Help me understand transmissions (especially AT545) and desirable model years

[RomaniGypsy]

Perusing bus auction sites, I see that the vast majority of offered buses come with the AT545 transmission. I've seen a good bit of "negative press" about the AT545 on this site and hoped that maybe y'all could help me out here.

Online I can find nothing about the transmission itself. Wikipedia doesn't explain what it is, and every other site that comes up in a general search seems to be a parts site.

What I've gathered so far is that the AT545 is a 4-speed automatic transmission with no lockup torque converter. What I know about regular vehicle transmissions is that a lockup torque converter helps with fuel economy, but also complicates the transmission (making repair, replacement, or overhaul more expensive).

Can any of you explain, in full detail, the pros and cons of the AT545? (After all, if it's THAT popular, how could it be so awful? The way I see it, if I'd get 9.5 mpg with an AT545 and 10 mpg with something that has a lockup torque converter, but the AT545 is bulletproof and will last longer than pretty much anything else out there... well, I'll take the AT545! I keep thinking about how GM still makes the Turbo Hydramatic 400 3-speed non-lockup automatic transmission and installs it in military Humvees. Well, heck, if it's tough enough for the military...!!!)

I'd also like to know what y'all think IS the best automatic transmission to seek in buses from the late 1990s and early 2000s. (I've also seen some offering an AT543 - what is the difference between that and the AT545?)

People also talk about buses with emissions control items from more recent years being extremely undesirable. Okay, I can rock with that, but why? A recent poster said he'd sooner not buy a bus at all than to buy one with EGR, DPF, and DEF. To the best of my knowledge, those mean "Exhaust Gas Recirculation" (cars have had that for decades and a malfunction in that system doesn't typically impact driveability nor fuel economy), "Diesel Particulate Filter" (seems that the only problem with that would be the cost of replacing it as well as the replacement interval, about neither of which I know anything), and "Diesel Exhaust Fluid" (now THAT one I can see being a pain in the butt... all I know is that regular diesel cars and trucks only started requiring DEF in 2007). So, the extent of what I know about these things would lead me to believe that I should avoid anything from 2007 onward if I don't want to deal with emissions stuff. But would it turn out to be a wash on account of better engines, better fuel economy, better transmissions, etc? Really, for example, if the best 2003 bus I could get returns 10 mpg, and I could get a 2008 model that returns 11 mpg just as reliably but requires DEF, you'd think the cost factor would be a wash.

I hate to seem like I'm just begging for help all the time but if I can't find this information easily online, I'd have to question its accuracy if I were to find it at all.

 

[EastCoastCB]

545's are weak, generate a lot of heat from friction, and have no OD.
Its the WORST transmission you can get for driving a long ways in a school bus.
ANYTHING is an upgrade from one. It has NO engine braking in the mountains, and less efficiency than a trans with lockup or a manual.
There's not really any "PRO'S" to list. Its just the cheapest transmission so that's what most districts ordered. Any other transmission is better.
But I own a bus with a 545- its a shorty. I'm not doing a conversion on it though, and when/if that 545 goes out I'll gladly spend the time and money to upgrade it to a 643.
Here are threads on transmissions- https://www.google.com/search?q=transmission+choices+skoolie+site:www.skoolie.net&sa=X&ved=0ahUKEwiBw-rSm7bWAhXFNSYKHS2CDpYQrQIIMygEMAA&biw=1035&bih=517

 

[Jacob-SKO]

Those are good questions and I don't think anyone will fault you for doing research. This information is probably located in the forum somewhere but it can be hard to find.

The 545 was a very popular transmission for a school bus for a long time, however they are rarely in buses designed for long highway trips. Other than fuel economy a downfall for most of the skoolie community on that transmission is the excess heat that is generated from never locking up the torque converter. Some have added a bigger cooler and a temp gauge and have good luck with them. What I personally disliked about the 545 was slowing 15-20 mph on a slight incline without any engine rpm change and getting no help slowing down from the transmission.

