# 02 2500 HD Allison Issues...



## exmark1 (Nov 30, 2005)

I have a 2002 2500 HD Duramax, Allison Transmission running a 8'2" Boss V-Plow

Last nite one of my guys calls and tells me that in clunks loudly and lunges when going into gear from park or neutral it does the same thing going into forward or reverse.

The truck has 139,000 miles on it and has never giving us any troubles, I have checked all the u joints etc... and they appear to be fine.

Any suggestions? The truck shifts and drives fine once it's in gear and going just engages bad


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## Tom1 (Nov 8, 2010)

Check engine light on? Possibly Torque Converter Clutch solenoid.


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## B&B (Nov 4, 2006)

Need to check and reference any DTC codes before doing anything besides checking fluid level.


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## exmark1 (Nov 30, 2005)

No lights came on ever it went from working perfect to what it was instantly, I went to pull it into the shop today and it was working perfectly fine again. It's going into the shop in the morning, thinking it had a sensor issue that sent it into safe mode.


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## chevyzrule810 (Oct 5, 2010)

check engine light on?


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## exmark1 (Nov 30, 2005)

chevyzrule810;1352635 said:


> check engine light on?


No it never came on


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## chevyzrule810 (Oct 5, 2010)

let us know what you find out


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## B&B (Nov 4, 2006)

The fact that the CEL isn't on doesn't mean there are no codes stored so I'm sure they'll find some when they scan it.


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## chevyzrule810 (Oct 5, 2010)

I've put a park neutral position switch in a 2001 2500HD because it wasn't shifting right and the check engine light was on.Not hard to replace at all right on the side of the transmission.


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## 496 BB (Feb 11, 2010)

Ive had a few issues with no CEL. Have it ran like B&B said. Youd be surprised what doesnt blink that light.


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## Schnabel Plowin (Feb 14, 2010)

exmark1;1352631 said:


> No lights came on ever it went from working perfect to what it was instantly, I went to pull it into the shop today and it was working perfectly fine again. It's going into the shop in the morning, thinking it had a sensor issue that sent it into safe mode.


Limp home mode for an Allison locks out all gears except 3rd and reverse.


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## exmark1 (Nov 30, 2005)

Schnabel Plowin;1353520 said:


> Limp home mode for an Allison locks out all gears except 3rd and reverse.


That's what happened to him when he drove it back to the shop when it started, I didn't get a chance to stop into the shop today and see what they came up with


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## samjr (Mar 18, 2008)

*I got the samething going on*



exmark1;1352120 said:


> I have a 2002 2500 HD Duramax, Allison Transmission running a 8'2" Boss V-Plow
> 
> Last nite one of my guys calls and tells me that in clunks loudly and lunges when going into gear from park or neutral it does the same thing going into forward or reverse.
> 
> ...


I took my truck to trans shop they said T-case i said nope but they rebild anyways.So they call and say we do not think we fixed it i LOL @ them.I told them its in the Differential in the back. 4X2 can feel it abit but when in 4X4 by pluck man can u feel and hear it O an i got the 6.0 and 4L80E. Thats y i think we got the same problem just dif setups


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## Philbilly2 (Aug 25, 2007)

I am going to let it all ride on an NSB switch. It will be a black or tan colored box attached to the drivers side of the transmission. There will be two wiring harnesses and the shift linkage attached to it. They are under $100 and you can change it in you driveway with a 13MM socket and a 15MM box end wrench. I have had those boxes go out on more of the 5 speed Allisons then you will believe. When they go bad, the truck and the transmission are no longer on the same page.


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## blowerman (Dec 24, 2007)

Philbilly2;1353990 said:


> I am going to let it all ride on an NSB switch. It will be a black or tan colored box attached to the drivers side of the transmission. There will be two wiring harnesses and the shift linkage attached to it. They are under $100 and you can change it in you driveway with a 13MM socket and a 15MM box end wrench. I have had those boxes go out on more of the 5 speed Allisons then you will believe. When they go bad, the truck and the transmission are no longer on the same page.


I'm betting on this one also. They can fix a million other parts and in the end it will be the NSB. If you feel like taking it in, shouldn't even be a $200 repair. Otherwise as stated, parts less than $100.


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## JD Dave (Mar 20, 2007)

An Nsbu switch does cause some funny symptoms. If the truck is stock I'd put my money on that also. B and B is right though you should get the codes read and go from there. Parts replacing gets pretty expensive.


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## exmark1 (Nov 30, 2005)

Truck is totally stock to my knowledge, I have done nothing to it! The transmission had a low voltage code in it, the transmission guy is trying to tell me that the truck needs new batteries and that would correct the problem... The problem is we just had the truck looked over for winter and both batteries passed a load test at over 900 CCA with no issues and both are only a year old.


Any ideas if that code could work with what you guys are thinking? The truck is driving great now and I havent done anything to it yet


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## got-h2o (Sep 26, 2008)

Another vote for the NSBU. See if you have a local Detroit Allison dealer. IIRC my last one from there was like $45.


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## White Duramax (Nov 8, 2010)

Like the others said, get an NSBU. They are known to act up when they get wet in snow/slush conditions.


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## Philbilly2 (Aug 25, 2007)

If NSBU goes bad, no codes are thrown. First time that it happened to me, it took me a couple of hours to figure out what went wrong, now I carry a spare on the service truck.


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## articstorm (Dec 15, 2005)

Check your grounds from anything wired to your ignition. I was having a problem with my plow last winter that ended up being a wired rubbed bare and causing it to blow the fuse. I pulled the wrong fuse and it was the ignition fuse while the truck was running and that caused the transmission to lose power. It then went into safe mode and was acting awful. Check engine light went on I shut it off. Trans went back to normal but light stayed on. I did some running around to fix plow then was on my way to have codes read and light cleared itself. It was still in the memory and it was for low voltage to Trans.


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## Schnabel Plowin (Feb 14, 2010)

When an Allison detects a problem, it usually goes into limp home mode and gives the P0700 code. Some common issues with the allison for that year are:

NSBU switch
Ignition cable (where the TCU gets its power from)
Leaking tranny cooler lines that go to the radiator causing low fluid levels and causing loss of prime.

If you can put some codes up, I might be able to help you some more. With the clunking, I'd suspect the cooler line problem. Check your fluid level with the engine warm and *still running* to see if it is still in the HOT range.


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## exmark1 (Nov 30, 2005)

Schnabel Plowin;1358053 said:


> When an Allison detects a problem, it usually goes into limp home mode and gives the P0700 code. Some common issues with the allison for that year are:
> 
> NSBU switch
> Ignition cable (where the TCU gets its power from)
> ...


The only code was a low voltage code, the fluid is full in the hot range and good red coloring etc... No visible leaking from anywhere on the lines or anywhere or the transmission


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## concreteguy (Nov 10, 2006)

I would bet its the NSBU switch. I replaced 4 on my '04 ali ready. Mine would bang into gear, like a u-joint, and would also feel like a cable is frozen and hard to shift feeling. The snow would always be built up around it and I think water would get in it and make it fail. Takes about 15-20 min to change and about $150(I cant remember) Also the PRD321 would also be out


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## exmark1 (Nov 30, 2005)

You were all right it was the NSBU switch we swapped that out and haven't had an issue since Thanks everyone for the advice


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