Stuck in 3rd, abs, service brake system, no gear indicator

Beatums · May 19, 2017

I'm new here so hello. I've tried searching here and Google and anywhere else on internet. Ive found many people with similar problems and their solutions but can't seem to find anybody with the same combination of issues as I have.

2003 gmc 2500hd 8.1 w/ Allison. Just bought this truck a month ago. It's got a shitty 3 month warranty on engine and trans. Dealer is 50 miles away, not trying to tow it if I don't have to but this might be out of my league.

I started it up one day and it's telling me to service brake system. Abs light is on, brake light is on. Its stuck in 3rd. It shifts hard into reverse and drive and there is no gear indicator light underneath PRD321. The speedometer and trans temp gauges aren't working, and it won't display the instant fuel efficiency.

This happened once, drove it two miles and it all of a sudden went away and drove like normal. Two days later it happened again and its been this way for a week now.

Current codes I'm getting are p0802, p1571, c0035, c0055, u1000

Codes stored in history are c0040,u1009, u1003,u1007, u1255, c0327, b0790, u1301, p0500

So far I've changed the neutral safety switch, vehicle speed sensor, and ignition switch. I attempted to change the wheel speed sensors but I can't get the damn brakes off, the whole assembly is so damn rusty it's ridiculous. The brake fluid level is good.

I don't know where to start, or where to go from here anyway since I've already replaced a few things. I'm guessing I have a bad ground somewhere or maybe my tcm is bad. Any advice would be greatly appreciated.

Adrian17 · May 21, 2017

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Edited May 21, 2017 by InvisibleBlack

C/K Man · May 21, 2017

Sounds like you are loosing electrical power to the transmission. ABS may show a fault if it can't see a signal from the transmission output speed sensor. Hard shift could be caused by maximum line pressure, which along with going into 3rd. are basically default modes I would start by checking the harness going to the transmission and the connector.

stan420 · May 11, 2023

I have this issue also 2001 chevy 2500 8.1 w / allison in limp mode shifts hard into drive n rev. stuck in 3rd gear has code p1571 also no gauges light up on cluster exept engine light but if I press the trip reset then all gauges work