Trucks & Trailers - 2500 HD Allison Transmission

I've always had a gas powered truck, however with my business I need a work truck that can handle towing very heavy loads so I've been looking into purchasing a used diesel. Came across an 02' GMC 2500HD with 147,000 miles. I know it can get the job done, but whats the verdict on the Allison transmission? Any current owners/previous owners out there of this model that can give some input on engine/transmission reliability etc..? Thanks!

The best transmission in any of the 3 trucks. Make sure the injectors were replaced and you should be fine.

It is considered a very good transmission.


The only known problem can be the pump bolt gaskets can deteriorate with the use of Transynd transmission oil leading to leaking oil and a need to change the gaskets. The cost is at about $1000 to remove the transmission and the parts run about $20-$30. You may want to stick with Dextron oil.

The older Allisons do not want to see big horsepower upgrades. Keep it to less than 350 hp and 600 tq and you will be okay.

Mine is a 2004 and I had to change the pump bolt gaskets. Our hp/tq is stock at 305/500.

I like the transmission.

Thanks for your input guys, below is an excerpt from the listing. Forgive my ignorance on diesels, but how much value would you assign to this kind of work done? I plan on test driving it soon, any additional parts of the engine/transmission I should question with regards to replacement and or current condition of?


Recently updated:
New Tires 2012
2011 New Disk Brakes all Around, Fuel Pressure Regulator, 2010 Fuel Injectors and Pipes,
2009 Front End Rebuild, Shocks and Air Bag Left Kit

Never had an issue whatsoever out of the 3 i have owned.

I have a 2002 Dmax / Allison I've owned since new. It currently has 262,000 miles. I have towed an incredible amount with it over the past 10 years. anything from maybe around 1,000 lbs up to 20,000+ lbs. I've towed my ~10,000 lb boat/trailer thousands and thousands of miles. I've towed my ~4,000 marsh boat / trailer thousands of miles, my 7'x16' cargo trailer all over the state, tractors on flatbeds, cars on flatbeds and my 30' 5th wheel camper and a virtual smorgasborg of other trailers/ loads... including once 18,000 lbs of mulch. Never once had a single problem with the Allison.

143,000 on my 06 Duramax w/Allison Trans and no problems. I am the third owner, towing a HS 2500Vx (8K lbs). Previous owner towed a 32' enclosed trailer everyday, seven days a week(auctioneer/antique dealer). Regular maintainence I think is the key to any used vehicle.

Get the transgo jr. shift kit installed if you are going to be towing heavy loads all the time

It is considered a very good transmission.


The only known problem can be the pump bolt gaskets can deteriorate with the use of Transynd transmission oil leading to leaking oil and a need to change the gaskets. The cost is at about $1000 to remove the transmission and the parts run about $20-$30. You may want to stick with Dextron oil.

The older Allisons do not want to see big horsepower upgrades. Keep it to less than 350 hp and 600 tq and you will be okay.

Mine is a 2004 and I had to change the pump bolt gaskets. Our hp/tq is stock at 305/500.

I like the transmission.

TRANSYND will not hurt the gaskets or seals in ANY ALLISON rans, DEX6 WILL take out the seals used in trannies built before mid year 06(yes GM knowingly put DEX6 in trannies with incompaitible seals in 06).

Get the transgo jr. shift kit installed if you are going to be towing heavy loads all the time

The Jr does virtually nothing for a 5 speed. The 5 speeds need the full TRANSGO kit which requires teardown to improve the apply piston.