M997 Ambulance Trans Problem

[BAW]

I have a 2015 M997 when we first got it a couple of years ago it was fine for about 6 months, then trans went in to default light came on. While diagnosing the problem the trans control module felt a little warmer than i thought it should and it would just blink code 12. replaced the control module after checking power ground and resistance of circuits. Ran good for six months and seldom drove, same problem again power, grounds and resistance all good but no 5 volt reference out of control module. Any one else had anything like this?

 

[TOBASH]

I don't know 4L80E BUT if you're suspecting electrics, make sure your alternator is within specs and your batteries are good and all grounds are functioning.

 

[AAVP7]

4L80 has a relay behind the commander´s seat that switches 12 volts to the TCM when you switch your Humvee to "Run". If you´re not getting 5 volts out of the TCM, I´d first check wether there is 12 volt input to the TCM (then again, without 12 volts, the trans check light shouldn´t work at all).

If you know about the 5 volt output, I guess you have the "4L80 troubleshooting manual" ? Invaluable for 4L80 owners.

Greetings from a fellow M997 owner

 

[BAW]

Yes all input voltages are good grounds good id make a bet that the TCM is out again, but don't know what has caused it again. This is a government deal to the city, and i keep telling them need to find someone that knows something about them. I was with GM for over forty years but some of the things they bring in here #$*%$# like their MRAP. I found some one to work on it but they don't want to spend the money. Anyway thanks guys, you never know when the littlest thing can make something pop into your head.