Why you don't want the Allison AT545 Transmission in the mountains. - Page 2

Versatile · 10-23-2016, 08:43 AM

Quote:

Originally Posted by GreyCoyote

The 545 isnt a world beater by any means, but it IS a fairly decent tranny for when it was developed, and for WHAT it was designed for (there. I said it). Many tens of millions of miles have been logged hauling school kids around with them, and they appear in all sorts of medium duty trucks. You dont get this kind of market penetration if you are building an Edsel. The 545 really isnt "bad". But it is old technology and getting long in the tooth. And its not made for pulling hills all the time in a 30k GWV bus.

Many of the people who roundly decry the 545 as "junk" have never owned one. Or if they did, they got it with the bus (at auction, with its socks down around its ankles) and it had 200k miles of wear. And then they refuse to use the correct, current generation fluid, they crank-up the injection pump, and then take it out on a hilly banzai run in the middle of summer without a cooler. And they wonder why there are troubles.

The truth is, its not a bad tranny. And if you have one, you would be well-advised to read the Allison service bulletins and learn to drive it and maintain it properly. It isnt idiot proof, and it does not suffer shock loads well at all. But it WILL perform to the nameplate values with basic care.

A fact I recently learned from a retired Allison engineer: full pressure on the internals (bands) doesnt occur until the input shaft hits 2400 rpm. If you run slower than this, the trans is slipping under load and under a heavy load the result is predictable. Many school districts in hilly areas found this out the hard way when they ordered taller rear-ends and set the rev limiters at 2300 rpm thinking they would save money on fuel by "better using" the torque curve instead. Those districts wrote a lot of checks to transmission suppliers.

I'm looking at upgrading from a 545 to a 643 or 3060. Not because there is anything wrong with the 545, but rather because I want a locking torque converter and something I can flog in the hills without cringing. And a couple hundred RPMs less at 60 mph would be nice too.

Otherwise I'd happily stick with the 545 and call it good.

Back to the article referenced: we dont know what happened there, but if you havent mastered basic bus dynamics, that could indeed be you. So take the time to train yourself how to drive a bus, to stop a bus, and to maintain a bus before you get too far down the road. Some accidents arent avoidable, but most are.

End rant.

Thanks that wasn't a Rant. You gave some newbies from very good info.

turf · 10-23-2016, 10:24 AM

ive heard a lot about upgrading to the mt643 or the 3060. after abusing my 545 like i do, and a few conversations with the mechanic i'd have to pay to swap them, when i kill my at545, i will happily replace it with a rebuilt at545.

imo, changing a 4 speed with a 4 speed (the mt643) doesnt gain you anything but lockup in 3rd and 4th gear, and isnt worth the cost. i'd rather have a rebuilt 545.
the 3060 with 6th gear unlocked would be cool, but again, the cost trumps reliability of rebuilt 545.

if i wanted a 643, i should have bought a different bus. unless i got a bigger motor than the cummins 5.9, i think that 545 will do me.

its fun to have pipe dreams with the bus, but its not fun when the costs are deeper than your pockets.

Versatile · 10-23-2016, 10:55 AM

Quote:

Originally Posted by turf

ive heard a lot about upgrading to the mt643 or the 3060. after abusing my 545 like i do, and a few conversations with the mechanic i'd have to pay to swap them, when i kill my at545, i will happily replace it with a rebuilt at545.

imo, changing a 4 speed with a 4 speed (the mt643) doesnt gain you anything but lockup in 3rd and 4th gear, and isnt worth the cost. i'd rather have a rebuilt 545.
the 3060 with 6th gear unlocked would be cool, but again, the cost trumps reliability of rebuilt 545.

if i wanted a 643, i should have bought a different bus. unless i got a bigger motor than the cummins 5.9, i think that 545 will do me.

its fun to have pipe dreams with the bus, but its not fun when the costs are deeper than your pockets.

As you said. Don't ruin your dream by trying to use a tranny that is older and so much better is out there if you know what you need and to look for. I have a 5.9 12valve mechanical pump and the MT643 and i have no buyers remorse.

somewhereinusa · 10-23-2016, 02:16 PM

I think it is time to remind/inform that any engine without a throttle, pretty much any diesel, won't slow anything down. Without a throttle plate there is nothing to create pack pressure. A "jake brake" modifies the exhaust valve timing so that it does create back pressure in the cylinder. An "exhaust brake" is some sort of butterfly in the exhaust usually where the exhaust pipe attaches to the manifold. A 643 isn't going to act much different going down, even if in lockup, it is attached to an engine that is basically freewheeling. Sure your bus slows down when you get off the throttle but, that's just because you quit giving it fuel. Other than the friction of parts involved, the engine does nothing to slow you down.