As far as emission equipment goes there are some pros to a newer bus and some cons (my bus is a 95 however). The emission equipment on a diesel Can negatively impact overall longevity of the engine and if it fails will leave you sitting until repaired. On the older diesels it's one less thing to break.... but a newer bus will have advantages as well. Everything is newer for one! And the computer can come in super handy if you are troubleshooting an issue. The older diesels are usually cheaper to maintain almost everything is less expensive depending on the engine. Like turbos, injectors etc.

 

[RomaniGypsy]

545's are weak, generate a lot of heat from friction, and have no OD.
Its the WORST transmission you can get for driving a long ways in a school bus.
ANYTHING is an upgrade from one. It has NO engine braking in the mountains, and less efficiency than a trans with lockup or a manual.
There's not really any "PRO'S" to list. Its just the cheapest transmission so that's what most districts ordered. Any other transmission is better.
But I own a bus with a 545- its a shorty. I'm not doing a conversion on it though, and when/if that 545 goes out I'll gladly spend the time and money to upgrade it to a 643.
Here are threads on transmissions- https://www.google.com/search?q=tra...WAhXFNSYKHS2CDpYQrQIIMygEMAA&biw=1035&bih=517

We certainly do intend to be putting a lot of highway miles on the bus. People are saying in one of those threads that the MT643 is better, but my question would be - is it a given that any bus with an AT545 can be found in the same year and model with an MT643 if simply spec'd differently from the factory? In my experience with regular cars, that ain't the way it always worked. For example, there was a time in the 1980s when you could get either the 3-speed slopbox TH-series transmission in a GM car, OR the optional 4-speed E-series overdrive / lockup transmission. There was SOME overlap, but not much. (In the 1970s, you couldn't get overdrive even as an option, as far as I have ever known.) Then toward the end of the 1980s into the 1990s, there was no option for the TH-series because the 4-speed lockup automatic became standard.

Essentially what I'm trying to figure out is - if I look on an auction or sale site and find, say, a 1998 Thomas with an AT545, should I pass and look for a 1998 Thomas with an MT643 or something "better"? Or would I be wasting my time because the MT643 didn't come out until 2001 or something?

 

[EastCoastCB]

It just depends on the bus' intended use and what the school district ordered.
Its not like cars. GM makes you buy package x to get one feature you want. When you order a bus you basically pick what you want for virtually everything. Most schools don't need freeway cruisers, so the 545's were the main choice for most schools.
The 643 and 545 were both around for decades. Then came the epa emissions regs, and electronic transmissions with overdives started showing up.
I've seen mechanical engines with 3060's in buses as late as 1998-1999.
Most of the buses out there from regular routes not in mountains will have a 545 until the early 2000's.

 

[turf]

the AT 545 and MT 643 are both mechanical run transmissions. both will be found on older buses... pre-obd2 years. after emotors came out electronic transmission followed and they are the 1000, 2000, 3000 series from Allison. most often we'll see a MD3060 on a bigger bus that replaces the MT643 and Allison 1000s on buses that replace the at545.

 

[brokedown-SKO]

Just to throw it out there.

Everything everyone has said about the 545 is correct. However.

I have the 542. It's the same as the 545 but with a smaller oil pan. It's not great, I'm not singing any praises, but ultimately it is adequate and works and I've put almost 25k miles on mine this year in every setting and every temperature across the US and I'm still rolling along fine.

Yes, more gears would be better. Lock up would be better. I'm generally rolling around 58mph because that's more or less the sweet spot of engine rpm vs making progress, another gear might add 10mpg or more to that.

The 545 is probably the most common transmission in these things in the 90s, and while it could be better, it could be a lot worse.

 

[EastCoastCB]

Just to throw it out there.

Everything everyone has said about the 545 is correct. However.

