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Limp mode/Transmission P/N 24208421

ahowell9

ahowell9

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My HMMWV M1152 A1 keeps going into limp mode. It's had this problem since the very first day I picked it up from Gov Planet.. im now 3 months into trying to diagnose the problem. I Replaced input and output speed sensors. Ive had a total of 7 different mechanics take a look at it. Most have never had hands on with a HMMWV so they aren't sure how to navigate the wiring. The last diesel mechanic suggested i swap out the TCM. I swapped it out with an identical TCM-2100AB but it hasn't resolved the problem.. Does anyone have any suggestions? Is it possible the wrong TCM is being used. My transmission reads GTP P/N 24208021. As much as ive searched the internet, I can't find the TCM that matches that specific tranny. 20210619_122138.jpg20210618_175045.jpg
 
A

Action

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1. Is your TCM just flopping around inside of the protective box?
1a. If so, you should probably secure it with some foam.
2. Does the TRANS light (by the shifter) stay on with the engine running?
3. If so, you can get the trouble codes from the TCM with a paperclip.
 
TOBASH

TOBASH

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Did you inspect the tranny fluid to ensure it is not a mechanical issue?
 
ahowell9

ahowell9

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Action said:
1. Is your TCM just flopping around inside of the protective box?
1a. If so, you should probably secure it with some foam.
2. Does the TRANS light (by the shifter) stay on with the engine running?
3. If so, you can get the trouble codes from the TCM with a paperclip.
Click to expand...
Action said:
Foam like this. This is just an older model (bigger)...

View attachment 837358
Click to expand...
Yes I have the foam to stabilize all sides the unit including the top so it's definitely secure. Also, indicator transmission lights work. We,ve connected a jumper wire to A and E to the box and got an diagnostic error code of I believe it was "86" , If I remember correctly. What I do remember for certain is the meaning of the code was " Undefined Ratio Error "
 
ahowell9

ahowell9

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TOBASH said:
Did you inspect the tranny fluid to ensure it is not a mechanical issue?
Click to expand...
Yes sir. That's was the first thing I checked. All fluids and filters have been changed. There was no sign of damage to the tranny within the tranny fluid. It appeared clear. No specs and cloudiness
 
ahowell9

ahowell9

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ahowell9 said:
Yes I have the foam to stabilize all sides the unit including the top so it's definitely secure. Also, indicator transmission lights work. We,ve connected a jumper wire to A and E to the box and got an diagnostic error code of I believe it was "86" , If I remember correctly. What I do remember for certain is the meaning of the code was " Undefined Ratio Error "
Click to expand...
Also the light stays on once it shifts into safe mode. It stays in safe mode until I turn of the vehicle and disconnect the battery. Once reconnecting the battery, trans light goes off, truck will drive as normal, shifts from 1st into 2nd, then right when it attempts to shift into 3rd, it will down shift into limp mode and the tranny light comes on
 
Retiredwarhorses

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First problem you have is expecting any shop that has no clue on how a hmmwv is wired and how 24 and 12v components play nicely together. Your transmission is the latest 4L80e, GTP is a wholly owned subsidiary of AM General. GTP “General Transmission Products”, just like GEP “General Engine Products” is a subsidiary of AM General.
You will have to start with swapping a new TCM in, you can use any known good TCM, the difference in TCM’s is the chip set, this is what sets the shift points, the later trucks have a later shift point.
you have left out any diagnostic info, like what does the Trans Lamp do when it’s in “limp mode“.
You could also have a failing Trans relay, a bad connection at the batteries 12v feed…don’t know what others have looked at, How long does it stay in limp mode, how is the ultimately corrected before next occurrence.
 
ahowell9

ahowell9

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TOBASH said:
You have a tranny sensor and or mechanical issue IMHO
Click to expand...
Yeah.... Kind of figured that much. Process of illumination has been the job for the past month and a half. Can't figure it out
Retiredwarhorses said:
First problem you have is expecting any shop that has no clue on how a hmmwv is wired and how 24 and 12v components play nicely together. Your transmission is the latest 4L80e, GTP is a wholly owned subsidiary of AM General. GTP “General Transmission Products”, just like GEP “General Engine Products” is a subsidiary of AM General.
You will have to start with swapping a new TCM in, you can use any known good TCM, the difference in TCM’s is the chip set, this is what sets the shift points, the later trucks have a later shift point.
you have left out any diagnostic info, like what does the Trans Lamp do when it’s in “limp mode“.
You could also have a failing Trans relay, a bad connection at the batteries 12v feed…don’t know what others have looked at, How long does it stay in limp mode, how is the ultimately corrected before next occurrence.
Click to expand...
Once it goes into limp mode, the trans light stays on the entire time. If I cut off the vehicle then restart, the trans light will remain on and the vehicle will continue to runs in limp mode. If I disconnect then reconnect the batteries, the trans light goes off and limp mode goes away.. Once limp mode goes off, the humvee will shift perfectly fine from 1st to 2nd, then at the higher end of 2nd, right before it goes into 3rd, it down shifts right back into limp mode again. When out of limp mode, it engages R, P, N, OD, 2, and 1 perfectly fine. When driving, It shifts smoothly from 1st to 2nd gear without a hiccup. Ive tried multiple TCMs, 2 of which are brand new but all three are the model # TCM 2100AB. Not sure if I'm replacing the TCM with he wrong model numer but I'm only replacing it with an identical unit to what was originally in the TCM housing box.
 

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NDT

NDT

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Sounds like the trouble is inside the transmission or related to the harness. What is happening is when the transmission is going from 2nd to 3rd gear, the software is expecting a range of values from a sensor and it is not seeing those values, and the software tells the program to stop. No program running is limp mode. You or a transmission shop needs to verify all sensors and solenoids and clutch packs involved with the 2-3 shift are working properly.
 
Retiredwarhorses

Retiredwarhorses

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You never really touched on 3rd or 4th…your trans in limp mode is locked in 2nd gear.
Pull codes…things like bad TPS, CPS can all causes limp mode issues.
a good trans shop should be able to hook up diagnostic gear and assume role of the TCM, shift manually, monitor shift points.
I’ve had in 2 instances a bad wire from trans connection to TCM. I’ve had bad 3-4 solenoids as well.
 
ahowell9

ahowell9

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NDT said:
Sounds like the trouble is inside the transmission or related to the harness. What is happening is when the transmission is going from 2nd to 3rd gear, the software is expecting a range of values from a sensor and it is not seeing those values, and the software tells the program to stop. No program running is limp mode. You or a transmission shop needs to verify all sensors and solenoids and clutch packs involved with the 2-3 shift are working properly.
Click to expand...
Thanks alot guys. You and "RetiredWarHorses" have given me a new starting point. You guys have a blessed day
 
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Action

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