DARK REAR DIFFERENTIAL LUBE
I recently purchased a Duramax/allison 2500HD from a dealer here in Olympia, Washington. I bought the truck to tow an 8,000-pound fifth-wheel travel trailer our family bought last summer. The gas engine in my previous truck just wasn’t up to the task with the mountains here, in Washington. As part of “baselining” the truck, I decided to service the truck’s differentials using synthetic lube. When the differentials were drained, we found a bright-yellow lube in the front and an opaque-black lube in the rear. We also discovered iron filings on the magnet of the rear drain plug. I now have all-synthetic lubricants in my Duramax and hope the truck’s next differential service will show better results. How can I determine if the differential has excess wear? Sean Jeffers Olympia, WA
Sean,
Ordinarily, the factory-installed 75W-90 synthetic lube in the rear differential should be changed after about the first 500 towing miles or 10,000 non-towing miles. The factory lube in the front differential is also a 75W-90 synthetic.
Like you, I found an alarming amount of metal on the magnetic drain plug when
I first changed the rear differential lube in my 2500HD. I was told by an industry source that this is normal for a first service. It shouldn’t be there, but I suspect some of the metal was leftover manufacturing debris, as well as early wear particles.
Excessive heat is what usually turns the lube dark. This can happen early in the differential’s life due to the heat generated during break-in or during heavy towing demands with high outside temperatures. If the 75W-90 synthetic continues to turn dark while towing, you might need to switch to a heavier-weight synthetic lube, especially if you tow during the summer months. Your cool western-washington location should be ideal for the life of the rear differential lube.
The magnets should have captured any ferrous metal particles. As long as the rear differential is quiet and the next service shows the lube to be normal, I wouldn’t worry about it.