LB7 in limp mode. Lots of codes

[Robby05]

LB7 shop plow truck has been at 2 different repair shops trying to figure out why its stuck in limp mode. Boss brought it back over from the last repair shop and wants us to figure it out.

Back story. Truck has had a hard life. Sits all summer and get abused in the winter plowing. New plow put on it and some underhood wiring cleaned up. Few days later. Truck goes into limp mode. Gets sent to trans shop and for a few weeks and they give up. Next shop it goes to they put a used trans in it, 4 TCM's and allegedly had a tech from GM shop look over wiring harness and try to figure out what was going on.

Gets to back to our shop and I scan it with my auto-cal:
P0380, P0405 (not too worried about those yet) P0802, C0035, C0040, C0055, U1000, U2106

Worried about all the communication codes.

So far I've cleaned all the grounds and repaired a few that were suspect. Looked over as much of the wiring harness and I could see. Pulled the underhood fuse box and checked the wiring underneath it.

Looking for a next step or problem areas where the harness may rub. Im assuming that this is a wiring or electrical issue because of the comm codes

Thanks

 

[Mike L.]

P0802 could be a bad ignition switch. The C codes could be wheel sensors if I remember correctly. The U codes might disappear when the other codes are fixed.

 

[Robby05]

Thanks Mike. I'll give that a shot first. Forgot to mention, the PRNDL lights up but no indication of when gear its in

 

[Budneeds2beers]

Sounds like what mike said. What your describing sounds like an ignition switch problem i had months back. Almost left us stranded in the desert.

 

[Robby05]

Update on truck. Replaced ignition switch and most of the codes have gone away.

Still have the U2106 code present. Trans stuck in limp mode. Anymore ideas? What wires at TCM harness should I be looking at?

Thanks for any input

 

[Mike L.]

Check for any loose wires at ECM and TCM. If they are all tight, go to the C1 ( Grey ) connector at the TCM and check resistance between wires 29 and 32. Resistance must be 60 ohms or less. If more, there is probably a disconnected wire. Wires 29/32 are the high/low communication wires between the TCM/ECM.

 

[Robby05]

Checked resistance at wires 29 & 32 -- 61 ohms. Verified ground, battery voltage and ignition voltage at C1 connector.

 

[Robby05]

Got it figured out. Double checked voltage at C1 connector. Noticed orange wire didnt have the same voltage as the battery. Pulled harness down through wheel well to get a better look at it. Found an old test light poke hole in the orange wire that was growing that nice blue corrosion. Cut out the damaged section and spliced in a new wire.

Cleared codes. Turned on and off a few times. Has not came back yet. Not in limp mode anymore. Gear indicator is back also.

As always, thanks a bunch for the help, Cheers.

 

[Budneeds2beers]

Nice glad to read the outcome and see the great suggestions from the man!

 

[Mike L.]

Good job. :thumb:

 

[TheBac]

Glad you got it repaired, Rob. I personally hate diagnosing electrical issues.

 

[Budneeds2beers]

^ I love it. But thats what i do for a living.