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2002 M1123 4L80E Strange Shifting

Hummer Guy

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For some odd reason my transmission is popping in and out of overdrive, it act as if it don't know what gear to go, the only way it won't do this is if I get up to 60Mph or really put my foot down. I've done a little research and see people say about some throttle control issue or the TCM, I'm kind of clueless with these automatic transmissions

Just to mention, I did change the transmission fluid, looks extremely clean, and the shifts are very smooth, it just seems like it doesn't know when it should actually shift between 3rd and 4th.
 
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Retiredwarhorses

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For some odd reason my transmission is popping in and out of overdrive, it act as if it don't know what gear to go, the only way it won't do this is if I get up to 60Mph or really put my foot down. I've done a little research and see people say about some throttle control issue or the TCM, I'm kind of clueless with these automatic transmissions

Just to mention, I did change the transmission fluid, looks extremely clean, and the shifts are very smooth, it just seems like it doesn't know when it should actually shift between 3rd and 4th.
TPS out of adjustment
 

Bulldogger

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In my case, the trans was jumping out of OD into neutral. I figured out after a lot of head scratching that the control rod between the shifter and lever on the trans had been bent a little during demil of the powertrain I'd had swapped in. While it seemed to shift through the gears OK, it had tension between some, for lack of a better word, and it was prine to jumping out of OG, I guess because the pawl couldn't screw out far enough for proper placement of the whole range of motion between the shifter and trans shift level. (I didn't do the swap and perhaps the garage that did hurried a little on that point.) Once I tweaked the rod straight and cleaned the threads, then had someone shift through the gears while I watched and listened to the shift detents on the transmission, I was able to move the connector an inch or so farther out and it shifted much better and didn't drop into neutral again. YMMWV
Bulldogger
 

Mogman

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Oddly enough, the truck is shifting perfect after I just mess with the wires at the brake pedal assembly, maybe a loose connection can cause it?
Yes the brake switch to the TCM opens when you step on the brake disabling the torque converter lock, so a loose connection or an intermittent switch can cause the torque converter to lock and unlock.
 

Hummer Guy

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I cleaned all of the connectors, now it shifts perfectly fine up to overdrive, it won't go into overdrive until I completely step on it up to 3,000 RPMs. 1-3 gear shifts flawlessly though. I'm assuming like RWH said, the TPS, does anyone have a good explanation on how to adjust the TPS or is it simple to just replace it and be done with it? I can't find too many things talking about this

Edit* my RPM is 55Mph at 2,300, I don't exactly know the difference between the torque converter locking up vs it going in overdrive, all of the transmissions I ever worked on has been a manual, I never actually been in a auto to figure out how they work
 

Retiredwarhorses

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I cleaned all of the connectors, now it shifts perfectly fine up to overdrive, it won't go into overdrive until I completely step on it up to 3,000 RPMs. 1-3 gear shifts flawlessly though. I'm assuming like RWH said, the TPS, does anyone have a good explanation on how to adjust the TPS or is it simple to just replace it and be done with it? I can't find too many things talking about this

Edit* my RPM is 55Mph at 2,300, I don't exactly know the difference between the torque converter locking up vs it going in overdrive, all of the transmissions I ever worked on has been a manual, I never actually been in a auto to figure out how they work
Are you actually checking to see at 55mph that you are in fact in OD by dropping the gear select into 3rd gear? If not….go test it.
 

Hummer Guy

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I replaced the brake switch under the dash, still the same, 55mph at 2,300RPM. I know for sure now that it's actually shifting in overdrive so my guess is something else electrically is affecting it from lock/unlocking the torque converter, the speed difference is only 5mph, it use to be at 60mph at 2,300 RPMs.
 

Mogman

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I replaced the brake switch under the dash, still the same, 55mph at 2,300RPM. I know for sure now that it's actually shifting in overdrive so my guess is something else electrically is affecting it from lock/unlocking the torque converter, the speed difference is only 5mph, it use to be at 60mph at 2,300 RPMs.
Is 2300 high? Do you have a 400A alt? I know the three speed trucks turn 2900@60MPH
 

Retiredwarhorses

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Has anyone put the torque converter lockup on a toggle switch?
From the looks of this video, it seems like he was running it off one
yes, i made one years ago as a diagnostic tool, all you are doing is denying 12v with a simple switch. The second you touch the brake pedal, you deny the 12v thus unlocking the TC, as soon as you hit about 40mph, apply the 12v and you should see a drop in RPM.
 

Hummer Guy

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yes, i made one years ago as a diagnostic tool, all you are doing is denying 12v with a simple switch. The second you touch the brake pedal, you deny the 12v thus unlocking the TC, as soon as you hit about 40mph, apply the 12v and you should see a drop in RPM.
Got it, where exactly do I send that 12V too? Do I wire it so I can send it to the wire that goes into the brake switch?
 

Mogman

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Got it, where exactly do I send that 12V too? Do I wire it so I can send it to the wire that goes into the brake switch?
You should get the same effect if you hold the throttle at say 55MPH and press the brake and then release, you should see the RPMs rise then fall as the torque converter unlocks then goes back into lock.
The switch RWH was speaking of would simply go in series with the brake switch to simulate stepping on the brake.
 
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