I have the 542. It's the same as the 545 but with a smaller oil pan. It's not great, I'm not singing any praises, but ultimately it is adequate and works and I've put almost 25k miles on mine this year in every setting and every temperature across the US and I'm still rolling along fine.

Yes, more gears would be better. Lock up would be better. I'm generally rolling around 58mph because that's more or less the sweet spot of engine rpm vs making progress, another gear might add 10mpg or more to that.

The 545 is probably the most common transmission in these things in the 90s, and while it could be better, it could be a lot worse.

For a full size bus they're even worse.

 

[brokedown-SKO]

Not to discount that in any way, just saying that it actually does function and you can drive them, despite every other possible option being better in substantial ways.

 

[Brad_SwiftFur]

. . .

People also talk about buses with emissions control items from more recent years being extremely undesirable. Okay, I can rock with that, but why? A recent poster said he'd sooner not buy a bus at all than to buy one with EGR, DPF, and DEF. To the best of my knowledge, those mean "Exhaust Gas Recirculation" (cars have had that for decades and a malfunction in that system doesn't typically impact driveability nor fuel economy), "Diesel Particulate Filter" (seems that the only problem with that would be the cost of replacing it as well as the replacement interval, about neither of which I know anything), and "Diesel Exhaust Fluid" (now THAT one I can see being a pain in the butt... all I know is that regular diesel cars and trucks only started requiring DEF in 2007). So, the extent of what I know about these things would lead me to believe that I should avoid anything from 2007 onward if I don't want to deal with emissions stuff. But would it turn out to be a wash on account of better engines, better fuel economy, better transmissions, etc? Really, for example, if the best 2003 bus I could get returns 10 mpg, and I could get a 2008 model that returns 11 mpg just as reliably but requires DEF, you'd think the cost factor would be a wash. . .

Other folks have covered the transmission fairly well so I won't get into that. I am a professional driver and have driven many trucks newer than '07, some with DPF systems and others with DEF systems. It is my experience that *BOTH* return slightly *LESS* fuel economy than their older, non-emissions counterparts. Both have "Regen" cycles which are supposed to "clean" the particulate filters ... the engines run at high idle and high temperatures to burn off the soot. They use a fair amount of fuel to do this. Some systems were known to prompt the driver to do a parked Regen while driving (and I recall a bus in this predicament in upstate New York, on the shoulder of a freeway doing a Regen being hit, killing a number of passengers). I have had trucks on which the DPF system would "Regen" but not clean the filter properly, necessitating a visit to a shop for filter removal and cleaning. The EGR cooling systems is one weak point on some of these engines. This is discovered when suddenly your cooling system "Low Coolant" alert goes off, and there is ZERO external sign of a leak (the coolant is sucked through the engine and burned). Care to buy a $2500 EGR cooler pipe? The DEF systems aren't any better. I saw one in the shop (on an IH truck, I believe it was with a MaxxForce engine) which needed the DEF exhaust system. Everything had to be ordered by the VIN specifically for the truck and the entire system was projected to cost around $8,000 (something about coolant contamination or some such). Need I give more examples?

Most of us here either have witnessed such examples, or have heard from reliable sources. Forums are full of similar stories. Why subject yourself to that if you can buy a slightly older vehicle without all that and save yourself the headache? Sure, many of us remember the 70's car emission systems and how terrible they were, and granted, over time manufacturers greatly improved them. We are at that same stage with diesels now and in the coming years the engines and systems will be better and longer lasting. Back in the day, folks would disable and bypass the car emissions systems, just as they can bypass/delete the EGR/DPF/DEF systems today (not legally, of course).

 

[Iceni John]

If you're going to put some serious miles on your bus, buy a bus that is set up for trips and distance work, not just for local around-town fixed routes. It will not only have underfloor storage (always good to have), but also a bigger engine (e.g. a Cummins 8.3 instead of a pickup truck engine) and an appropriate transmission with lockup. That was one reason I chose my bus - it has the same driveline as a Greyhound MC9 bus, so it drives like a Greyhound bus. Same engine, same transmission, same driveshaft, same axle. Don't faff about with marginal drivetrains better suited for pickup trucks, but buy what will do the job. If you live or drive where there are real mountains, this is especially important.

