Servicing your Allison 100 transmission on your 2001-2019 GM 2500HD & 3500HD trucks is vital to keep your transmission running properly for years to come. Before you start you need to verify how much fluid you need. Two factors determine this, the first is the transmission pan that you have on your truck, and the second is what service you would like to do. Today we will be covering a standard drain and refill while changing the small external filter, this is the recommended service at 30,000-50,000 miles per the Allison 1000 handbook. Allison does not recommend using a machine to filter and /or flush the transmission.

How much fluid do you need?

To do this you will need to confirm what transmission pan you have, the factory-installed pan is a stamped steel pan and it will have the Allison logo stamped on it and have a slope, the pan gets shallower towards the rear of the transmission. Allison also makes a deeper pan that is available and it is also stamped steel with the Allison logo on it, but this pan is slightly bigger than the factory one and maintains the same depth from the front to the back of the transmission. The final option will be an aftermarket deep transmission pan, such as a DmaxStore High Capacity Pan, Mag-Hytec Pan, PPE deep pan, as well as many others. For a stock pan, you will need 2 gallons, for a deep Allison brand stamped steel pan you will need 3 gallons, and the aftermarket pans vary a little bit more but will generally take 4-5 gallons for a drain and refill service.

Now that you know how much fluid you need you can now go purchase your service kit. DmaxStore recommends using Castrol Transynd 668 synthetic transmission fluid, along with a DmaxStore Performance Allison Transmission filter or an Allison branded transmission filter. This will ensure proper fluid longevity and performance over time to keep your Allison shifting great for years to come.

Once you get your service kit in your hands it is time to service your Duramax.

1. Remove the 15mm drain plug, aftermarket pans use a different size, from the transmission pan and drain the old fluid. This should yield approximately 7.4 quarts from a stock pan, unless you are low on fluid to begin with. Then re-install the plug into the pan.
2. Remove the old spin on filter as well as the magnet located on the top of the old filter and clean it with a rag. Install the magnet on the new filter and fill the filter with transmission fluid. Install the filter onto the transmission.
3. Using a transmission funnel, fill your transmission with 7.4 quarts of transmission fluid, check the level. Once the level is within the cold hash marks you can proceed to warm the truck up and check the level once the fluid is up to temp.