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Allison transmission not right. Please help.

Discussion in 'Chevy Trucks' started by DuramaxPowered, Dec 23, 2007.

  1. DuramaxPowered

    DuramaxPowered Member
    Messages: 86

    I have a 2002 Chevy crew cab 2500 HD Duramax/Allison. Going into reverse with a "SLAM" now and i am pretty sure it is stuck in 3rd gear. Will not shift. Please read my description and tell me what you think it is. Do you think it is the (NEAUTRAL,SAFETY,BACKUP,SWITCH). Could this be? Any help would be appreciated. I'm going to try and get it looked at tomorrow at a dealer but the holidays are here. Man what a bad time for this.

    2002 Chevy 4x4 2500HD short bed crew cab, Duramax,Allison
    95000 miles. Well taken care of, 1 year comm. plowing now.(Which was drove by me and i would say light plowing).
    I bought it new and drove all the miles. 5% towing miles.5000-12000 # Trailers. Heavy loads but taken care of.

    Just last week i jumped in the truck to back it out of the shop. I noticed that when i put it in park the doors didn't unlock like they are supposed to. I then noticed the little square which tells you which gear you are in was either in the wrong position, or didn't show up at all. But the truck would still go in the correct gear it was in... And it shifted/acted normal. Only thing wrong was it wouldn't unlock the doors for you when it went into park.(Sometimes it would) but not all. And the gear position was in the wrong position or wasn't displayed at all.
    The next day we got alot of snow so i had to plow for 3 days straight. Wet heavy stuff. Transmission worked fine other then what i just described. And i had to plow. Since then i removed the plow and drove it a few times and the same thing.
    And today (Sunday) my wife drives it 20 miles away to the Mall to pick up a present. Anyways she got to destination and all was fine. She is on her way back home and she discovers the truck is going 45 - 50 mph and reving 3000+ rpm's. She knew something was wrong and called me. She described it and i told to do a few things. I knew she had it in drive but it was still not shifting... So i had her baby it home. I get in it and start it back up. I notice right away the Service Engine Soon Light is on,(WAS YOURS ON?) The engine ran fine. I then put it in reverse and "SLAM" i thought oh boy.. I back it out of my shop and put it in drive. It goes in but felt like it is in a higher gear. Had to give it alot of gas to make it go. I drive it down the road and back. And i think it is "stuck" in 3rd gear. I manually shifted it from 1st on up but it didn't shift at all. And would rev high as she described... Does this sound like yours? I need to get this fixed ASAP for the next snow. What do you suggest i do? Is this internal or a external part? What was the total cost? Any help would be appreciated. Thanks alot. Chad
  2. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    Sounds like all the classic symptoms of a failing NSBU switch.. the fact that it stopped locking the doors was your first indication of the NSBU going bad as it wasn't sending the signal up from the switch to the body control module...so the BCM didn't know to unlock/lock the doors...

    The fact that the check engine lamp is on and it's stuck in third gear means that it self diagnosed the problem and now it's in what is referred to as "limp mode". This is what the transmission is designed to do when it encounters a problem of some sort... it will only have 3rd and reverse gear, nothing else and trip the CE lamp..

    Another good indication of a failing NSBS is the malfunctioning gear shift indicator..

    Now also, all these problems could also be due to a problem with the wiring harness that travels up from the trans to the transmission control module... as they did have some issues with this harness in the early D-max/Allison trucks....

    If it turns out that the NSBU switch needs replaced, they're not to expensive... around $60-$75 or so. Not too tough to change either.
  3. ticki2

    ticki2 Senior Member
    from NH
    Messages: 175

    If you can find an Allison dealer you may do better than chevy. Their are two version of the NSBS , one black , one tan . Get the tan one it's more weather proof . Good luck
  4. DuramaxPowered

    DuramaxPowered Member
    Messages: 86

    Thanks alot for the help! I spoke to my local gm dealer this morning. I talked to a lady who didn't know to much, but she gave me 2 prices. One was $73 and the other one was $213 and she said it was a kit of some kind and the description also said it has a shield... So i don't know what the difference is yet. Does anyone know? (By the way the high price part is Part# 29540479 , but she wouldn't tell me the part# for the cheaper one.