John

 

[RomaniGypsy]

If you're going to put some serious miles on your bus, buy a bus that is set up for trips and distance work, not just for local around-town fixed routes. It will not only have underfloor storage (always good to have), but also a bigger engine (e.g. a Cummins 8.3 instead of a pickup truck engine) and an appropriate transmission with lockup. That was one reason I chose my bus - it has the same driveline as a Greyhound MC9 bus, so it drives like a Greyhound bus. Same engine, same transmission, same driveshaft, same axle. Don't faff about with marginal drivetrains better suited for pickup trucks, but buy what will do the job. If you live or drive where there are real mountains, this is especially important.

John

I've seen some school buses that appear to have underbody storage compartments. Are you saying that school districts buy some buses to use primarily for around-town routes and some to use primarily for long-distance trips such as for field trips or when hauling a competition team or marching band to a relatively far-off location?

I guess I never paid attention to that when I was in school. I always thought that they just plucked a few buses from the yard and used them whenever we had to go anywhere.

 

[RomaniGypsy]

the AT 545 and MT 643 are both mechanical run transmissions. both will be found on older buses... pre-obd2 years. after emotors came out electronic transmission followed and they are the 1000, 2000, 3000 series from Allison. most often we'll see a MD3060 on a bigger bus that replaces the MT643 and Allison 1000s on buses that replace the at545.

One would think that a fully mechanical (hydraulic) transmission would be ideal. The more electronic gadgetry you have, the more that can break and the more expensive the repairs will be.

I had a 1967 Buick Skylark with the 2-speed slopbox Powerglide - okay, Dynaflow - transmission, and when it blew, I wanted to get a 4-speed overdrive transmission installed. The mechanic said that my best bet would be to get a 3-speed TH350 installed, as its guts used the same bellhousing as the Powerglide. So, I did that. Cost me less than $1,500.

About ten years later, I blew the transmission on a 1996 Tahoe - one of those 4-speed electronically controlled overdrive transmissions. The replacement cost me almost $3,000.

Given that a retired bus may not have many miles left before its transmission bites the dust no matter what it is, one would think that replacement cost would be a concern.

I really do want the best information that's out there... I just try to put everything in perspective. These days cars are using 9 and 10 speed automatic transmissions, CVTs, you name it... if one of those things blows, the replacement cost is stratospheric. Some people say that fully mechanical transmissions with a low number of gears are best due to simplicity and ease of repair / replacement. I tend in that direction. I'm generally wary of modern technology. What I'm looking for is a transmission that will last a long time and do its job. I'd sacrifice a bit of fuel economy if a transmission like the AT545 would far outlast its newer, lockup / overdrive stablemates.

But if the longevity is the same... well...

 

[Jacob-SKO]

I've seen some school buses that appear to have underbody storage compartments. Are you saying that school districts buy some buses to use primarily for around-town routes and some to use primarily for long-distance trips such as for field trips or when hauling a competition team or marching band to a relatively far-off location?

I guess I never paid attention to that when I was in school. I always thought that they just plucked a few buses from the yard and used them whenever we had to go anywhere.

Yes some buses are for picking up the kids around town and some are for taking kids around to events and such. They will typically have storage underneath for gear. The 545 was typically in the around town buses. If you want an older transmission with lock-up the 643 was most common but are typically in the out of town type buses.

 

[Brad_SwiftFur]

I've seen some school buses that appear to have underbody storage compartments. Are you saying that school districts buy some buses to use primarily for around-town routes and some to use primarily for long-distance trips such as for field trips or when hauling a competition team or marching band to a relatively far-off location?

Yes, in fact, this is exactly what we are saying.

One would think that a fully mechanical (hydraulic) transmission would be ideal. The more electronic gadgetry you have, the more that can break and the more expensive the repairs will be.