As an example, if you have two identical buses except for one being gas and the other diesel. Drive them side by side and let off of the throttle, the gas bus would slow down MUCH faster.
The third method of slowing a diesel engine vehicle down, the "tesla" has nothing to do with the engine or transmission. It is mounted around the driveshaft. It uses magnetism to slow the driveshaft down but doesn't touch it thus, nothing to wear out.

Dick

cadillackid · 10-23-2016, 06:23 PM

I dont understand... in my observations it is the compression of air in the cylinders but niot being converted to energy that slows things down.. that compression stroke... the jake break indeed alters the valves and allows the compressed air to release before the "power stroke" would happen so you are then at close to a vacuum on your "power stroke".. which gives you more stopping power...

my little diesel DT-360 dropped into L2 or L1 in my 545 pulls that monster bus down quite quickly.. muych quicker than dropping the 5.3 gas in the little silverado down to L2 or L1 pulls that truck down.. only .6 liters difference and a HUGE difference in weight.. and im talking slower speed pull downs so wind resistance isnt as much of a factor...

so to me the diesel is pulling down much better than a gas motor...

-Christopher

jazty · 10-23-2016, 06:39 PM

Here's a YouTube video that explains engine braking in a quick and simple manner (4:49 minutes, to be exact

):

https://www.youtube.com/watch?v=o8Cta2cC2Co

Iceni John · 10-23-2016, 09:23 PM

Excellent video clip. The Jake works by preventing the compressed air in the cylinder from springing the piston back down again, by wasting that air out before it can return energy to the crankshaft. The pistons are then sucking against a vacuum, like in a gasoline engine with its throttle closed. That's why a straight-piped truck with Jakes sounds LOUD - you're hearing however many hundreds of PSI air being suddenly exhausted to atmosphere, essentially lots of little explosions. Another self-explanatory term for a Jake is a Compression-Release brake. He also talked briefly about exhaust brakes such as those made by Williams or Banks - they work reasonably well on smaller engines, but if there's any weakness in the exhaust system they'll find it!

When used with a transmission that locks up in gear, a Jake provides very positive and predictable braking. For my engine it gives almost 200 horsepower-equivalent of decelerative force, and for large 4-strokes a Jake can provide more braking force than the engine can produce power! My friend's Gillig tandem with a big 14-liter Cummins has a 3-stage Jake, and when we took it up to Wrightwood ski resort in the San Bernardino mountains near his house (we were having to slow for the curves on the way up!), on the way back down he didn't touch the brakes at all, just switching between the Jake's Low, Medium and High settings to control his speed. Fun!

If I ever straight-pipe my bus it will sound like this:

https://www.youtube.com/watch?v=Wz_VJB3y89M

It seems to slow this bus better than mine, and I don't think he's using his wheel brakes at all - I don't see his brake lights coming on, but maybe they just don't work! (It's in Mexico . . .)

Anyway, back to the subject of this discussion. Personally I would never buy a non-lockup transmission if I had to deal with any serious up or down grades at all. Maybe a 545 is OK for Florida and other flat states, but for most of the country I think it's a potential safety hazard unless a Telma is fitted, and that's very unlikely with a 545.

John

cadillackid · 10-23-2016, 09:27 PM

ive logged 11000 miles on my 2 545's through the smokies, appalachains, adirondacks.. i agreee maybe its not a rockies transmission and maybe im biased because my busses are shorties.. but all the 'AT545 SUCKS' talk frankly is just total BS... its fine for anything east of the rockies

I get you its probably not proper for the rockies but theres been more than one skoolie on this site thats run them over the rockies and they havent crashed in a pile, nor did theur bus blow up...

a really good part of it is just plain learning to DRIVE your bus.. and not just occupy the left front seat..

-Christopher

Iceni John · 10-23-2016, 11:31 PM

Here's an interesting point by Lin in his second paragraph: Transmission oil cooler? He has a MC5a with a MT647 transmission.

