Range shift inhibited problem in 03 gmc 3500hd duramax Allison trans

[mrmiano1989]

Hey guys I'm having a problem with my 03 GMC Sierra 3500 crew cab dually 6.6l lb7 duramax/ Allison trans ! At first start up when the tranny is cold it shifts into gear smooth and the indicator line shows up under all the gears on display after it warms up abit the indicator disappears and clunks when I take it out of drive ! Ends up in neutral or what I assume to be neutral ! But when I give it a lil tap of the accelerator and "range shift inhibitor" shows up on the display so after a while it will then dump into 3rd and stay ther without being able to up shift anymore ! So that's my issue any help on this would be awesome I've only had the truck for a year now but it's up ther in the mileage bought it from my uncle and when he had it didn't experience this issue whatsoever


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[Mike L.]

Get some codes. You may have a bad NSBU switch. I would change the switch anyways as it's cheaper than trying to diagnose it.

 

[mrmiano1989]

Thank you I'll try that first I was also told it could be a dirty ground but I can't seem to figure out were that ground could be


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