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Thread: Allison Transmissions

  1. #1
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    Default Allison Transmissions

    This may not be the right place, but I figure a bunch of guys tow their toys around with GMC trucks. My transmission is acting up. Twice now when I have placed it in gear the "D" has just flashed and it did not go in gear. Does anyone know what the problem could be?

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    Member AKDoug's Avatar
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    What year of truck? The earlier trucks had an issue with what's called and NSBU switch. You should have a check engine light coming on in relation to it.
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    It is a 2002. I don't have any other lights. It has only happened twice. I took it to the dealer and they said there was nothing wrong with it. I'm just concerned they would like to see it crap out so they can make some big money.

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    Member AKDoug's Avatar
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    Sounds like the NSBU switch connection may have gotten some moisture in it. There is a pile of information out there on this issue. Google time
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    Just had to have a trany rebuilt on my RV...The guys and gals at B&B on the corner of C and Diamond are the best. Great service and just down right good people and HONEST.

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    Default Thank You

    Quote Originally Posted by AKDoug View Post
    Sounds like the NSBU switch connection may have gotten some moisture in it. There is a pile of information out there on this issue. Google time
    I looked it up on the internet and it sounds like my problem. They sell for about ninety dollars. The ad said easily installed with comman hand tools. Is it the switch located on the top of the transmission? The one I have looks like a rock or something may have hit it. I am able to do some minor repairs, but I have never really messed with a transmission before.

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    The NSBU is on the left side of the transmission. Below is a picture. Yours may be black. First try unplugging it and looking for any corrosion or melting. If none, then apply some dielectric grease and plug it back in. The most common problem is water getting in either in heavy rains or winter snow storms while driving. All the ones I've heard of also had a check engine light on. If you replace it be very careful to follow instructions. May not come with any though The important thing is to not move the shifter position that the NSBU comes in. If it gets moved I believe the truck will need to be put on the computer to rematch the shifter positions. I've never done this to mine, but have done a ton of research when my tranny went haywire. Mine ended up being frayed wires in the trans harness.
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    Just an FYI be careful pulling those connectors they can be tight especially if caroded. If you succomb to the temptation to pry them apart they can be broken! I am not saying that you shouldn't use something to help pull the connectors, just that if you do be very careful, especially w/ the little clip ears.

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    Thanks Bro. I'll let you know how it turns out.

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    When I had my 02 GMC I would carry a NSBU switch in my glove box as a spare. They are notorious for going out at the wrong time due to the extreme conditions faced in Alaska. Takes about five minutes to swap out once you've done it a couple of times. Also when you put the new switch put some of that dielectric grease in the plug to keep the moisture out, and it makes it a whole lot easier to unhook if you have to change it out again. I bought the truck new in 02 and when I traded it off in 08 it had 223,000 miles on it and changing that stupid switch was the only problem I had with that truck.

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