I had a 1967 Buick Skylark with the 2-speed slopbox Powerglide - okay, Dynaflow - transmission, and when it blew, I wanted to get a 4-speed overdrive transmission installed. The mechanic said that my best bet would be to get a 3-speed TH350 installed, as its guts used the same bellhousing as the Powerglide. So, I did that. Cost me less than $1,500.

About ten years later, I blew the transmission on a 1996 Tahoe - one of those 4-speed electronically controlled overdrive transmissions. The replacement cost me almost $3,000.

Given that a retired bus may not have many miles left before its transmission bites the dust no matter what it is, one would think that replacement cost would be a concern.

I really do want the best information that's out there... I just try to put everything in perspective. These days cars are using 9 and 10 speed automatic transmissions, CVTs, you name it... if one of those things blows, the replacement cost is stratospheric. Some people say that fully mechanical transmissions with a low number of gears are best due to simplicity and ease of repair / replacement. I tend in that direction. I'm generally wary of modern technology. What I'm looking for is a transmission that will last a long time and do its job. I'd sacrifice a bit of fuel economy if a transmission like the AT545 would far outlast its newer, lockup / overdrive stablemates.

But if the longevity is the same... well...

I have a '98 Suburban in which I had to replace the transmission a couple years ago. Cost was about $1200.

A transmission's biggest enemy is heat. Friction and resistance generate heat. The AT545 does not have a lock-up torque converter and while it's going down a flat road, there's relatively little slippage so it generates relatively little heat. Start going up a mountain and watch it start getting hot. Once or twice may not be a major problem but do this frequently and eventually you'll not be going up any hills any more with that transmission. When a torque converter locks up, there is no slippage, reducing the heat generated. The AT545 is only designed for a certain amount of horsepower and torque, which is one reason you don't see it in buses with larger engines. Much has been written about the AT545, especially the ones running on mountainous routes. The gist of it was accountants spec'ing the buses with lower RPM governors in an attempt to save fuel, and subsequently burning out the transmissions. Allison basically said they had to turn the RPM governors back up or they would void the warranty on every last bus on the lot (it has to do with the transmission building up full pressure at around 2500-2700 RPM). Also consider the AT545 is(was) commonly used in buses on stop-and-go routes, so even in the mountains it gets a chance to pause and cool down every so often.

 

[cadillackid]

Just to throw it out there.

Everything everyone has said about the 545 is correct. However.

I have the 542. It's the same as the 545 but with a smaller oil pan. It's not great, I'm not singing any praises, but ultimately it is adequate and works and I've put almost 25k miles on mine this year in every setting and every temperature across the US and I'm still rolling along fine.

Yes, more gears would be better. Lock up would be better. I'm generally rolling around 58mph because that's more or less the sweet spot of engine rpm vs making progress, another gear might add 10mpg or more to that.

The 545 is probably the most common transmission in these things in the 90s, and while it could be better, it could be a lot worse.

here's my deal on the AT545..

Yes my upgraded allison 1000 in my RedByrd is INCREDIBLE to say the least.. I get 25-30% better MPG, my engine runs MUCH cooler and the bus just sails right along..

BUT!

I previously drove my DEV bus, what 18K miles all over the place (not the rockies.. but all over the eastern mountains). on a DT-360 and an AT545. and it got me there... yes i took it easy on the hills.. slow going up and down.. but nothing broke, i didnt careen off the side of a mountain,

I drove the redByrd 8-10k miles before I upgraded its AT545.. it got me there just fine too.. HOWEVER it DID break.. the modulator was no good and I didnt know it.. which caused me harm to the transmission.. no it never put me along the road, but it did always want to run hotter than i liked(engine), and eventually i started having valve body shift issues. (never slipped)..

going to an allison 2000 series or a bus with MD-3060. is a double upgrade as you gain Overdrive along with a lockup.. diesel engines like lower RPMs.. they just do...

more and more as the months and years go on we will see busses with these transmissions at the auctions.. most everything built after about 2002 or 2003 will have one of the overdrive models..

yes its true they are more complex.. with electronic computers and solenoids.. however they are designed to last a long time.. the fact that they have lockup means they have likely always run cooler (heat KILLS transmissions), havent had the slips associated with hot thin fluid and overall can easily last 300K or more miles..

