Service Tips: The care and feeding of GM HD differentials
Front and rear differentials may look like they are sealed and would last forever. Not so.
Gear lube tends to naturally break down over time and it also absorbs moisture on its own accord in all differentials according to GM master technician Rod Halsten.
On 4x4s the diffs also can get water in them through the vent tube when the truck is fording or plunges into deep mud holes — or they rear diff is submerged while launching a boat and the differential is hot from towing.
Halsten, who specializes in GM HDs, says diffs should be checked every 30,000 miles to be sure the lube is doing its job.
“It’s not uncommon for the gear lube to have a yellow-tinged, milkish appearance instead of clean fluid,” says Halsten. “That means the fluid is compromised and needs draining and replacing. If the fluid looks clean, then all it needs is topping off.“
Halsten likes to use Mobil 1 Synthetic Gear Lube LS 75W-90 to maximize lubrication and minimize fuel-robbing friction.
Service tip: Fill the rear differential a little higher on the HDs than GM recommends, bringing the lube up just below the fill plug lip. “That way the lube is getting into the axle shafts and out to the bearings,” says Hlasten.