Allison 5sp: No talky to TCM help!!

[DieselDummy]

Checked all connections at both ends, all looked good but cleaned anyway. Was told a there are 3 trouble spots for harness, gotta check those and pull harness to check it all. All codes point to a wiring issue but could nsbu do it or a bad TCM?
It happened on start up first got it to come out of limp drove to work fine, left work did again, shut off couple times came out if limp drove 20mi normally then right into limp at a snap of a finger under no load, drive another 15mi in limp and boom right out of limp and drove home with no issue. 1 hour later start up in limp and been there since.

 

[Mike L.]

Codes?

 

[DieselDummy]

U2106 is main one

 

[DieselDummy]

And u1024 and u1026

 

[DieselDummy]

Ok went thru harness completely from TCM to trans, found no issues at all reassembled and nothing still. Changed out TCM and no change, anybody else have this issue?

 

[DieselDummy]

Any help would be great, need truck back ASAP!

 

[jc1843]

Does the shift display work OK-? If not, replace the NSBU switch.

Many have had a ignition switch problem, but I don't think that will cause a "U" code.

The "U" code is a communication error, and can be caused by a lot of different things.

 

[Swisstyphoon]

My buddy just had a similar issue the other day, he said one of the ground connections was bad

 

[DieselDummy]

Display shows selections, but no hash mark under them.

 

[DieselDummy]

Well whole harness is on bench and we can't find anything wrong with it, need to know if a ignition switch will cause the issue ?

 

[Gmclbz]

Also, when issue is present ABS light is on.....tried NSBU switch and checked speed sensor...

 

[Gmclbz]

Anyone?

 

[GeneralTJI]

Check the grounds under the drivers feet on the outside of the frame.... remove, scrape, sand clean etc.

Even if the harness is good, it may not be making good connection on one of the connectors:

My truck once showed the check brake system message, all warning lights on the dash on, the obdII wouldn't work, couldn't flash truck, went into limp mode and had no power etc. ... ended up being either that ground I mentioned above or pin# 27 on the tcm (it looked like it was opened up a little too far down inside the plug.. so I got a small sharp tool and bent the little sides back in a bit... did those two things and plugged it back in and all those issues were instantly gone!! real PITA for a few hours trying other things before that...

Just a thought....

 

[DieselDummy]

Well I've put a new/used harness in it and new nsbu problem is still there. Will work fine for a when, then I will shut it off for 45-60min and go to leave and it starts up right in limp mode. Tried different,working TCM and no change, thinking about a new ECM . Driving me nuts because I can't trust it to go anywhere, need help or ideas!!!

 

[ryanvanl]

DieselDummy, did you ever figure this out? I'm having the exact same problem. I can start it by jumping power to crank fuse and it goes straight to limp mode. No dash under PRNDL. Harness is fine, ALL fuses are fine and such.