Problem with Allison transmission. Is there any Allison experts out there?

RDUKW

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I have a Allison ht 740 behind a Detroit 8v92 in a mk48 that I an having problems with. When I put the truck in forwards the converter locks and stalls the truck. When put in reverse the converter is unlocked and operates normally. I have bypassed the external lock up selinoid and checked to make sure the modulator pin is installed. Does anyone have any other ideas before I drop the pan and pull the valve body. Any help is greatly appreciated.

Thanks
Ryan
 
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LanceRobson

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Yes, I have an idea.....call your local Allison dealer and see what they say. They've always been helpful to me. You might have to hunt around a bit to find a service tech familiar with the older HT series stuff but it's worth it.

Lance
 

ZZ4x4

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Sachse, TX
This may have no bearing on what you are seeing but as I looked at page 2075 in the -20 manual for an unrelated issue, I saw that an air line runs from the Axle lock lever to the TC lock-up. Not sure why they are connected but just in case it helps...
 

RDUKW

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I will have to go check that out later. I drained the trans last night and I'm planning on pulling the pan and valve body tomorrow.

Thanks
Ryan
 

RDUKW

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Well I have pulled and inspected the lock up cup off valve and all looks good there. Now I am at another stopping point not knowing where to proceed to. I am learning that Allison dealers really don't want to talk to customers. I have calls into 3 different dealers and Oshkosh directly with no response. If any one has any other ideas it would be greatly appreciated. Also when I pulled the pan the oil was very clean with very little to no clutch material and no metal particles.

Thanks
Ryan
 

FASTNOVA

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I'm not sure if the switch is the same as a M939, but I had a problem with mine as soon as I would put it in gear it would kill the engine. Turns out the lock out switch was pushed in and had to pull it out to get it to work properly. I took it apart cleaned it out now it works great.
 

RDUKW

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I'm not sure if the switch is the same as a M939, but I had a problem with mine as soon as I would put it in gear it would kill the engine. Turns out the lock out switch was pushed in and had to pull it out to get it to work properly. I took it apart cleaned it out now it works great.
Are you talking about the front wheel drive lock switch?

Thanks
Ryan
 
Clean fluid can be a good or bad thing. Can't tell you how many times I've had customers come in with a trans problem that required a rebuild after they did a fluid and filter change to see if that helped, only to wash away any evidence of what was going on in the pan. I would contact ATSG and see about getting a manual for the trans to see what's up. Unfortunately, I've never worked on the big Allisons, even though I've been meaning to drop my M35A3's pan someday to do a service on it.
 

Kayne

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My old Shop Foreman now runs the local Allison dealer. I have to call him Tomorrow about my trans. Id be willing to call for you as well. Just need what ever info you can give. Hes been great in the past about getting me pdfs of dealer only info. Hope he can do it again for these older transmissions.
 

RDUKW

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My old Shop Foreman now runs the local Allison dealer. I have to call him Tomorrow about my trans. Id be willing to call for you as well. Just need what ever info you can give. Hes been great in the past about getting me pdfs of dealer only info. Hope he can do it again for these older transmissions.
Thanks that would be great if you could ask your buddy. The only extra info since the first post is that I did pull the pan to inspect the converter lock up portion of the valve body and it is operating perfect. I had one deale tell me that I could be having a leak from forward clutch lock up to the converter lock up selinoid but had no idea how or what would cause that. Any extra info would be great because I am about to pull the cab and remove the engine and trans package to go deeper into it.

Thanks
Ryan
 

Kayne

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Well they have no info on either trans. There worth less i guess. Dealer that is. Had a brain storm to day at lunch. If your valve body has a spool valve or clutch pack that the converter lock up and forward clutch share they could be bleeding over. trying to find the Tm for your trans now to see if i can find a cominality between the two. If they do then it could bleed over or bypass into the converter lock up and cause it to stall out. Hope this helps
 

RDUKW

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Well at this point I am going to move the truck to a location that I can pull the cab and yank the motor/ transmission out. I found a lead on a rebuilt trans, so I guess this will motivate to do my swap and get this truck all painted up nice.
 
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