AT545's are cheap.. for one if you are mechanically inclined at all you can re-clutch one pretty easily.. for another, people give them away practically.. a fully rebuilt one will cost you about $1200..

if you burn up an allison 2500 PTS (pretty standard school bus trans) you can find them for $2400 + shipping online remanufactured..

changing a transmission in a conventional dognose bus is actually pretty easy if going Like-for-like. an AT545 can be swapped in a day.. maybe less if you have a helper..

upgrading a transmission isnt so easy.. (follow my redbyrd transformation thread to see how its done).. (or maybe Turf;s AT1545 thread), and you can see how it can be tough to perform an upgrade..

-Christopher

 

[RomaniGypsy]

here's my deal on the AT545..

Yes my upgraded allison 1000 in my RedByrd is INCREDIBLE to say the least.. I get 25-30% better MPG, my engine runs MUCH cooler and the bus just sails right along..

Is it a bolt-up swap, where you can just drop the AT545 and bolt up an Allison 1000-series without having to modify anything else?

going to an allison 2000 series or a bus with MD-3060. is a double upgrade as you gain Overdrive along with a lockup.. diesel engines like lower RPMs.. they just do...

Does this mean that the Allison 1000-series does not have overdrive? (You called it a "double upgrade"...)

more and more as the months and years go on we will see busses with these transmissions at the auctions.. most everything built after about 2002 or 2003 will have one of the overdrive models..

Fair enough, but then as the years get on, I've noticed that the engines tend to change (the MBE-series engines come into view a lot in 2004 and beyond) and then we wind up with emissions control stuff. (I still haven't gotten an answer as to what year that stuff became problematic. With passenger vehicles it's easy - 2007 and later, avoid 'em. 2006 and earlier - no DEF.)

upgrading a transmission isnt so easy.. (follow my redbyrd transformation thread to see how its done).. (or maybe Turf;s AT1545 thread), and you can see how it can be tough to perform an upgrade..

-Christopher

I'd like to know which swaps can be done as bolt-up jobs. (For example, in the 1967 Skylark example I gave earlier, the Powerglide to TH350 was a bolt-up swap.)

 

[EastCoastCB]

There will be no cheap, easy, bolt up transmission upgrade.
Chris is a mega-tech-guru and was able to pull it off, but its definitely not a plug and play job.

 

[RomaniGypsy]

There will be no cheap, easy, bolt up transmission upgrade.
Chris is a mega-tech-guru and was able to pull it off, but its definitely not a plug and play job.

Okay, then what would you say is a good price premium to pay for a better transmission? Say that I could get a bus with an AT545 for $3,000 and we will assume that that's a "good deal" given the condition and characteristics of the bus. The exact same bus with an MT643 or some other "better" overdrive lockup transmission - what would you pay for that?

 

[turf]

I'd like to know which swaps can be done as bolt-up jobs. (For example, in the 1967 Skylark example I gave earlier, the Powerglide to TH350 was a bolt-up swap.)

same transmission for same transmission.

the problem is the bell housings on the transmission and the engine adapter.

a cummins 5.9 that has the MT643 already mounted to it has a #2 bell housing. a cummins 5.9 with a AT545 already mounted to it has a #3 bell housing.

when they order the setup, the vehicles are different. MT643 is much heavier and has its own support. the AT545 just hangs off the back of the motor. the overall size of the transmission is different and therefore the drivelines on each are different.


if you get a 643, be sure its off the same engine you are putting it on and get the adapter parts. don't take a trans off a t444 and put it on a 5.9.