay.
I bears mention that my specification calls out the use of "Unistrut" also known as "Hardystrut" and a couple other names. It is a steel product, a "U" shaped channel with the open ends curled around. I comes in ten foot lengths, punched with holes or not. You might see it in a warehouse or an auto parts store used to construct shelving. It is an incredibly strong product, plated for rust resistance, and is weldable. I used it on my first conversion, and will use it on this one.

Over the rest of last Summer, last fall and Winter, I did the layout for each room, the detailed design for walls, counters and appliances, while acquiring the items. None of my design work was first cut success, rather most having multiple re-design efforts. In my case I used a spiral notebook to document the design, and calculations need to refine the design. All this may not be exiting reading but I found that doing this first has saved me much aggravation.

In the next post I will talk about a problem that almost ended the project before it really started.

flattracker 08-22-2020 11:37 PM

The conversion of my 86 Crown Supercoach
In this post, I talk about an expensive failure that almost ended the project before I started it.
Back in January I went to the storage yard in Bly, and started the bus as I had been doing on a semi regular basis. It didn't run right and would not idle. I first attributed the problem to cold temperatures. It had oil pressure.
In March I attempted to start the bus, and found it would not even turn over. I had the starter rebuilt (about $250), no help. I replaced the batteries, still no help (about $400).
I called a mobile truck mechanic who troubleshot the problem and determined that one of the main bearings were spun, meaning that the bearing was welded to the crankshaft and the bearing had turned in the block, damaging it.
Such failures do not mean that the engine is ruined but to repair the damage requires line boring of the block and replacement of the crankshaft. This also requires complete stripping of the block. This is very expensive.
Fortunately the mechanic knew of a guy that has a tow service for big trucks and that he had a Cummins Big Cam engine he might sell. I went and saw the potential donor engine, it started up fine, ran OK (it was installed in an old "Pete" tow truck), so I was able to purchase that engine and got a big price break on towing as a package deal.
The engine cost was $3700 and the tow $350, going 60 miles to Klamath falls, a great deal.
The mobile mechanic also had a shop, where the Crown would be repaired.
The replacement project entailed removal of the engine and transmission, stripping the new engine to a long block, replacing the rod and main bearings (a good idea since the oil pan had to come off). The fuel pump on the replacement engine was sketchy so the fuel pump from the Crown's engine was gone through and re-calibrated for a Cummins Big Cam 400. The original engine was rated at 300 HP and the replacement engine is rated for 400 HP. As the original bus engine was a Big Cam III with mechanical valve timing (MVT) and the replacement engine a Big Cam I with fixed valve timing, all external components from the original engine were installed on the replacement engine. It turns out that the differences between the Cummins 300 and the 400 are the fuel injectors and the fuel pump are different. The difference between the injectors is slight and the fuel pump can be re-calibrated for 400 HP.
There are improvements that came with the Big Cam II and Big Cam III engines and all of these except the MVT (which is known to be problematic) ended up in the engine. Of note, the compressor used in the Crowns with a Cummins Big Cam are special, and the regular Cummins compressor cannot be used.
After some initial problems were worked out, I drove the bus back to Bly. One thing I noticed immediately was the extra horsepower. This bus was a rocketship.

Fixing a configuration problem with the Crown.
This bus was a school bus, made to stop every few blocks and either pick up or drop off kids, and usually did need to got faster than 50 MPH.
The Rockwell differentials in this bus came with a 4.10 gear ratio, great for school bus usage or drage racers, but not good for interstates, especially in states like Nevada, Idaho, Utah, Colorado, Montana, etc. where the speed limit is 80 MPH. With a 4.10 gear ratio the engine reaches 2100 RPM at 63 MPH. I don't want to be THE GUY going 60 MPH in an 80 MPH speed zone. Also if running a Cummins at 2100 (redline) isn't great for it and fuel economy is NOT good.
My solution:
The Rockwell differential used by Crown are available with different ring and pinion gears, with the tallest gear ratio being 3.42. I had the same shop remove and replace the "pumpkin" in each differential with rebuilt units with the 3.42 gear ratio. Cost for this was about $6200, but worth the cost.
Ultimately the Crown ended up with a 25% increase in torque (and HP) and a 16% taller gear ratio. Top speed is calculated at about 77 MPH.

The Crown had some air system issues to fix and those will be discussed in the next post.

