Buddy_M Posted November 23, 2011 Share Posted November 23, 2011 I seem to have been punished for being asked to look at this, at least to look at it with what little GM experience I have.. Its a 2005 GMC C4500, 7.8 diesel, with the allison auto(LTC1000?). Only complaint is the PRNDL letters show on the cluster, but the box does not appear around the letters to indicate what gear is selected, and it feels to stay in either 1st or 2nd gear limp mode and will not shift from that gear. All I have for a scan tool that will hook up is the OTC Genisys, it pulls a code U0101, from the engine pcm, which is a TCM communication problem from what I can find about that code, but scan tool will not communicate with the TCM. Checked all fuses, all are good and present in the fuse boxes. Is there anyone here with some GM medium duty experience that can give me some insight on if this is a known or common issue? Otherwise it will probably be getting trucked to the other corner of the state and they would like to avoid that expense if possible right now. Quote Link to comment Share on other sites More sharing options...
joshbuys Posted November 23, 2011 Share Posted November 23, 2011 Check the powers and grounds at the TCM - be sure to LOAD test these circuts. There are 3 power wires, and IIRC 3 ground wires. I'll check for you - just helped one of the guys at the shop with this exact issue - the ground wire in the kick panel was 'good' on an ohm check, but could not carry any load. Repaired the ground and voila. Have also seen chaffed harnesses at the crossmember where the TCM is mounted. We're an Isuzu dealer also, so I have access to all these manuals Quote Link to comment Share on other sites More sharing options...
Bruce Amacker Posted November 23, 2011 Share Posted November 23, 2011 Speaking in very general terms, 3/4 of IH Allison problems are harness related. Good Luck! Quote Link to comment Share on other sites More sharing options...
joshbuys Posted November 23, 2011 Share Posted November 23, 2011 Speaking in very general terms, 3/4 of IH Allison problems are harness related. Good Luck! 100% Agreed!! Quote Link to comment Share on other sites More sharing options...
Clark Posted November 26, 2011 Share Posted November 26, 2011 Definitely sounds like you're not getting power to the TCM or possibly TCM failure. The 1000/2000 will get reverse and third with no power. Speaking in very general terms, 3/4 of IH Allison problems are harness related. Good Luck! You said it! Quote Link to comment Share on other sites More sharing options...
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