M74 Allison LCT 1000

akduramaxgirl · December 31, 2019

Hey y’all, I own a 02 duramax 2500hd with the Allison 1000 tranny. I’m having shifting issues or tranny issues, my truck went from going into 3rd gear limp mode to now it throws itself into neutral limp mode when I put it in drive, it’s throwing p0872 DTC, I’m hearing I have a short to ground in either my external or internal wiring harness, I have changed the NSBU ( put the upgrades one on), I replaced the Trim solenoid B, replaced the output speed sensor and replaced the TFPS inside the transmission pan, it’s mounted on the valve body. Has anyone had tranny issues that throw out random DTCs? Sometime it shifts like a dream and other times she’s very temperamental. Thank you

Mikebtte · December 31, 2019

As you say sometimes it shifts great and other times it sucks that would tell me it's ether an electrical fault (ground or wiring) or you have something sticking in the valve body. If you have serviced the transmission internal parts I assume you put in new fluids. Did you see any contaminated fluid or old fluid prior to replacement of the internal parts?

How did the wiring look on the inside?

Have not worked on an Allison in a few years. Last stuff I worked on I all required a computer to troubleshoot.

davester · December 31, 2019

Did you do anything to see if there is a problem with the wiring harness to the transmission or the one in the transmission?

akduramaxgirl · January 1, 2020

I checked as much as the harness as I could, the wires looked ok, lots a dirt and dust in them. I didn’t look at the wires in the internal harness yet. The fluid smelled a little burned, I checked my fluid level today and it’s over full, so I need to drain some out. I drove it today and it did so good, shifted great had no issues, then came home, shut it off and went to leave like 30 minutes later and back into neutral limp mode it went. Totally different codes this time, p0776, p0700, p0500, I’ve already changed the Trim Solenoid B, I’ll open the pan up tomorrow and check the connections and wires and change the fluid again.