Allison Problems - PN Safety Switch

snyps · October 23, 2006

2002 2500HD

8.1L / Allison

While out plowing in the early snow storm we got in Buffalo, NY I started to have problems with my Tranny. I noticed that my auto unlock feature wouldn't work and that my indicator on my dash was not telling me what gears I was in. Park, Netural, and Drive were not working.

Shortly After that I got a SES light, and my tranny was shifting hard from Park, Reverse, and N. The indicator on the dash also was blinking in third gear. It also seems like it only will go in first gear, I cannot feel it upshift (might be a limp mode).

I got the SES codes downloaded and this is what I found.

P0700

Trans Control Sys Malfunction

p0872

Trans Fluid Press Sensor/Switch C Circuit Low

P0875

Trans Fluid Press Sensor/Switch D Circuit

P0708

Trans Range Sensor Circuit High Input

Basically with a ton of searching I found that these all point to the NSBU Switch. My questions are, do all of these codes corospond with that switch, and only that switch being bad? With the purchase of the upgraded switch does the computer need to be reporgramed, or can I just toss it on and have it work? With the hard shifting does it cause extra wear and tear on the tranny and or staying in 1st gear should only be an indicator that it is in limp mode correct? I am hopeing that I can put the switch in, and have everything go back to normal. I don't want to hear that I might have blown up my tranny.

Any help would be appreacated.

Robert

GM-Tech · October 23, 2006

This almost certainly a bad NSBU. I doubt there's any other damage done.

Be sure that when you put the new one on, it slips EASILY over the shaft. The shaft tends to get mushroomed on the end but can easily be cleaned up with a file. If you don't clean this up, you'll damage the switch putting it on

snyps · October 25, 2006

Ordered the Part from http://www.gmdieseltech.com for $65 plus shipping. Local GM Dealer installed it for $76. Everything worked out perfectly. Truck totally back to normal, shifts better then I can remember before this happned.

Thanks again for all your help.

Rob

snyps · March 21, 2007

Ugggh!!!

So last night my tranny started to act up again!!

This time, the P,N,D&2 will not light up when I put it in that gear. In 2 and Drive it seems the truck is in 3rd gear. When in 1 or 3 it seems to be in the proper gear. When putting it in park the doors will not unlock or the DRL will not shut off. Also there is no SES light is not going on.

Now this is happening very intermintentlly when I first turn the truck on it seems to work fine, but after driving for a while the D light goes out and then seems to go into 3rd gear. If I keep driving it might eventually go back to normal. Also when the light is not on but it is in P, eventually it will go on when the truck is sitting and the doors will unlock.

My two options is bring it to the dealer and have them look at it or have the switch replaced under warente and replace it again myself. What do you think..

Any help would be appreacated..

spackelman · March 21, 2007

There was a TSB on the problem. Left hand tire, when turned to the left will directly spray road water onto the switch causing corrosion. If you do alot of plowing and with the salt, it just may be the switch again. Happened to me two years ago.

The new switches, I thought corrected the problems.

I pulled my old switch apart, expecting to find a circuit board - ALL mechanical. Try another switch, install it yourself, easy fix.

Also, I talked to a gm tech about the allison transmissons- he said he has never seen one in the shop for any problems other than the switch. To me that was a comforting statment.

snyps · March 23, 2007

I called Eric at Merchant Automotive (GMDieseltech.com) and told him that my new style switch went bad. He exchanged the part under warranty through Allison and overnighted me a new switch.

I towed my truck out to my buddies shop yesterday to get ready to replace the switch, but when the part came it was damaged pretty bad. Broken off pieces of plastic in the box. I e-mailed Jen over at gmdieseltech.com and told them about it and am waiting to hear back on the claim against UPS.

I have to say after all of this Eric and Jen were very helpful I would order from them anytime I had the chance. Their customer service goes above and beyond any other company I have dealt with. It just sucks that UPS killed my chance of getting my truck fixed today!!

Here are some pics of the towing experience. This is my friends 2007 classic with the d-max in it that he picked up last week. The goose neck hitch was installed 2 hours prior to this picture being taken. That truck has about 500 miles on it and towed my truck like a dream!!!

fast1elk · April 2, 2007

I have a 2003 2500HD 8.1 with the Ally, Would this NSBU stop my truck from starting I believe that I have a bad switch but my truck will not crank either it all happened at the same time if I manually press the starter relay after breaking the cover off it will start fine but it still has the bad NSBU symptons. Just wondering if it wont start because it is not showing in park also tried it in neutral and nothing that way as well Thanks for any help.