    One thing i will add. I looked at my truck and my factory NSBU is Tan in color...

    One other thing. I called another dealer and talked to a guy and he said They need to know the VIN # of my truck to determine which part i need. He said there are 2 options which the VIN # will tell him which one i need. I gave him my VIN and he said it takes a $200+ part. Can anyone verify if this is true?

    Another dealer i just called said give me the vin # and he looked it up also and said $199.98. He said it goes by the VIN #... Any Tech goes know for sure??
    My VIN # 1gchk23122f180515 Thanks for any help.
  5. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    Even though your truck already has the tan switch, they still go bad sometimes...

    There is actually three different NSBU switches for the Allisons..
    1) The original "black" switch that they had all the problems with.
    2) The updated "tan" switch that replaced the "black" switch (p/n 29540479).
    3) And theirs also a different switch that they used when they went from 2 separate 6 pin connectors, to a single 12 pin connector..and they don't interchange that's why they needed your VIN.. to determine whether your truck has the two 6 pin connectors, or the single 12 pin.

    Now I'm not sure if you got your numbers mixed up or the dealer is high on his prices but the tan two 6 pin switch (the 29540479 which includes a shield) for the switch is only $100 or less, so I'm not sure whats up with your prices.
  6. DuramaxPowered

    DuramaxPowered Member
    Messages: 86

    Well my local dealer 1 mile from home said. Part# 29540479 which is a kit with a shield. (2 phone calls. 1st call they said $213 and 2 hours later same place different person said $199.98)

    Another dealer 30 miles away said the same part was $200.

    So i didn't buy today. I will have to wait until Wed. and see what else i know from another dealer which i bought my truck from in the first place. I will let you know though. It makes me wonder what is going on with the extreme difference in my price versuses your price.
  7. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    I think the prices they gave you are the list price... I've never paid more than $80 for one (I do get a good deal though)....Go to GMpartsdirect and type the p/n in.....they're only $103.00 even there.
    Last edited: Dec 25, 2007
  8. ticki2

    ticki2 Senior Member
    from NH
    Messages: 175

    Try Merchant Automotive , 1-866-399-7169 , 82 and change shipped.
    Or you nearest Allison dealer .
  9. DuramaxPowered

    DuramaxPowered Member
    Messages: 86

    Thanks alot for this information to this place. I called today (12-26-07) and they said that is the Part# i need for sure and it is in stock. $77 shipped to my door. It is supposed to be delivered to me on 12-27... Thanks for this everybody. I will update when it is fixed...
  10. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    That's a good deal. So your going to replace the NSBU switch yourself?
  11. Philbilly2

    Philbilly2 PlowSite Veteran
    Messages: 3,797

    Here is the write up on the NSBU switch for anyone else viewing this thread:

    Oh yeah, that is without a doubt in my mind the NSBU switch! Everything you discribed is all things my truck did that night.

    Ok, if you roll under your truck about even with the drivers side door, you will find your NSBU mounted to the side of the transmission. Your box is going to be black in color most likely, those are the ones that go bad. You will need to do to an Allison dealer to get one at a reasonable price.

    The part number is 29540479 and I paid $55.63ea.
    It is a completely exturnal part that can be changed in under 45 minutes on your first one.

    Here is what you need,
    13mm Deep Well Socket
    15mm Box End
    A small extension and ratchet
    Possible a flathead screwdriver:

    1: Put your truck in Nutral with the Parking Brake on. (your new NSBU switch comes shipped in the nutral position)

    2: Try to unhook the two electrial connections ( this is where the screwdriver comes in handy. the top connector on the box is almost like glued in, just crack the plastic around it on the old box) as I did to this one.

    3: Undo shift linkage nut and two bolts holding the NSBU to the side of the Transmission.

    4: Pull old NSBU off and throw it away!
    This is what you end up with.

    Here is a new one.

