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- 1,633
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- Location
- Wilson County, Texas
Everyone,
I've been troubleshooting this one for a few weeks now & am going to make a nice, long detailed post.
Truck is a 1988 M1097R1 w/ A2 powertrain in it from it's 2012 rebuild.
The first obstical was putting a new TCM in the truck when I finally got it. If you want, read up on that one here as it had a similar issue, but that TCM was 100% hooked to 28v or higher power & burnt out;
https://www.steelsoldiers.com/threads/transmission-diagnostic-mode-issue-i-couldnt-find-here.221920/
After solving some other issues that came up, I went to go take the truck on a test drive. Got out on the highway, and it didn't get to 4th gear. Look down to a lovely TRANS light again!
Where I'm currently at.
-TRANS light is on solid when truck is running. Truck is acting like no 4th gear & no TCC lockup, but no code.
-Hooking up my SuperFlow Supershifter, it doesn't get past the power on screen. This is my only 4l80E truck, so I honestly don't know if it works & I just got it!
-It will go into diagnostics mode via jumper. Only gives a 12.
-TCM was installed from a sealed military foil wrapper 2025FEB07 this light came on 2025MAR30. I had put maybe 50 miles & a few hours on the truck.
-Has brand new batteries in it right now
-All grounds are clean & tight
-Confirmed the TRANS light is not a short. It will not come on if TCM J1 is disconnected.
-Spent significant time testing wires & circuits for test as the Supplement guide calls for. Have not found any bad wires, connectors yet. Truck overall is very clean.
-Speedometer (80mph electronic) was off & I changed to 41 tooth at the transfer case. It is spot on now. Wasn't sure this effected anything but needed to be done.
-Found the brake switch had a dead spot. Replaced (with a AM General part) & adjusted properly. Had previously adjusted it not realizing the dead spot was on the brake light side.
-Breaker & power relay are good. Confirmed power readings right at the connector that the TCM goes to, not TCM connector J1.
-Used DeoxitD5 on all connectors to be sure they're clean.
-Have tried removing the TCM & let it sit without power for a few days.
-Kascar supplemental grounding wire kit installed.
I'm sure I did more.
Here is a list of things that I'm thinking MAY had triggered the issue from least to worst.
-Transmission fluid was grossly overfilled when I got it Drained some out. It developed a leak when I got it out on it's first drive before this TCM issue came up. Went 80MPH no problem.
-When I adjusted the brake switch, I didn't realize the dead spot was on the brake light side & only was using the lights as the means to adjust it. This could (and probably) made it so the switch was triggering a MIL due to the switch being open. This could cause my symptoms but the issue should reset with a power cycle of the ignition.
-I had let the truck sit for a week. When I came to move it, batteries were weak. I attached a jumper pack & didn't disconnect the TCM. The reason for the no-start was not the weak batteries, the IP decided to get stuck closed. In my frustration with the situation, I tried cranking a few more times before disconnecting the TCM. However, I did not have any TRANS light issues until what I mentioned above.
-After getting home when the TRANS light came on, that is when I decided to just replace the batteries. They were only 3 yr old Hawkers that I revived that came with the truck BUT wanted to just get something new in there to eliminate a problem.... the batteries shouldn't have drained so maybe a cell was bad? When replacing the batteries... found the bolt for the main ground at the shunt was loose! I was mortified.
So, here is where my head is at right now with the issue, even though I don't want it to be. The loose ground was the issue. It could have caused voltage to travel back up a wire to the TCM or something like that? The grounds for the TCM were good & tight. Maybe power was "chattering" on & off to it as I was bumping down my county roads, similar to when you have really dead batteries & slave start? Could have "half baked" the TCM & it is stuck in a NO 4th NO TCC LOCKUP type fault that may have been triggered by my bad brake switch adjustment while it was getting hammered with a bad connection driving down the road?
I really would just light to find the root cause so I don't just toss a TCM in & end up with the issue again in 50-100 miles. I do have another, brand new one here but am reluctant to just toss it in.
Figured I'd come here to see if I missed something or get another idea as I'm frustrated with it. Got EVERYTHING else good. Well, I say that, & the starting issue is coming back again... kinda. But, that is not for this thread. The truck is super clean & worth the work for what I want it for. Engine has 0 blowby. Body is very clean. Once it's squared away, I plan to mount a very nice military radio system in it that I have.