Crown_Guy 08-22-2020 11:50 PM

Congrats. You have a very good Crown to do a long distance build on.
I think it's the best combination of overall size and massive power to make any road trip enjoyable and drama free. For me, most of the reason for having and driving the Crown is the sublime experience of Just Driving it on the road. I have many more miles driving commercial coaches over the years but the Crowns are hands down the most fun to drive and keep you in touch with the country you're passing through and the feel of the road under you like a sports car.

Since you're a previous Crown owner I don't have to tell you how they are on the road. And that monster Cummins will make a believer out of you for sure. As much as I love my Scream'n Jimmy's I have to say that the Cummins is much quieter and easier on the ears for long hours on the road. You also don't need to thrash it and keep your foot buried in it all the time to keep your road speed up(non-turbo'd). Or watch your temp gauge on hot days and steep hills. Crowns don't usually get hot but on really hot days and steep hills a 6-71 will need to be watched and probably down-shifted to keep it cool. An extremely well engineered and well balanced cooling system for the engines installed by Crown.

The most common reason for downshifting on hills is just plain running out of torque to pull any longer in that gear. It'll drop to 1600rpm where you must downshift or risk lugging and hurting the engine. Some say to downshift at 17-1800 and keep it up close to, or on, the max governor speed at around 2200 or so. The 2-strokes demand this driving technique or you WILL do damage to them. I say you need to drive them like you're always mad at them. They love it and thrive on it.

I'm sure you'll find that Cummins won't be going slowly up hills. In fact I'd guess it only does about 63-65mph max at about 2100 rpm. I know the engine will turn as high as 2400rpm without shutting down like the 6-71 governor would....well that's the way Cummins work. So you may be getting a usable highway speed with it and the normal 4.10 rear-end gears. Very normal speeds for the originally ordered school bus operators with 4.10's was a max of about 63 at governed engine speed.


Additional edit**** I was doing my reply when you posted before I was done. Your solution of changing out the rear end gears was what I was going to mention to you......but Nevermind as Gilda used to say.

You now have one of the ultimate Road buses and it still won't notice what a hill is. I know because mine has a 300hp Big Cam III with a 10spd overdrive transmission and I rarely take it out of 10th (o.d). Good for you and you're right it's worth the cost to have that highway speed on the 80mph posted interstates.

Depending on the exact setup of your engine you may be able to cruise with it at insanely low engine speeds (for us with many 2-strokes miles under our belts). I suspect you can easily maintain about 16-1700 rpm at 65-70 or so which will not hurt the engine. My engine is rated for 1050ft/lbs at 1300 rpm and it does it too. Unless you have a Big Cam III (which I doubt it is) you may need to keep it up above 1600 or so for max torque.


More update.... I just read the WHOLE post above about the engine swap so please ignore some of the stuff I'm off about. It's not worth re-doing it. I have a mild concern about the differences in the 80 degree laid down pancake configuration used in the Crown, and that all the tricky oil return and other little internal things were done properly. You found one of them, the fuel pump is a different part number, as I found when I re-built mine. I'm told there are other deeper differences that should be heeded in order to lay one down, and I believe them. I know personally of one guy who tried what you did but did none of the appropriate modifications to lay it down and he got about 5 miles down the road before a piston flew out the side of the block due to oil starvation.

Rocketship, I'll bet, I know how mine drives and with 400 rated HP I'd only worry that I was taking a hit in fuel mileage. I plan on looking into this one day to see if I can gain anything useful. In the mean time I fly over Cajon pass with 10 or so pax and lots of onboard parts and such, at almost 65 mph in 10th over and about 1400rpm and it doesn't drop any lower, just digs in more, kind of like the Caterpillars do. Awesome setup really and a true joy on the open road.

I'd check with Cummins direct and see if you can find out all the modifications for that special engine model used in Crowns to be totally safe. I look forward to talking with you.


But that Cummins won't even slow down or show you it knows what a hill is. Torque to Die for. Try to verify exactly what the engine is. A Big Cam I, II, III, or something else. It matters due to the Big Cam III having an increased torque rating over the previous builds. The 350 is in fact an earlier 743 block and not the 855 which the Big Cams are all built on.

I have a listing of the various Crown specific Cummins model/HP engines with their torque/rpm ranges I can send you if you contact my email. That way you can begin to get a handle on what exactly is in your Crown. There's a lot of well meaning and unintentional confusion on the Cummins, and I was among those until I did some digging to sort out what I had in mine. There should be an engine serial number plate on it someplace and with that you can contact Cummins service direct and get the details for your engine. You'd be amazed at all the little things they can tell you about it that you're going to need in the future, I know I was.

