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Hi All, Newbie here. I was able to troubleshoot my 02' 2500HD Dmax LB7 5spd auto, approx. mileage 150,000K/90,000Miles .Thanks to all of the awesome articles posted - big thanks. The symptoms were as follows: over the last week the shift display wasn't showing that I was in Drive. I could shift into 3 and it would light up over the 3 but when shifting back to D, it remained unlit. The problem then amplified while plowing snow, I shifted from fwd to rev and there was a delay, then big clunk. It did this a couple times then went away. The lack of indication problem occurred through the week culminating with blocking traffic in two directions trying to back into my driveway. I shifted her into reverse and she clunked again then nothing. tried to shift from drive to reverse, didn't matter what gear I chose, it wasn't moving. The check engine light came on this time as well. Worse yet, I had to pull it into the driveway with the wife's Ford! I had thought the worst, the transmission was pooched! Thats when I googled the problem and these forums came up. It took only a few minutes and I saw a similar problem had occurred and the NSBU (Neutral Safety Backup) switch was the culprit. With a little more googling I found some pictures and the actual part number (29540479) heres the link: (If it doesnt work just google NBSU switch.)Allison 1000 Series NSBU Switch pictures from cars photos on webshots
I replaced the switch, the problem was solved. It costs approx $280 - alot cheaper than a new tranny! Replacement instructions came with the switch kit, easy to follow and only two plugs, two bolts and the linkage arm to disconnect.
Cheers guys! :drink
Sally
 

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Oh...... Squirrel!
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Thanks Mike L - problem I have is the truck will not shift into high gear - and the dealer can't figure it out - says go to Allison (at 35 mph). I suspect Neutral Safety Backup Switch as they do get crudded up and I haven't put back my fenderwells since other work. I have a p/n 29541852 - 1 connector. Trying that tomorrow as it is a $50 part.
Would like to put an aluminum cover on but someone said might get hit by a rock - steel is better (????). I did lose my tranny cooler, had to suck it up and have dealer replace - looks like a mickey mouse unit.
 
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