I've been troubleshooting this one for a few weeks now & am going to make a nice, long detailed post.
Truck is a 1988 M1097R1 w/ A2 powertrain in it from it's 2012 rebuild.
The first obstical was putting a new TCM in the truck when I finally got it. If you want, read up on that one here as it had a similar issue, but that TCM was 100% hooked to 28v or higher power & burnt out;
https://www.steelsoldiers.com/threads/transmission-diagnostic-mode-issue-i-couldnt-find-here.221920/
After solving some other issues that came up, I went to go take the truck on a test drive. Got out on the highway, and it didn't get to 4th gear. Look down to a lovely TRANS light again!
Where I'm currently at.
-TRANS light is on solid when truck is running. Truck is acting like no 4th gear & no TCC lockup, but no code.
-Hooking up my SuperFlow Supershifter, it doesn't get past the power on screen. This is my only 4l80E truck, so I honestly don't know if it works & I just got it!
-It will go into diagnostics mode via jumper. Only gives a 12.
-TCM was installed from a sealed military foil wrapper 2025FEB07 this light came on 2025MAR30. I had put maybe 50 miles & a few hours on the truck.
-Has brand new batteries in it right now
-All grounds are clean & tight
-Confirmed the TRANS light is not a short. It will not come on if TCM J1 is disconnected.
-Spent significant time testing wires & circuits for test as the Supplement guide calls for. Have not found any bad wires, connectors yet. Truck overall is very clean.
-Speedometer (80mph electronic) was off & I changed to 41 tooth at the transfer case. It is spot on now. Wasn't sure this effected anything but needed to be done.
-Found the brake switch had a dead spot. Replaced (with a AM General part) & adjusted properly. Had previously adjusted it not realizing the dead spot was on the brake light side.
-Breaker & power relay are good. Confirmed power readings right at the connector that the TCM goes to, not TCM connector J1.
-Used DeoxitD5 on all connectors to be sure they're clean.
-Have tried removing the TCM & let it sit without power for a few days.
-Kascar supplemental grounding wire kit installed.
I'm sure I did more.
Here is a list of things that I'm thinking MAY had triggered the issue from least to worst.
-Transmission fluid was grossly overfilled when I got it Drained some out. It developed a leak when I got it out on it's first drive before this TCM issue came up. Went 80MPH no problem.
-When I adjusted the brake switch, I didn't realize the dead spot was on the brake light side & only was using the lights as the means to adjust it. This could (and probably) made it so the switch was triggering a MIL due to the switch being open. This could cause my symptoms but the issue should reset with a power cycle of the ignition.
-I had let the truck sit for a week. When I came to move it, batteries were weak. I attached a jumper pack & didn't disconnect the TCM. The reason for the no-start was not the weak batteries, the IP decided to get stuck closed. In my frustration with the situation, I tried cranking a few more times before disconnecting the TCM. However, I did not have any TRANS light issues until what I mentioned above.
-After getting home when the TRANS light came on, that is when I decided to just replace the batteries. They were only 3 yr old Hawkers that I revived that came with the truck BUT wanted to just get something new in there to eliminate a problem.... the batteries shouldn't have drained so maybe a cell was bad? When replacing the batteries... found the bolt for the main ground at the shunt was loose! I was mortified.
So, here is where my head is at right now with the issue, even though I don't want it to be. The loose ground was the issue. It could have caused voltage to travel back up a wire to the TCM or something like that? The grounds for the TCM were good & tight. Maybe power was "chattering" on & off to it as I was bumping down my county roads, similar to when you have really dead batteries & slave start? Could have "half baked" the TCM & it is stuck in a NO 4th NO TCC LOCKUP type fault that may have been triggered by my bad brake switch adjustment while it was getting hammered with a bad connection driving down the road?
I really would just light to find the root cause so I don't just toss a TCM in & end up with the issue again in 50-100 miles. I do have another, brand new one here but am reluctant to just toss it in.
Figured I'd come here to see if I missed something or get another idea as I'm frustrated with it. Got EVERYTHING else good. Well, I say that, & the starting issue is coming back again... kinda. But, that is not for this thread. The truck is super clean & worth the work for what I want it for. Engine has 0 blowby. Body is very clean. Once it's squared away, I plan to mount a very nice military radio system in it that I have.