The 6-71 Crown probably had the same school bus geared rear end as well so you may not see any real increase in road speed. Also I suspect the 6-71 may have only been naturally aspirated with no turbocharging. In that year range it was still an extra cost option seldom bought. It's true a 6-71 without turbo charger is typically sluggish, like 30-35 mph, up a decently steep highway grade. That's just the way they are and we all got used to it. The Turbo'd 2-strokes and series 60 4-strokes in the coaches certainly spoiled us forever with the old ways of going up hills, watching the temp gauge and downshifting to keep the engine in the torque range and circulating coolant to keep it cool.

You have an outstanding Crown with the best combination of power and never ending long distance highway driving, plus that Allison automatic makes it as easy as a car to drive so you both can enjoy the thrill of driving the Crown on the highway. Nothing else comes close to that. The air ride rear is a great feature to have since the fully sprung tandems were pretty hard riding until they got a real heavy load of bodies on board.

One thing I bet you'll find, like I am, is that those rear wheel humps are taking away from usable flat interior floor space and where to locate things. I'm still not sure how or where to place some basic items like the toilet, and how to run the plumbing without cutting any holes in the floor. That extra 5ft over the 35ft two axle Crown comes with a penalty called the second rear axle hump. It appears to be a complete wash. I think we're the ones being humped actually. Oops. Like you I don't like cutting holes and will do everything I can to not have to penetrate the existing structure as best I can. Contact me so we can compare notes.

flattracker 08-23-2020 01:37 AM

Hi Mike,
My understanding from our local Cummins expert was that other than oil pan and compressor the horizontal configuration is identical to the vertical one. I have put about 130 miles on the bus since the re-power and it still run perfect. I have and will put holes through the floor for plumbing, gas and electrical. It seems impossible to not have to do that in a conversion. I will need to cut through the side for the water heater and furnaces. I had no issues on my first crown. I like the crown in your picture.

Crown_Guy 08-23-2020 02:11 AM


Originally Posted by flattracker (Post 402001)
Hi Mike,
My understanding from our local Cummins expert was that other than oil pan and compressor the horizontal configuration is identical to the vertical one. I have put about 130 miles on the bus since the re-power and it still run perfect. I have and will put holes through the floor for plumbing, gas and electrical. It seems impossible to not have to do that in a conversion. I will need to cut through the side for the water heater and furnaces. I had no issues on my first crown. I like the crown in your picture.

Good. I'm glad you made the effort and checked into it. My problem is I have so much time and miles on the 2-strokes I haven't yet found a true Cummins "expert" I can trust and query about the Zen of Cummins engines. I definitely have that in spades when it comes to Detroits. I have one of the best guys around whenever it involves anything Detroit 2-stroke. He works on others of course but he's a master when it comes to Detroits.

My engine runs and purrs so good I haven't had any need to look around for a really good Cummins guy..yet. Except for the previously mentioned fuel pump issue, which started out as a simple slow fuel leak around the throttle input shaft, and it's silly little internal seal which is a known issue apparently. It needed to be removed and opened up to do the seal and of course while at the diesel injector shop it made no sense not to have the whole thing rebuilt and zero timed. This was when they told me I'd have to wait to rebuild my pump instead of merely getting one off the shelf because it was a different part number with several internal differences due to the horizontal configuration.

I always remembered this as well as what others have said through the years about the engineering differences to lay the engines down. I've also heard over the years it took a lot longer for Detroit Diesel to do the horizontal configuration on the 6-71's than it was for the Cummins. Which might explain the ease for the Cummins and the early Crown diesel conversions were only available with Cummins 220's then 262 turbo's. Mostly due I suspect to all the issues I've heard about with the 6-71's. Once they got the 6-71 figured out it became more popular and that's why you see so many more of them in the Crowns. Cheaper initially and back in the day much cheaper and easier for parts and labor to repair compared to the Cummins. Not so much today with all the experienced 2-stoke mechanics retiring and not being around.

You've got an outstanding Crown and have solid plans for getting the most out of it. Feel free to email me so we can swap numbers and we can keep in touch and I can answer any questions you might have and help you get in touch with other Crown owners who would like to know about you also. There's more of us every day.

