Allison Issue

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NickInMN

Member
Aug 30, 2021
44
3
8
I have a problem with my Allison 1000 6 speed transmission. When I put it into drive it doesn't seem to go into gear. Sometimes it will after a second or two. For a while I had this issue with no codes but today I finally got a code, P0731 - Incorrect 1st Gear Ratio. When I put the truck into gear, monitoring with a Tect 2, I see a ratio of 8:1 which is wrong according to the p0731 diagnostic sheet I have. Also, when I put it into gear the turbine speed drops to about 300rpm and then goes back up to about 700. I can hear the engine unloading when this happens. According to what I've read it should go to zero. It does when I put it in reverse.

Since I got this truck there has always been a shudder when the torque converter locks if I'm at more than 1/4 throttle. I don't know if they are related or not.

The fluid is full and looks good. I've never seen the transmission fluid temp over 170 or so.

Is there anything I could check or is this a job best left to a transmission shop?
 
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NickInMN

Member
Aug 30, 2021
44
3
8
PureHybrid said:
Yep agree on c1 pack. Was it recently apart? Any valve body work? Truck tuned?
Click to expand...
I haven't had it apart since I got it about 30k miles ago. There is a tune on it but I only ever use the "Optimized Stock Setting". I'm not sure what the previous owner did with it. One thing to note, this is an LBZ/Allison swapped into a 1997 Hummer. I wouldn't think that would matter much though, it can't be that much heavier than a 2500 or 3500 and I never tow with it.

I was talking to a guy at a local shop and this is what he said: "The symptoms that you are describing are usually indicative of a valve body issue that has caused progressive damage. The shutter that is coming from the torque convertor was probably the lockup clutch dragging some and causing the damper springs to be excited causing the shutter. There is a second potential cause as well but not as common, the oil pump/regulator." I don't know enough to know if that is correct or not, it sounds logical to me.

I'm going to pull the transmission this weekend and drop it off for service next week.
 
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