    5: Put new one on, hook up shifiting linkage, and hook up electrical connectors

    I split this one to see what is inside this thing, you can see why they fail. Alot of contact points and spots for corosion in my mind!


  12. fatboy

    fatboy Senior Member
    Messages: 222

    Its the same switch as late 4L80e's and there allot cheaper when you order it for the
    4L80e, this our little secret though.....SSSHHHHH!!! Oh yea , if you just use a little heat, like from a hand held butane torch on the plug it will
    slide right out, another secret for ya!
    Last edited: Dec 26, 2007
  13. DuramaxPowered

    DuramaxPowered Member
    Messages: 86

    Well i wanted to update everybody on this. I ended up ordering a new part (the same part # as listed in the above pictures) from Merchant Supply in Michigan. $70+$7 for shipping. So $77 total. They had it in stock and shipped it the same day i ordered it. The lady was extremely nice! I recommend this place. She verified it was in stock before i hung up the phone. (She had it in her hand within 30 seconds of me being on hold). She assured me it would ship that same day and i would receive it the next day. She used USPS Priorty mail. And this place is atleast 200 miles away from me. So that went well...
    I received it as promised and i installed it today by myself. Was very easy and was step by step as described above in this thread. Very simple job. It fixed my door unlocking problem and the gear position problem but my service engine soon light was still on and it was still in the "Limp" mode. So i drove it up to my local Advance Auto Parts store and they scanned it and cleared the code out. That fixed it and it is like new again.

    So i wanted to say Thank You to all who helped me fix this problem. Also thanks for recommending the few places to buy cheaper parts. If i bought at the local GM dealer i would of had $213.48 invested. But you guys got the part for me for $77. I saved $136.48 and alot of running time... The internet message boards are awesome! Thanks again Chad from Ohio
  14. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    Glad to hear it's working good and you did it yourself. Congrats!

    Just a note though:

    Clearing the codes with the generic parts store code reader will remove the codes from the engine ECM but won't remove any codes it has stored in the TCM (trans control module).. but as long as it works good it will clear them out on it's own after a minimum of 40 hot/cold run cycles. So you may find that it acts a little different at times for a while but no big deal.
  15. santelikk

    santelikk Member
    Messages: 67

    FYI, Even the updated switch may cause this problem, however not as frequently. Prior to changing to the tan switch I had a problem every time it rained/snowed.
    I have only had it happen to me once since I the r/r. I sprayed the area with Fluid Film to help prevent any moisture from getting in there again.
  16. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    That's why they then updated from the twin 6 pin connectors to the singel 12 pin.. and sealed the switch better..

    Good idea and I'm sure GV would be happy to hear that :D. After all he is the "FF king" here.

    BTW, good right up and pic Plilbilly... I'm sure it will make this thread very useful for guys in years to come.
  17. ticki2

    ticki2 Senior Member
    from NH
    Messages: 175


    Nice right up , have mine on the bench waiting to go in , if I ever get a day off . It's been one heck of a December and it's not over yet . BTW , I was thinking some silicon on the seams might help.
  18. ticki2

    ticki2 Senior Member
    from NH
    Messages: 175

    Way to go , good place to keep in mind for anything Dmax or Allison.
  19. Philbilly2

    Philbilly2 PlowSite Veteran
    Messages: 3,797

    Glad I can help!
  20. sparky8370

    sparky8370 Senior Member
    Messages: 234

    I just found this thread because I was searching Allison and NH looking for a rebuild manual in NH, since I forgot to order one and am rebuilding it this weekend.
    Anyways, I'd like to add something to this.
    The reason the connectors seem like they are glued in is because the poor connections from the corrosion cause excessive heat and melt the reason, effectively glueing them in. I heard the heat trick, but decided instead of risking damaging it I broke my switch around the connectors and cleaned out the connectors thoroughly with electrical cleaner, then used di-electric grease, and used a spray silicon on the outside of everything. I should have re-applied it but never have. Also, if you're in NH check Liberty truck on south willow. I don't remember how much cheaper it was than the dealer, but I wanna say it was around $70.