Allison shifting problem, 3rd gear.

[162tcat]

I have bad luck with automatic trannys.

One of my trucks, M923, has just developed (in the last 100ish miles) a strange shifting problem. When shifting into 3rd, it will rev up like it's going to neutral then it will shift into 3rd. It doesn't seem to happen when the tranny is cold or if my foot it mashed to the floor. I was going to re-adjust the modulator cable but it didn't seem to be out of adjustment in the first place. All other gears are fine.

About 3,500 miles on truck
Fluid is dex III
Less than 1k on fluid filters.
Old filters and fluid looked good.
Operating temp hasn't changed.

Any thoughts before I take it to the allison dealer and spend a fortune?

 

[Castle Bravo]

I had a M925 that more or less did that, I think it was also into 3rd gear. It was like it was delayed from shifting somehow. Mine was on 10wt oil, but otherwise about the same conditions as yours. I never did fix it and it didn't seem to have any apparent negative effect on the truck. Sorry I don't have a solution for you - just that mine did it also.

 

[162tcat]

Did yours always do that or did the problem develope as you drove it?

 

[73m819]

I am guessing you have a worn shifting valve or piston, when the oil is cold it is thicker so would not bypass as easy, when hot, the oil will be thinner and tend to bypass more so you need more volume/pressure to make up for the bypassing oil to make the shift, so the shift seems late, when you are at full throttle the trans pump is pumping enough volume/pressure to over ride the bypassing oil, hence the normal shift.

 

[Castle Bravo]

Did yours always do that or did the problem develope as you drove it?

Always did it from when I got it to when I sold it.

 

[162tcat]

That may be, does it require complete disassembly to replace?

It's strange because it just started one day with no warning.

 

[73m819]

Depends on where the valve is, if in the valve body a good chance no, but if it is in the trans it self or the piston then yes

 

[Suprman]

Try turning up the trans pressure a bit maybe that will overcome the shift gap.

 

[162tcat]

How do you adjust the pressure? I'm going to drop the pan and change the fluid and filters one more time. Some crud could have broke loose since the truck only has about 3,500 miles since 1985. While I'm in there, I'll give it a visual inspection and see if anything looks abnormal.

 

[162tcat]

Update

It was the "trimmer valve". If anyone else experiences the same issues, this is probably the culprit. I pulled the valve body and with the help of the Allison manual, figured it out and fixed it. Parts required were a baldwin filter kit and a trimmer valve assembly from ebay. About $170 total including fluid.

 

[jonesal]

Pictures, man! How about a part number too? Thanks!