Yes another Limp mode post! 03

[Jerrdanjohn]

Bought a 03 cab and chassis Topkick. Ex ambulance 4 door c4500. Slight damage to pass side doors and quarter fender, put it out of service. I want to install a rollback bed.
Truck runs great, but stuck in 3rd. (Limp Mode). Replaced NSBU, replaced wire pigtails for NSBU and replaced all three speed sensors. Searched the wiring for hours with no luck in finding any issues.
When I start the truck the line for the shift indicator is on then as the engine idles down the tranny light comes on and indicator goes out and don't work. Unhooking the battery does not reset it, so it has never (since I bought it) came out of limp mode. I have left the battery unhooked for a month and it still does not reset. I am assuming it is picking up a code as soon as it starts and putting it into limp mode. The truck did sit for at least a few months. Fluid level is fine, just not sure were to go from here. Don't have anyone to read the codes for the TCM on a Allison. Any guesses, I'm ready to just start throwing money at it. John

 

[Mike L.]

Need codes.

 

[Jerrdanjohn]

Need codes.

I called the local dealer, they said they couldn't do it. I towed it to a shop about 25 miles away because they said it was no problem on the phone, but there computer (a bluepoint) wouldn't show any codes.
I had the codes ran before and it showed speed sensor and NSBU switch, but it also showed a engine codes, (mass air flow low voltage or something like that) Will a engine code send the transmission into limp mode? Both the engine light and tranny light is on. I called the fellow I had check the codes before (GM mechanic) and he is going to try to come by this weekend and check them.

 

[clrussell]

Call the shop and see if they can get into transmission by going to 03 3500 truck instead of medium duty.. I think I've been able to cheat and do that

 

[Mike L.]

You might have 2 connectors under dash. OBD11 and one for the Allison. OBD11 will work for you.