I have cleaned it up some since I took that pic on picking it up. The huge black patches are gone and the paint has been buffed out to remove the oxidation, Crown used the most superior paint I've ever seen. It seems to last forever. I really haven't done much more than clean it up a little and drive it every month to keep it charged and limbered up. It awaits my attention and $$$ to really get started on it but I'm getting closer to being able to do that. I might as well try updating the avatar if I can find a good one to use. Thanks for reminding me.

flattracker 08-23-2020 11:51 PM

The Conversion of my 86 Crown Supercoach
Tonight I will speak to the problems found in air system of my Crown:

Symptoms found in the air system of the Crown were:
1) Slow buildup of air pressure.
2) Overnite leakdown of all air pressure
3) The bus would be level after use, but one side slumped down the next day.
(loss of pressure on one side of the air ride suspension)
4) A slight hissing noise under the front left area of the bus.
5) problems maintaining air pressure.

Fixing the problems:
Problem 4 was traced to a slow venting of pressure from the air dryer.
Problem 5 was traced to not one but two different bad governors.
Problem 3 was traced to an intermittently failing check valve
Problem 1 seems to be somewhat lessened by addressing problems 3, 4, and 5.
Problem 2 is also lessened but one more valve is still leaking. The next day 10 psig will be found in the air system. I consider at a minimum that 50 - 65 psig would be acceptable the next day.

I had what I thought was an acceptable level of understanding of how air systems work, but the Crown let me know I needed to learn more.
For those with air system in there bus, Bendix has an excellent tutorial on air systems at:

Of note Cal Fire considers 65 psig present the next day in the air system to be acceptable (for older fire appuratus).

My next post will address the first part of the conversion of my Crown, installation of the Onan 6.5 DKD RV generator.

Iceni John 08-24-2020 01:57 AM

Hi, fellow Crownaholic,

You mention wanting to use flexible solar panels. My suggestion (for what it's worth) - don't! They simply won't last very long: how long's their warranty, compared with regular panels' 25-year warranties? You also need an air gap under the panels, otherwise they'll get so hot that their power output will be seriously affected. I suggest you do some more research on PV panels before you plunk down your hard-earned cash for them: the Northern Arizona Wind & Sun forum has plenty of good reliable info for anyone to learn from.

FYI, I have a smidgeon over 2kW of individually-tiltable panels on my Crown, and still space for two solar water-heating panels whenever I finish building the damn interior! PM me if you need any pointers on how I've done mine - they're very swish!


flattracker 08-24-2020 01:41 PM

The Conversion of my 86 Crown Supercoach
8 Attachment(s)
In this post I document the process of installing an Onan 6.5 DKD diesel generator in the Crown:

I happened upon the opportunity to purchase the generator from a wrecked Bounder 35 ft? motor home from a friend and neighbor down the road. Of course as part of the deal, I had to remove it from the motor home.
It was configured with two 120 VAC output circuits, one with a 30 AMP breaker and the other a 20 AMP breaker.
Once my brother and I removed it from the Bounder, and transported it to the bus, we started it up, using a 1 gallon fuel can and a hose. Sure enough it put out the expected voltages, but not in the configuration I wanted, which is 240 VAC, like the power coming into your RV at a park or to your house.
I found the Onan manuals for this generator on-line and through careful analysis of the schematic determined what wires to change and where to. Onan provides the schematic for the 240 VAC configuration but the labeling of the schematic is confusing. A look in the control box helps some by looking on the inside of the cover at the diagrams. A look at the actual wiring is not the clearest way to understand the changes (see picture). Fortunately Onan does label the important wires so that one can figure out which wire is which. After configuring all wiring out and changing the needed wiring (in the process finding a serious factory defect, a wire to one of the circuit breakers not tightened with obvious heat damage), we started up the generator and checked the voltages and all was good.

Of note, in the model Onan used two seperate breakers for output, different from each other. Other versions of this generator used a dual 30 amp breaker. When researching a replacement dual breaker from Onan it was listed as obsolete not available, but Onan used breakers made by Airpax. A quick trip to eBay and I had the right breaker on the way. The breaker needed has 1/4-20 studs and does not plug into a panel. If you go to an normal electrical supplier looking for a breaker like this they will likely look puzzled and say they don't have them. A marine store or boat store will likely have them as this style of breaker is used on boats.

Based on successful results with my first Crown, I determined that the spare tire compartment was the best place to put the generator on this bus. There is plenty of room in there with excellent airflow so it lends itself to this purpose. In the first bus my son made me a mounting tray and mounted the tray on slides and all fit nicely in the bus, but this generator is taller the the gas engine version of 6.5 Kw Onan in the old bus so a different solution was needed.