John

slidewayzm5 · 10-24-2016, 12:27 AM

Food for thought:

I've looked for an Exhaust Brake for my 91 7.3L IDI which will be turbo'ed by the time the conversion is finished and I'm on the road. They regularly run $1000 CDN +.

Since I have an AT545 and will be in the Rockies VERY often, I found an APEXI ECV (Exhaust Control Valve) which does not fully close (I had read some people had concern of a fully closing exhaust brake would potentially cause the exhaust valves to float).

Basically it is a $200 flap that will be welded in just behind the turbo and when I'm on a grade, I'll start slow, use 3rd or 2nd gear and pull the ECV to its 80~90% or whatever it closes to. Stab braking for maximum cooling.

The ECV valve will provide back pressure and restrict the engine's rpms which should create drag into the transmission.

The question is - because the trans does not lock up - will it freewheel or will it slow me down a bit?

I'm assuming it will slow me down a bit because I don't think it just freewheels now when I let off until it is close to idle. I haven't driven it enough to know - only scooted it home 20 miles from where I bought it until the conversion is done.

I'll likely be putting in a larger capacity cooler for the runs up as well to keep the thermal shock to a minimum.

Thoughts??

turf · 10-24-2016, 01:18 AM

the AT545 just freewheels! you can only control your speed with the brake.

ive had an at545 for 4 years and been up and down the rockies and beat the crap out of it. the 1st few trips were white knuckle, change your shorts trips, but once you get the hang of it, it gets better.

coming down a hill is a balance of braking and trying to keep on the throttle to keep the transmission engaged while not speeding up. i may shift down to keep it in 3rd, but only the brake will control your speed. and go slow!

the difference in the engine rpm and the driveshaft rpm comes out as heat in the torque converter, so i can see how you could overheat a non locking t/c coming downhill.

before i added my transmission cooler, i was giving myself an ulcer over replacing the transmission. that fix was the tlc that the transmission needed to show me how good it really is. i no longer worry about replacing it.

cadillackid · 10-24-2016, 05:52 AM

ive done quite a bit of playing around with my 545's.. one bus is a GVWR 27500 and T-360 the other bus is a GVWR 17500 and T-444E

4th gear zero holding power
3rd gear - decent holding power in the lighter bus
2nd gear - pretty good holding power but max speed of 25-30 before the trans will override your shift and just go to 3rd anyway on its own..

trans cooler seems mandatory in a big bus on big hills... on a small bus it doesnt strain that hard and may very well may not get hot on you with the stock setup..

-Christopher

turf · 10-24-2016, 09:36 AM

i just checked my weights.

'93 tc2000, 30' converted bus - 21,000 lbs
and i sometimes pull a 3,000 lb trailer.

i've driven down monteagle without any problems. its the long, steep grades in the rockies where transmission will shred and give you ulcers. the roads are curvy and your always in the middle of a traffic jam at 65mph.

the trans will hold a just a wee bit before letting it freewheel, thats why i try to be on the accelerator on the way down, to keep the trans engaged. if you let gravity accelerate you out of the speed range for that gear, the transmission respond with noises you never want to hear. you just have to keep that 12 tons under control by braking and not exceeding the rpms for that gear range.

i have freewheeled down the mountain (scary) i have caught my brakes on fire coming down the mountain (less scary). since then, i've learned to slow down and control the bus on a descent. patience and a heavy brake foot get you down to the bottom safely. you just keep an eye (or ear) on the air, dont ride the brake, but use stab braking when you need it. i think im doing good if i hear the air dryer purge while i'm headed down hill.

cadillackid · 10-24-2016, 09:48 AM

ive driven monteagle in the DEV and it seemed easy to me.. sure it was a nice descent but I ran it nice n slow in 3rd gear of my AT545.. and only tagged the brakes every so often... I never once smelled hot pads or felt like I was running too fast...

are you guys saying monteagle is a huge deal? like im rockies-ready if my bus can handle monteagle?

to me monteagle doesnt seem near as steep and heavy duty as on I-68 in maryland...I think that grade is 7% and the 545 has run that..
-Christopher

Iceni John · 10-24-2016, 09:57 AM

In Lin's post of the thread I linked, he's saying that slippage, even when going downhill, is causing heat. That sounds entirely plausible - slippage is slippage, whether in forward or reverse. My guess is that non-lockup transmissions need a good transmission fluid cooler even more so than lockup transmissions, especially if used in mountainous terrain. The BCM thread was talking about coolers, so it may be pertinent for folk here using 545s. The Thermal Dynamics cooler I just finished installing is rated at 33,000 BTU/hour so it won't be adequate by itself for a big transmission like a HT740, but maybe it would suffice by itself for a 545? However, the best way to install such remote coolers is to have them reduce the fluid's temperature straight out of the transmission before it's then further cooled through the engine coolant's heat exchanger. The benefit to doing this is that the transmission fluid is quickly warmed by the engine coolant when starting in cold weather - cold fluid can be as damaging to a transmission as overheated fluid.

