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Old 01-09-2019, 02:16 PM   #1
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Allison AT 542 Upgrade

Greetings. I'm new to the forum, but I've done a lot of research here and on other forums in regards to my desires to upgrade my transmission. Here's what I currently have: It's a 97 Newmar Kountry Star FRED with a 5.9 Cummins 12V and the Allison AT 542 transmission. It's kind of a dog, especially at the top end. It's hard to hold a steady 71-72 MPH on flat ground, much less even the slightest grade, and pulling the serious grades through Wyoming is a joke. I suspect the engine is making pretty good power, as it has a Banks Stinger kit in it, but I'm losing a lot of that power through the non-locking torque converter. I know it's also costing me fuel mileage (average 7.5 on a 5,000 mile cross-country trip). So I've come up with three possible plans:

1. Upgrade the torque converter to something with better power transfer. This would help somewhat, but I suspect that the $800 - $1000 I'd spend would be disappointing in the end.

2. Swap in an Allison MT 643. This would give me a locking torque converter and keep the installation simple, as the transmission is mechanically controlled. Still no overdrive, but I think I'd be able to hold a steady 75 MPH and maybe pick up a few MPG.

3. Swap in a 1000 or 2000 series Allison. This would be a little more complicated, because now we're dealing with electronics, but since the engine is mechanical, there's no interface with an ECM. I've read elsewhere that I could use an early (2000 or 2001) Duramax TCM and wiring harness and make the wiring really easy. This would give me overdrive, a locking torque converter, and probably a serious increase in MPG. I read somewhere that I can control it with my dash-mounted shift lever, which would be a bonus as well.

I'm not afraid of turning wrenches (I can fix or build just about anything) but the Allison transmissions are new to me, so I would appreciate any expert advice you could give me about anything I might not be thinking clearly about (bellhousing, torque converter, wiring, etc). I plan to keep this motor home for a while, so I'm willing to invest a bit of money into it, but I also don't want to break the bank (in other words, I'm kind of cheap).

Thanks for your help!

David
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Old 01-09-2019, 07:05 PM   #2
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My 5.9 with the MT643 still only gets 8mpg on the best days. May help you a little with pulling power. I still slow to 40 or so on any fairly steep grades. Normal interstate driving it will hold speed most of the time. I never expect to be the first one to the top of the hill
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Old 01-10-2019, 11:57 AM   #3
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There have been a lot of motorhomes upgraded from the AT540/542/545 to the MT643. It's a fairly easy swap. You have to get the right adaptation hardware for the back of the engine, the same selector could probably be used, and I believe there is a slight change in driveline length. It does make a much better driving vehicle, especially for downhill braking.
As you stated, the 1K/2K swap is more complicated but you do get overdrive. The early pickup truck TCMs are usually programmed as five speeds. The transmission has the hardware in it to be a six speed but you have to be careful with driveline speeds anytime you put in an overdrive, especially if you try to go to six speeds. A complete driveline analysis should be performed. I believe you can still find the Dana Expert analysis tool free on Dana's website. Allison dealers and distributors have access to an Allison software to check driveline acceptability also.

The selector you use has to match what's called the shift mask in the TCM you are using. The selector positions on the selector must match what's available in the TCM. There's D-4-2-1, D-4-3-1 and D-3-2-1.
You'll have to also have a TCM that is programmed to use a mechanical throttle position sensor. The TPS should be mounted with a yield link connecting it to the engine throttle so if the TPS cable sticks, the engine isn't stuck a full throttle....yikes.
The TPS should also be mounted with a generous bend in the cable. The TPS body should be above its connection to the engine throttle to prevent moisture from making its way down the cable into the TPS itself.
And of course you have to wire in the transmission control system. Make sure you use battery direct power and ground where it's required.

I'm sure there's more, but that's all I can think of right now.
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Old 01-10-2019, 07:30 PM   #4
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Thanks for your input!



I'm more interested in improving my top end speed (not anything crazy, but a steady 75 MPH would be nice) than I am in fuel economy. Anyone have experience with a low stall converter and how much difference that made?



I'm still learning, so keep it coming, please.
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