We started by removing the spare tire by unscrewing the bolts holding down and removing the tire and wheel. On most crowns there is a substantial U shaped steel support welded to the frame that acts as a cradle to support another substantial steel piece welded to it and the re-enforced horizontal outside body part at the bottom of the access hatch. (see picture).

We cut the welds and removed the steel piece connecting the U shaped support and the outside body support framing (the spare tire mounted to this piece) (see picture)

After cleaning off any rust or undercoating so the surfaces would be ready for welding, and temporarily removing the dump valves (air system) from the body support, we were ready for adding the new generator mounts. Like the first Crown I determined that the generator needed its own fuel tank with a filler in the compartment

First two 4"x1.5" C channel lengths of steel were welded from the U shaped frame welded to the frame rails to the outside frameworks at the hatch opening. (see picture)

Second two more C channel lengths of the same material used in the first two were added inboard on the other side of the U shaped support, with a cross piece at the end of those C channels, and a vertical piece tying the end to the support for the radiator ducting. (see picture) The lengths of C channel on either side of the U shaped support were tied together with additional steel plates on top. To some this may seem overkill but the generator weighs 515 lbs. and a 20 gallon fuel tank full of diesel has about 140 lbs of diesel in it. When you encounter pot holes or bumps (especially speed bumps) the resulting forces could make the approximately 650 lbs. load in the compartment seem to weigh over 1000-1200 lbs., so build your mounts accordingly.

The fuel tank was installed next on the inboard supports. Because the fuel tank's sump stuck down 1.5" below the rest of the tank special considerations had to be taken. The fuel tank requires support under all of it and can not support all of its weight on the four mounting points (it was made for use in a race car), a wood mount was constructed out of some OSB I had in the garage. To make it tall enough we used three layers and saturated the outside with clear polyurethane to seal them up.

The filler system was added next. The tanks filler was about the middle of the top, and consisted of a large circular opening that a flange was mounted to. Two inch exhaust pipe was used for the filler, with diesel certified hose with 90 degree turns where the filler needed to turn. It is important to use the correct hose for this. Do not use radiator hose. An old filler from a junked Dodge pickup was used for the filler and a vented cap topped it off. (see picture)

Of note: before the fuel system was added all of the added support framing was painted with Cummins beige paint. The exhaust pipe is plated for corrosion resistance. To be installed later is a fuel transfer system that will connect to the output of the engine's fuel pump through a solenoid valve and a small fuel pump. The fuel transfer system will monitor how full the generator fuel tank is and stop transfer automatically when the generator fuel tank is near full, and will also stop transfer if the main fuel tank get below 1/3 full.

The actual generator install was a challenge, as the generator is taller than the hatch opening to the compartment. Ultimately we had to remove the cooling system from the generator as well as the mounting cradle. It took all day to install it and get the mounting cradle back underneath the generator. The distance from the mountings we installed and the floor of the bus was sufficient to meet the Onan distance requirements listed in the installation manual. (see pictures).

The exhaust system was some effort, as I didn't want the muffler hanging below the generator and reducing ground clearance. On factor that must be considered is that the generator is on stiff rubber mounts. For my plan to work the muffler had to be mounted the part of the body framework. You do not use rigid connections that connect a fixed mounted piece to a rubber mounted piece. The exhaust pipe now goes forward instead of down, but not too long a distance before turning towards the outside of the bus and connecting to the muffler rigid mounted to the bus body, through flexible exhaust pipe brazed to short lengths of 2" NPT pipe. It is flexible enough to handle any movement between the rigid pipe from the engine to the muffler. (see picture)

After re-installing the cooling system, servicing it, connecting up the fuel system, connecting the generator to 12 VDC (at the alternator), I started up the generator which ran for a couple minutes and quit from fuel starvation, I had another problem to solve.

Reading the service manual was very useful as it contains troubleshooting trees. I determined the the generators engine protection mechanisms were not shutting it down, and the fuel control solenoid only released after it stopped, I noticed I didn't hear the fuel pump. Connecting the fuel pump to 12 VDC showed it did not work any more. These days Auto parts stores don't carry the old style pumps like the Onan had, but I was able to find a usable diesel rated low pressure pump at an AutoZone. The pump needed to have NPT fitting at both ends. The one I bought fit perfectly and I was able to use the Onan hard lines without modifications.

