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Pelican Technical Article:

Differential Fluid Change

Kerry Jonsson

Talent:

*

Tools:

14mm Allen head socket with ratchet, Allen key or and or wrench

Applicable Models:

Mercedes-Benz R129 (1990-02)

Parts Required:

Differential fluid

Hot Tip:

Drive the car first to heat up the fluid. It's thick and will take time to drain

Performance Gain:

Extend the life of your differential

Complementary Modification:

Check the differential mounts

     Your differential is one of the final driveline components in your vehicle. It uses oil to lubricate it. The oil typically used in a differential is gear oil. It tends to be much heavier oil because it only needs to protect the ring and pinion gears from mechanical contact as the gears mesh. It does suffer thermal stresses or combustion contamination like engine oil does. It is usually forgotten about since it does not require regular service intervals. Sometimes it is even referred to as fill-for-life. Now if you have a leak then you are going to need to keep an eye on it. Even if you don't have a leak it is such an inexpensive fix that is just makes good sensor to change it at least once on your car with all the miles you put on it.

In order to service your differential you will need to lift and support the rear end of your vehicle. See our tech article on jacking up and supporting your vehicle. Drive your vehicle for a few miles to warm the fluid.
This photo illustrates the bottom of the differential looking up at the left side.
Figure 1

This photo illustrates the bottom of the differential looking up at the left side. This is where the fill plug is. Insert your 14mm Allen key and loosen the fill plug. My Allen head socket is long and the exhaust and parking brake cable mount is in the way of putting on a ratchet or breaker bar. I used a 14mm wrench over the Allen head socket to loosen the fill plug.

This photo illustrates that we are working under the rear of the car by the rear differential.
Figure 2

This photo illustrates that we are working under the rear of the car by the rear differential. Remove the 14mm drain plug with a 14mm Allen head socket or key.

Allow time for the rear differential fluid to drain out into a drain pan.
Figure 3

Allow time for the rear differential fluid to drain out into a drain pan. Gear oils are typically thicker and drain slower. Once completely drained, reinstall the drain plug.

Use a long hose to insert into the differential fill hole (green arrow).
Figure 4

Use a long hose to insert into the differential fill hole (green arrow). As you squeeze the bottle the gear oil will be forced through the hose and fill your differential. Keep adding gear oil until it starts to run out of the fill plug. You will probably need 2 quarts of gear oil to fill this differential. Once full, reinstall the fill plug. Double check that both plugs are tight and you are done.

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Comments and Suggestions:
King John Comments: Just make sure you loosen the fill plug before you drain the differential! You don't want to have a differential drained with no way to refill it
April 24, 2015
  Followup from the Pelican Staff: I agree! Great tip. - Nick at Pelican Parts  

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