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Replacing Your Differential Fluid
 
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Pelican Technical Article:

Replacing Your Differential Fluid

Steve Vernon

Time:

1 hour1 hr

Tab:

$35

Talent:

*

Tools:

14mm Allen, fluid pump

Applicable Models:

Mercedes-Benz W123 (1977-85)

Parts Required:

Differential Fluid

Hot Tip:

Do not over tighten the plugs

Performance Gain:

Smoother drive train operation

Complementary Modification:

Check your CV joints

A differential is just like any other mechanism with moving parts. It generates heat and friction while in operation. Just like in an engine or transmission, regular oil changes help to keep the bearings and gears inside well lubricated and clean. Over time, the differential wears just like any other component. However, it has no internal filter to capture small particles of metal as the gears wear. Usually, differential oil is good for up to 80,000 miles without changing, however it is commonly overlooked. Failure to change the fluid at regular intervals can eventually cause the bearings in the differential to fail.

Get the oil in the differential warm before you drain it, it will help get the impurities out.

Next safely jack up and support your vehicle. You will need to jack up both ends of the vehicle as the fluid level needs to be set with the vehicle level. Please see our article on safely jacking up and supporting your W123. If your differential is dirty, clean around the plugs so no dirt or debris will get into the diff while you are working.

There are two plugs on the differential.
Figure 1

There are two plugs on the differential. The fill plug (yellow arrow) is on the rear right side of the diff higher than the drain plug. The drain plug is on the lower right side (red arrow).

As with any fluid change, always make sure you can remove the fill plug BEFORE the drain plug.
Figure 2

As with any fluid change, always make sure you can remove the fill plug BEFORE the drain plug. If you drain the fluid and then cannot get the fill plug out you will have no way to drive to a store and get anything to help you remove the fill plug. Use a 14mm Allen and remove the fill plug (red arrow). These plugs can get set pretty tight in the diff over the years so make sure the Allen is properly seated.

Remove the drain plug.
Figure 3

Remove the drain plug. Let the differential fluid drain completely. Before reinstalling the drain plug, check it for any dirt, debris or metal shavings (red arrow). Small metal shavings are normal but any large bits or pieces are a sign of serious trouble. Reinstall the drain plug and torque to spec. Do not over tighten.

Due to the location of the fill hole you will need to use a fluid pump to get the fluid into the differential.
Figure 4

Due to the location of the fill hole you will need to use a fluid pump to get the fluid into the differential. Fill the differential until the fluid starts draining out of the fill hole (red arrow). Reinsert the fill plug and torque to spec. Do not over tighten.

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Page last updated: Mon 12/5/2016 03:04:48 AM