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Old 01-21-2007, 11:27 AM   #1
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How to tell AT545 from MT643, w/pix & specs


I'm no expert, and this may not be all you need to know.
But here are photos of the two most common Allison automatic transmissions in
older school buses. The side mounting pads may be a good clue. On my buses
they were not used and easy to see.

Here we go:



AT545. Note simple mounting pad with two holes above each other near the front.
Nothing on oil pan.




MT643. Note complex mounting pad with four mounting holes.
Temperature sender on oil pan.


The MT643 is the one to have. It is stronger and the converter locks in 3rd and 4th.

(Edited with this additional info):

AT545 -- max 235 HP, 445 lb ft, 30.000 Lbs GVWR; no lockup, SAE 3 bolt pattern.

MT643 -- max 250 HP, 640 lb ft, 73.280 Lbs GVWR; locks in 3rd and 4th, SAE 2 bolt pattern.

More info here: http://www.industrialautomatic.com/html/onhiway.htm
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Old 03-08-2009, 11:22 PM   #2
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Re: How to tell AT545 from MT643, w/pix & specs

These automatics are new to me and I know you said you were'nt a expert But Ive got 366 chevy drag'in 545 alice trans .after I get my bus done I'm wanting to hook a 16ft. travel trailer to it and leave Miss. and try and make it B.C. Canada. I dont know what my rear gears are but I'm gonna check them out come daylite. I had a 55 white back in the mid 70's It had a 5speed with a 2 speed rear,mucho ponies and good gear selection to chose from. I'm think'in I would be alrite with a outside the radiator cooler with a elect fan blow'in on it. Everyone knows heat is every auto's enemy. the other choice that I have is convert it to 5 speed with a 2 speed rear. I've got a friend thats got a 5 and 2 set up that will bolt up to my 366. It's better to change all this stuff up before I leave the barn, been on the side of the road before with a hand full of nuts and bolts-- no fun! Or do you know if I can put a 643 were a 545 was. We got a lot of bone yards around here and I think I could find a 643 here I've got my own metal fab shop so if its just a bracket here and a angle iron there we got that covered. But I gotta tell ya I'm lean'n toward a manuel gear box. I guess its because its what I know, Thanks a lot and may god bless, Russell Hardtimeswelding@hotmail.com
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Old 03-09-2009, 01:51 AM   #3
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An auto trans are the best for many reasons

Auto transmissions and especially Allison, do a great job. Connected to correct rear axle gears, an auto trans will make the best power, fuel millage and longevity. A few changes to the auto trans system will improve everything. These changes are my idea for building a great bus or heavy duty truck. First change is to install a heavy duty stacked plate trans cooler, remove cooler trans lines from radiator, install large remote trans filter, change all cooler lines to at least 1/2, install trans temp guage, and install larger trans oil pan for greater capacity. Having a rear axle ratio of about 4:10 would be great. A ratio of 3:55 or 3:73 is better but the engine will need to develope more horsepower. A bturbo will be good but propane and alcohol/water works well too. Frank
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Old 03-09-2009, 12:35 PM   #4
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Re: How to tell AT545 from MT643, w/pix & specs

Hey Frank. Where's the best place to install the sender for the trans temp gauge in your opinion? I've heard arguments for in the cooler supply line, in the cooler return line, in the mainline pressure port, in the pan, etc so I'd like to hear your respected opinion.
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Old 03-09-2009, 01:17 PM   #5
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Re: How to tell AT545 from MT643, w/pix & specs


Russell, swapping to an MT643 may or may not be possible. You would need a bell housing that matches the 643 on the back side (SAE 2 pattern) and your Chevy engine on the front side. Hard to say if any factory ever put an SAE 2 tranny on that little engine. Best of luck to you!
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Old 03-09-2009, 02:12 PM   #6
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Allison trans da best

Probably the best location for a temp gage is in the trans oil pan. This is the place that reflects the fluid temp under load. The hot and cooled fluid mix and make a good average temp. My usual methor is to remov oil pan and weld in a place for the sender and a large drain plug. With a remote filter, internal filter require minimal replacement. The sender location is at the front side of the pan and drain is at rear of pan on the side so no road trash can affect the drain .
On several occasions, I have wires 4-5 senders to a rotary switch to a single dash meter. I installed senders in the radiator, trans pan, engine compartment, and rear axle. A good gage is also a pressure gage for the cooling system. This gage will tell any cooling system loss of coolant and pressure. Franki
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Old 03-09-2009, 08:32 PM   #7
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Re: How to tell AT545 from MT643, w/pix & specs

Thanks for the come back,I'm going to install a good cooler on 1/2 in. lines with a remote spin on filter and I will either deepen my pan or pull out my aluminium furnace and cast one with a bung hole for a temp. sending unit. Your wisdom and experince and wisdom show Frank, thanks again may God bless you and yours Russell
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Old 03-09-2009, 08:42 PM   #8
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Re: Allison trans da best

Quote:
Originally Posted by frank-id
On several occasions, I have wires 4-5 senders to a rotary switch to a single dash meter. I installed senders in the radiator, trans pan, engine compartment, and rear axle. A good gage is also a pressure gage for the cooling system. This gage will tell any cooling system loss of coolant and pressure. Franki
Thanks for the reply, Frank. I figured you had a trick or two. The multiple sender, single gauge one is a a cool trick.
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Old 03-09-2009, 09:29 PM   #9
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Re: How to tell AT545 from MT643, w/pix & specs


Just for the fun of it:

On my first hot rod car, many moons ago, I installed all the usual gauges. But I also installed warning light senders, and I wired all those to one lamp -- by running each wire thru a diode. Thus, if the lamp came on, I would know to check all my gauges, for one of them was out of the normal range.
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Old 03-17-2009, 04:54 PM   #10
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auto trans improvement

I recently advised a guy his truck and RV needed more fluid capacity. He was gonna buy or make a deep trans pan. Too much money. An old air brake tank was gotten from a big rig truck. Some brackets were made and tank installed with some additional fluid lines. The increased capacity of about 3 gallons allowed the fan cooled cooler to keep the fluid at about 185 degrees while climbing up our 12,000 ft mountains with a full payload. Don't ya love it when a plan works? Frank
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