Allison Transmission Interchange

[sojourner58]

In my thread 'Searching For Mr Good Bird' I described how I recently purchased a 1982 Eagle Model 10 with a 1994 Cummins N-14 500E engine. While driving home from the west coast, the Allison HT740 reverse went out completely, 1st gear started slipping when hot as well as 4h gear. I was able to nurse it home and have been weighing my options whether to rebuild the current transmission or upgrade to an electronic 6 speed overdrive Allison. With the current HT740 the engine turned 1500 rpm at 55 mph in 4th gear which is probably the sweet spot for fuel economy. If I want to run 65 mph at 1500 rpm I'd need a higher overall drive ratio.

There was an Allison HD4060P on Ebay out of a low mileage wrecked fire truck. Unfortunately I discovered the P stands for pto. The pto version is several inches longer than a non-pto version and would increase the likelihood of having to shorten the driveshaft to make it fit.

A wrecked '02 Spartan motorhome is for sale in my area with a 400 hp Cummins and Allison, most likely an HD4000MH. A salvage yard also has an HD4000MH for sale online for a reasonable price that I'm considering. I'm thinking the HD4000MH should be strong enough to handle the hp and torque my 500 Cummins produces since the Spartan chassis has a 400 hp Cummins but I'm not sure. I also wondered if the Allison MH series is engineered as rugged as a commercial heavy duty series since the MH looks smaller in photos I've seen.

Some questions I have are: Is the bellhousing bolt pattern the same between an HT740 and an HD4000MH? Will the HT740 flex plate on my Cummins bolt up to an HD40000 torque convertor? Will a 2002 Allison Transmission Control Unit communicate with a 1994 Cummins Select Engine Control Unit? Will it need to be reprogrammed or will I need to find another TCU?

I'd appreciate hearing from anyone who has knowledge regarding these issues. -Jim

[blue_goose]

The 4000 and 4060 are both 6 speed transmissions that have two overdrives. That is what your engine needs, but you will need to change the rear to be able to use the overdrives.
If your engine is electronic no problem. if not you will need another unit to tell the transmission when to change gears.
Jack

[sojourner58]

The 4000 and 4060 are both 6 speed transmissions that have two overdrives. That is what your engine needs, but you will need to change the rear to be able to use the overdrives.
If your engine is electronic no problem. if not you will need another unit to tell the transmission when to change gears.
Jack

Thanks for the info blue_goose.

What ratio axle gears does it need to be able to use the overdrives? I don't know the current ratio but with 24.5 tall rubber and the HT 740 transmission the engine turns 1500 rpm at 55 mph. I could go with 11-22.5 wheels & tires to reduce the overall final drive ratio but I'd have less ground clearance.

The local Allison shop wants $16000 to rebuild and install the HT740 or $7500 for a Weller reman plus labor for removal & installation. I found an HD4000MH for $850 with 18,000 miles on it. It comes complete with the torque converter, the TCU, the touch pad shifter and wiring harness plugs. Even if I have to re ratio the rearend it would still be cheaper to go with it rather than the HT740.

[beltguy]

The world transmission has two overdrives: .74 and .61. I am told that many world transmissions have sixth gear (.61 overdrive) locked out. In the best of all worlds, you would use something like a 4.11 rear end and then you could take advantage of the 6th gear.

The two most popular Eagle rear end ratios are 3.36 and 3.73. If you ratio your 1500/55 it is equal to 2100RPM and 77 MPH. That would suggest you have a 3.36 gear ratio. That gear along with the .74 overdrive will something like 74 MPH at 1500. I like to drive at 68 MPH which would be about 1380 rpm with 3.36 gearing. That would a great combination for a 4 stoke engine and reasonable road speed. All of these numbers are based on a typical 11 24.5 tire.

So, I don't think you need to change any gearing. You will not have the advantage of 6th overdrive, but that is not a big deal.

At least one of our members put a 4.11 in his Eagle and was able to use all the gears. The 4.11 was not a standard gear that I am aware of, but apparently you can find aftermarket gears ($$$$$$$).

Jim