Another idea I had for a transmission fluid cooler is to use an old floor heater. They typically can output up to 80,000 BTU/hour, way more than any remote cooler with fan such as I bought. The main reason I didn't go this route is because heaters' inlet and outlet hoses are a lot smaller than the 1" hoses from my transmission, and I obviously don't want to restrict fluid flow. Also, I wasn't sure what the pressure ratings are for both my transmission fluid cooler loop and for a typical heater core - a burst heater core would not be good. Coolant is usually pressurized to 10 PSI or so depending on what cap is used, but transmissions could be pushing their fluid out to an external cooler at much higher pressures than that: there's probably a good reason that Crown used expensive SAE 100R5-16 hydraulic hose and fittings for the transmission fluid.

John

turf · 10-24-2016, 10:19 AM

the passes/grades in Colorado are different than Monteagle. i dont know why its different from Monteagle, but it is. I'd guess the main difference to me is that the grades are longer, there are more curves and more traffic. they're still 6-7% grades.

i'd agree with your link about the slippage. over rpms from a grade makes an awful noises in the trans as it copes with it.

before i got the trans cooler, on the way up the grade, the allison would downshift and still slow down. now with the cooler, it downshifts and has power to speed up. i can at least maintain 45 on the way up the grade unlike prior to the cooler. on the way down, i put it in third and you just have to brake to stay under 45.

my bus came from Arizona, and unlike most Colorado buses, has no retarder. an at545 with a retarder would be a nice combo to over come the lockup problem.

come to Colorado. the at545 is capable, just not optimal. drive it like a really heavy car, when you hit the grades, turn on the 4 ways, slow down, sit back and let everyone pass.

cadillackid · 10-24-2016, 10:29 AM

yes i agree on heat to some extent... when you are on a heavy ascent you are putting all oyur horsepower and TQ into the Torque converter at its stall speed typically...

when on a descent the compression slowdown of the engine doesnt pull down at the TQ and HP rating of your engine so you dont generate as much heat..

heat is generated both from TQC "stall slippage" and also from gear friction... the more HP and TQ your engine generates the harder those metal gears press against each other thus more heat,... as well as the higher differential in speed between the engine crankshaft speed and the transmission input speed..

the most heat is generated when the engine is running at full power in its sweet spot.. (ie 2300 RPM on a T-444E) and the transmission being lugged.. 45 MPH in 4th gear up a hill... you are making tons of heat...

the physics of it all says that more "work" is being performed on the fluid in the converter during this time..

on descent you arent performing as much "work" on the fluid in the converter so yes you will make some heat but not as much as running on a power run.

-Christopher

Iceni John · 10-24-2016, 12:41 PM

Here's some specs for the different Allisons: http://www.haydenauto.com/upload/Hay...il-coolers.pdf Scroll down to page 68 (the 6th page of the pdf).

John

slidewayzm5 · 10-24-2016, 01:01 PM

So installing the ECV will help SOME as long as I keep the transmission engaged in the gear that I'm descending in, but not a ton? Is that correct?

EastCoastCB · 10-24-2016, 01:11 PM

Quote:

Originally Posted by cadillackid

ive driven monteagle in the DEV and it seemed easy to me.. sure it was a nice descent but I ran it nice n slow in 3rd gear of my AT545.. and only tagged the brakes every so often... I never once smelled hot pads or felt like I was running too fast...

are you guys saying monteagle is a huge deal? like im rockies-ready if my bus can handle monteagle?

to me monteagle doesnt seem near as steep and heavy duty as on I-68 in maryland...I think that grade is 7% and the 545 has run that..
-Christopher

In a 24 year old school bus you just paid two grand for and hit the road in, Monteagle is a bit butt-puckering.
The brake adjustment station and runoffs are a little intimidating for a tropical flatlander in a forty foot used bus.

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