Now the generator was running and producing power (see picture)

In the next post I will talk about the electrical connections from the generator

flattracker 08-24-2020 02:00 PM

Hi John,
I appreciate your advice on the solar panels. I bought the flexible solar panels last fall, 10 total. One major consideration I had was how the bus would look with solar panels on the roof, and the flexible ones have the least impact on the shape and look of the bus. I believe I have a solution for the heat problem. You know these political campaign signs we see every election? They are basically a plastic form of cardboard type material. After the election they have no use, but sandwiched between the panel and the roof they actually would allow airflow between the panel and the roof, as well as insulation.

Iceni John 08-24-2020 03:53 PM


Originally Posted by flattracker (Post 402255)
Hi John,
I appreciate your advice on the solar panels. I bought the flexible solar panels last fall, 10 total. One major consideration I had was how the bus would look with solar panels on the roof, and the flexible ones have the least impact on the shape and look of the bus. I believe I have a solution for the heat problem. You know these political campaign signs we see every election? They are basically a plastic form of cardboard type material. After the election they have no use, but sandwiched between the panel and the roof they actually would allow airflow between the panel and the roof, as well as insulation.

The important thing is to allow a free flow of air under the panels, even when parked - PV thermal deratings make a big difference to power output in hot weather, and with only a thousand watts you don't want to waste any potential power at all. Just a few inches of air gap should work well, in effect making a tropical double-skin roof like old Land Rovers had, and like the MC5 buses that were used by Aramco in Saudi Arabia (complete with extra rear doors for the women!). In hot weather my PV panels, and the central walkway to which they are hinged, are shielding my roof from almost all direct radiant heat, making the bus's interior noticeably cooler because the roof itself is at little more than ambient air temperature. At some point you have to decide what's more important - aesthetics, or functionality and efficiency. Le Corbusier said that a house is simply a machine for living in, so by that same notion a converted bus is also still just a house machine, albeit a mobile one - I generally gravitate towards functionality as the primary determinant of design, but that's just me!

Also bear in mind that the panels, if matching the very arched Crown roof contour, will make the half of them on the lee side produce very little or no power in the winter when the sun is lower and their angle is approaching perpendicular to the sun, or even partially shaded by the curve of the roof. Partially-shaded PV panels produce very little usable power if any at all, so by effectively blocking half the panels' insolation you'll be down to probably only a few hundred watts at best from a nominal 1kW of panels. That's why my panels can be raised up to 21, 33 or 45 degrees from horizontal, to maximize solar harvest even in the winter at higher latitudes during shorter winter daylight. And if half your panels are producing significantly less power than the other half, then you'll need two separate MPPT charge controllers because if you feed differing inputs into one MPPT CC its MPPT function will get mightily confused, resulting in less usable power reaching the batteries. That's partially why I have two Morningstar TS-MPPT-60s, one for each array of panels, so it doesn't matter if the arrays are producing different power. Can't waste those photons!


flattracker 08-24-2020 04:55 PM

Hi John,
Reading your post prompted me to look at the panels that I have. I measured them and found all of them would use up 30 feet front to back on the roof of my bus, they are only about 27 inches wide so they would only cover the center part of the roof of a crown, where there isn't that much curvature to worry about. the plastic campaign signs are still a good bet for insulating the roof from the panels. I plan to try placing one on the bus in the next couple days to see how it would look. I know the rigid panels get warm also, based on temperature measurement I took last winter. I have one of my panels that were damaged rejects from a local solar plant project I got a few years ago. I read 100 degrees in the sun when outside temp was about 40 degrees. I don't think I will actually get the full kilowatt most of the time, but the bus will have multiple power sources. I have calculated my expected power usage and I know I won't be using a whole kilowatt most of the time. My biggest power consumption will occur when running both furnaces and the refrigerator, but none of these are constant power users. Your points are valid though. By the way, I have a second 250 amp alternator mounted on the engine, so when I am driving, that is up to about 2 Kw (only when load is sufficient to cause 250 amp draw)
Thanks though

flattracker 08-29-2020 06:28 PM

1 Attachment(s)
Hi again John,
Today I brought one of my flexible solar panels to the bus for a fit check. I found that when the panel was placed with the longer dimension front to back the curvature is negligible as far as sun angle. When placed with the longer dimension side to side it's still not a major concern. My brother told me that he could not see the panel place on the roof when standing on the ground. I plan to connect the panel to a charge controller and a battery to produce a load, and then monitor the temperature of the panel under load with good sublight. I will let you know of the results. Your input gives me interest to know all operating results of the panels. I will try to add a picture of the panel.
Thanks again

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