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Old 03-28-2016, 10:33 AM   #21
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I have seen AT500 series transmissions spend their whole service life in a bus without ever having a lick of problems. And I am talking about well in excess of 250,000 miles.

For low torque and low HP engines in school buses you couldn't ask for a better transmission. Since most school buses spend their life at 35 MPH or less you don't need a lock up torque convertor for speeds in excess of 50 MPH.

I will admit the AT500's behind gas engines were better matched than the AT500's behind diesel engines. The first time I went down hill with a Cummins 5.9L and AT543 I almost messed my pants. By the time I got to the bottom of the hill I had smoke coming out all for brakes. I had driven an IHC/Thomas Type 'C' for eight years on that route. It had an MV404 gas V-8 and AT540 and I used 2nd gear to go down that particular hill. It held back without any problem. New bus with diesel and the same transmission in 2nd gear felt like I was free wheeling. I learned that I had to go down the same hills in 1st instead of 2nd.

Once I learned the limitations it wasn't a problem.

If the choice was between two identical buses with the same engine with the only difference one had the AT540 and one had the MT640 I would go with the MT640.

But if your bus that you already own has the AT540 I wouldn't spend the time and $$$ changing anything. With proper care and maintenance (don't let it get hot, change the oil regularly, change the filter regularly, and don't let it get hot) it should last as long as anything else on your bus.
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Old 03-28-2016, 11:54 AM   #22
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With any automatic, you really need to run a tranny temp gauge if you want long life. Somewhere online Allison has a chart indicating transmission fluid life as it relates to temperature. It makes it clear pretty quickly that it can only take a few minutes to totally ruin both the fluid and the tranny if you let the temps get away from you. I'll see if I can find it again. Very informative.
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Old 03-28-2016, 12:05 PM   #23
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With any automatic, you really need to run a tranny temp gauge if you want long life. Somewhere online Allison has a chart indicating transmission fluid life as it relates to temperature. It makes it clear pretty quickly that it can only take a few minutes to totally ruin both the fluid and the tranny if you let the temps get away from you. I'll see if I can find it again. Very informative.
120% agree on temperature!!

AND thats one place where a Lockup transmission EXCELS.. an unlocked torque converter creates MEGA amounts of heat esp if you are "stalling" your transmission..

by that I mean you are lugging up a hill, foot to the floor.. (your diesel engine will run all day like that if its radiator is big enough).. but you are running your trans against its torque converter stall speed RPM..

its amazing how quickly your fluid temperature will soar... and once you over-temp your fluid.. its done.. change it ASAP! it has been destroyed.. (while the transmission may still be fine at this point.. runnign that once-overheated fluid in it will kill the transmission.. simply cooling the oil down does not 'un-destroy' it..)..

-Christopher
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Old 03-28-2016, 12:24 PM   #24
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Well, I couldn't find the official Allison chart, but below is one that reads pretty much the same. The story is simple...to keep it working...keep it cool. And it does not take much time on a hill (especially with an older trans like a 545) to wreak havoc on an otherwise OK transmission. Synthetics hold up better at higher temps but you still need to stay on top of what the tranny is going through.


Hayden - Transmission and Engine Oil Coolers
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Old 03-28-2016, 12:43 PM   #25
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do diesel busses do what the old gassers did, and just have a small cooling loop in the radiator downside?
-Christopher
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Old 03-28-2016, 12:59 PM   #26
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Most larger rigs with automatics, whether gas or diesel, really need a substantial trans fluid cooler system to stay happy. Even then, a temp gauge is essential.
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Old 03-28-2016, 03:29 PM   #27
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Most larger rigs with automatics, whether gas or diesel, really need a substantial trans fluid cooler system to stay happy. Even then, a temp gauge is essential.

I agree on both... even on my high performance cars i always put a cooler on...

im really surprised busses dont have a gauge.. heck even my silverado truck has one..
-Christopher
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Old 03-28-2016, 06:16 PM   #28
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We have a full size DT360 with an AT545. Our foot is always on the floor, unless we are stopping or going downhill. There is virtually no hold-back from the tranny downhill, no matter how low of a gear it's in. Pretty scary doing the 5000ft to -200ft elevation change in Death Valley. We take hills full throttle at a roaring 25mph (if that). It shifts pretty hard between 1st and 2nd, especially when coming to a stop, but I've heard that's common for this transmission. Going uphill we never have overheating issues as long as we keep the RPMs above 2000 or so. Normal highway RPM is 2700.
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Old 03-28-2016, 07:52 PM   #29
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Most larger rigs with automatics, whether gas or diesel, really need a substantial trans fluid cooler system to stay happy. Even then, a temp gauge is essential.
I don't have my bus here yet Tango but what are the chances that it already has an xmission temp gauge for the MD3060 trans? It is safe to assume that if it does not already have one that the threaded opening is there (and plugged) just waiting for one to be installed?
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Old 03-28-2016, 08:07 PM   #30
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Originally Posted by onenationundergoat View Post
We have a full size DT360 with an AT545. Our foot is always on the floor, unless we are stopping or going downhill. There is virtually no hold-back from the tranny downhill, no matter how low of a gear it's in. Pretty scary doing the 5000ft to -200ft elevation change in Death Valley. We take hills full throttle at a roaring 25mph (if that). It shifts pretty hard between 1st and 2nd, especially when coming to a stop, but I've heard that's common for this transmission. Going uphill we never have overheating issues as long as we keep the RPMs above 2000 or so. Normal highway RPM is 2700.
That sounds like my experience with the 545, although I didn't drive mine across the country like you two did.
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