While most Mercedes owners remember to change the engine oil often enough, not everyone remembers taking care of the other lubricants in the car. This article focuses on the most seriously neglected fluid: gear oil in the rear differential.
Most Mercedes 126 cars have "open" differentials, although some of the latest models have limited slip differentials. The former do not require much attention and are, in fact, extremely robust. Hot-rodders have found that the rear differential on the 380SE can handle up to 800hp through the rear wheels. But that does not mean we can forget it completely. Gear oil must be changed at intervals of about 100,000 miles, or every 50,000 in severe service. If left too long, it will eventually lose its lubrication ability and start to look like frothy pea soup! (It does not smell too good, either!)
The capacity of the fluid is only about 1.5 pints, so there is no excuse for do not use the "good things". Amsoil, Redline and Royal Purple all manufacture high quality synthetic gear oils, generally 75W-90 viscosity. Just as with a change of oil, it is best to do it with the fluid at the operating temperature. The two differential plugs – top fill and bottom drain – take a hex key of 14 mm and are usually extremely hard to move. A long circuit breaker bar will be needed. Make sure to loosen the filler cap first, because if you remove the drain plug and you find that the shutter is stuck, you will have a small problem. Melting an ice cube in the 14 mm fitting of the filler cap can help relax it by thermal contraction.
Filling is much easier with a small oil pump. Just fill up until the gear oil is at the same level as the fill port. when it starts to drain wires, you know you're done. To ensure accurate filling, the car must be level during this operation, although it is not critical. If you leave the back of the car on candles, a slight overflow will ensue. Tighten the plugs to 50 Nm (37 lbs / ft).