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Old 03-06-2014, 10:49 PM   #1
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Allison HT740 shift problem

I have a allison ht 740 trans that was sitting for over a year (dipstick was off). I Was having problem with no reverse with all forward gears working fine, when I would shift to reverse the truck would not shake or anything. I check the governor no problems there. I checked the modulator pin and was still intact inside the trans (cable modulator). I tried manually moving the shifter on the side of the trans into reverse and it did nothing. I pulled the pan and inspected the the shift detents, all looked fine (no broken teeth). Transmission fluid level was fine as well. I put the pan back on and filled with fluid.
Still without reverse I went for a test drive, when i returned miraculously reverse started working! I took it out for another test drive and now the truck will not shift into 4th gear, it just sits in 3rd and will not shift. It goes through 1st, 2nd and 3rd fine but then will not get out of 3rd! I tried different throttle speeds and just cant get it out of 3rd. I am leaning towards pulling off and inspecting the valve. Any help is greatly appreciated.
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Old 06-27-2014, 11:08 AM   #2
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I know this is an old thread, but maybe I can help. When you had no reverse, did you have fourth gear?

In this trans, reverse has first clutch on.
First gear is first and forward clutches.
Second gear is second and forward clutches.
Third gear is third and forward clutches.
Fourth gear is fourth and forward clutches.
Reverse is first and fourth clutches.
The clutch apply pressure cascades through the relay valves in this valve body. It's possible if you had no reverse, but had fourth gear the 1-2 relay valve was stuck in its stroked position. Reverse pressure goes underneath the 1-2 relay. However if that were the case, you would be shifting 2-3 and 3-4 in forward with no first gear when in drive.
If you didn't have fourth, and had no reverse, the 3-4 signal valves or the relay valve could be sticking in the de-stroked position.
It's also possible fourth clutch itself has a problem. It could be a burnt clutch, failed seals, or check balls in the clutch housing leaking. It could also be the sealrings on the forward hub of the center support.
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