Limp mode I guess

[Duranator]

Hopped in the truck today and noticed shift indicator not highlighting N and D, then door locks would lock when placing in drive instead of waiting until I started rolling and not unlocking automatically when placed in park. Now the check engine light went on and it won't shift in forward gears and it hits hits a little hard when placing in forward gear. Won't be able to check code for couple days, friend suggested unhooking battery for an hour. He told me it was in limp mode. Anyone have a guess what the problem might be?

[Duranator]

Found a thread describing my syptoms except for the door lock problem and appears to be a NSBU (PRDNL) switch which will be about $50 and a 1/2 hour installing in my garage. Whew! Hope this does it but will be sure to get the code also.:p

[THEFERMANATOR]

If this is an ALLISON, then yes it is most likely the NSBU switch on the shift shaft of the transmission. Buy the switch from ALLISON as it is ALOT cheaper from them than GM for the same switch. Or if you have the dual plug model(01-roughly 03 or so), you can use one off of a 4L60E/4L80E as it is the same switch. The ALLISON switch is a better switch though as the ALLISON TCM is VERY picky about that switch being 100% accurate with no fluctuations in outputs.

[Duranator]

Thanks for the info Thefermanator. Borrowed a inexpensive actron OBD11 PocketScan and it showed P0700 and P0708 for codes and pending codes of the same number. Cleared them, the MIL showed OFF, but on the View I/M moniters it read COMP with an up & down arrow which has me concerned. Checked connections on battery and 1 pos. took a 1/2 turn to snug on 1 battery. Pulled plugs off NSBU and looked good except for the larger plug, the 2 larger connectors had a touch of corrosion, cleaned and reconnected both with dielectric grease. Took out for drive and shift indicator still not working perfect nor doorlocks but it shifted! Turned around 1/2 mile down road, shut truck off and restarted and is back in limp mode. Checked for codes and showed none except for the COMP on the I/M Moniter so now have disconnected batteries and will wait an hour and see if anything changes.

At this point will get a switch from Allison dealer if nothing changes.

[Duranator]

Reconnected batteries and still getting the COMP off the I/M Moniter mode. No idea what this means but ordering NSBU switch in AM.

[Kennedy]

When asking questions like this it helps to note the model year and vehicle details so that the guesswork is minimized.