4L80E locked in second gear

[mojo2013]

I recently acquired an M1123 with a rebuilt GM transmission dated 9/10.

Truck originally shifted fine but is now struck in safe mode-second gear.

Fluid is in good shape, connections are together. Speedo is off by about 10mph (faster) than actual. Transfer case works well.

Any suggestions on where to start?

Thanks


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[papakb]

Check the relay that provides +12v to the transmission control unit as a start. If you lose it the truck goes into limp mode. After that it's all the usual suspects.

 

[infinitymotors]

For future reference where is that relay located?


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[86humv]

For future reference where is that relay located?


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Behind the battery box inside the panel with the circuit breaker.

 

[BLK HMMWV]

You can read all about it in the tech manual found in the technical manual library here on the forum.

 

[Retiredwarhorses]

It’s a 4L80e, not 4L60e...verify that your transmission lamp goes off after startup

 

[mojo2013]

Lamp goes out, no issue there...I know it’s something simple I haven’t found yet.


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[AAVP7]

Did you read out trouble codes from the transmission ?

As said above, there´s an amazing 4L80 troubleshooting manual out there, which is a very detailed reference and includes step-by-step instructions.

The tranny might default to safe mode if one of several sensors gives unplausible results. The manual will guide you how check them all.

 

[gringeltaube]

It’s a 4L80e, not 4L60e

Title has been edited/corrected, now.