Mercedes-Benz W124 ASD differential oil and whine
Which oil goes in the Mercedes-Benz W124 ASD locking differential?
Any oil that meets the Mercedes specification 235.7 is suitable; Mercedes-branded oil is not mandatory. The current viscosity is 75W-85, whereas 85W-90 was used earlier, so the specification has changed. The same specification is also fine for a normal differential.
How do you change the oil in a Mercedes-Benz W124 ASD differential?
1. Raise the car level and clean around the plugs. 2. Open the filler plug first, so you know it will open. 3. Drain the oil and blow compressed air into the filler hole to make sure the breather (the differential's vent) opens. 4. Fill in new oil through the filler hole until it is level with the hole.
Is the hydraulic oil of the Mercedes-Benz W124 ASD system the same as the differential oil?
No. The hydraulic oil (ZH-M) in the reservoir in the engine bay controls the locking and is a different fluid from the gear oil in the differential. The same reservoir also serves the level control. Hydraulic oil is topped up as needed and is not usually changed. If the reservoir runs empty, check the lines for leaks.
Why does the Mercedes-Benz W124 ASD differential whine or leak oil?
The most common cause of whine is worn bearings, and replacing the bearings quiets the noise. If the pinion gear is worn, replacing the bearings alone is not enough. Leaking seals (shaft seals) are replaced, and the breather is kept clean. The centre nut is returned to the same position after opening, because too tight a setting loads the bearings.