How to Change Differential Oil in W140 S-Class

Apart from power steering fluid, this has to be the most neglected fluid change on a car. The intervals are usually long, sometimes in excess of 60.000km, but if you have no history on when this was last changed it is probably a good idea of changing it. It might give you a smoother ride with less rear end noise. At least it gives peace of mind and the job is fairly simple.

Preparation

You will need to get some differential oil from the store. 1.4L to be exact, but in practice that means two bottles of fluid. The differential can either take 75W-90 or 85W-90, you have to decide what is best for you. It should be API GL5 quality at least. I chose to put in 85W-90 since I had good experience with that before. Some bottle comes with a little tube which you can use to fill the oil with. This oil is very thick and is a bit tricky to squeeze into the differential. Consider getting an oil pump which will make this job much easier and faster.

As for special tools you will need a 14mm Hex Socket for the drain plugs.

14mm hex socket
14mm hex socket for the drain plug

Also prepare some brake cleaner and rags to clean up underneath the diff while you are there.

Procedure

Make sure the differential oil is fairly warm, if it is summer it will drain ok without driving it first, but if it is a bit cold outside take it for a short spin before draining the oil. The oil will drain out much easier when it is warm.

Start by raising the entire rear end of the car up. Make sure to block the front wheels from rolling since there is no parking brake there. Secure the rear with jack stands so it is safe to work under. It is possible to put the jack stands int he area behind the rear axle, so you can use the jack normally to raise the car.

Raise rear end of car
Raise rear end of car

Inspect for leaks on the differential, they usually are a bit dirty. I hardly ever see a leak free differential, and they seem to seep no matter what, often they stop seeping by themselves too. No hurry in changing seals unless you see a puddle under the car when it is parked for a few days. Yours is probably looking something like in the picture below. Here the oil was very caked on and old, not fresh as far as I could see.

Dirty differential
Dirty differential, clean with brake cleaner and paper/rags
Differential cleaned
Differential cleaned

After the differential is fairly clean. Locate the upper drain plug on the differential. It should be on the left side just in front of the axle. Be sure to loosen this top drain plug first. This trick is to ensure that you can stop the job and still keep driving if you are unable to get the upper or lower drain plug open. If fluid runs out from the top drain plug, the level is topped off. If not, feel it with a finger to estimate how much oil is missing.

Upper differential oil plug
Upper differential oil plug. Located on the left side in front of the axle

When the upper drain plug is loosened. Now it is time to drain the fluid, open the lower plug and keep clear from the flow into the drain pan. Try to hold on to the nut so you do not have to fish for it in stinky oil afterwards. Differential oil is very stinky stuff, and more so when old.

Open drain plug
Open drain plug to let old oil out
Draining differential oil
Stinky and filthy differential oil pouring out

The oil from my car was absolutely stinky and very filthy, it had turn into a totally brown goo. The oil level was not completely full either it seemed. I think the differential oil here was never changed in the car’s lifetime. Good thing I got to it before differential wear started happening. Now it is a good time to check this oil in the drain pan if it has any metal shavings in it. Also stick your finger into the drain hole and see if you can feel any shavings inside. You can also stick in a magnet and check if you find any. Luckily my car had zero metal shavings which I could find.

Inspecting oil pan for metal shavings
Inspecting oil pan for metal shavings. Here there were none

Remember to put back the lower drain plug. No need to torque it, but it should be torqued to snug with a normal sized ratchet, do not over tighten this. Clean the area around so you can spot leaks after.

Now get your fresh oil and squeeze it into the upper drain plug. Either the bottle you bought have a filler tube, or you have a large syringe or a gear oil pump. I use Mobil1 85W-90, but you can choose of course whatever the brand you like, I do not honestly care.

differential oil bottles
Mobil1 85W-90 API GL5

After you have refilled the oil and lowered the car back to the ground. Take it out for a short spin, come back and look under the differential for leaks. Hopefully the ride will be a bit smoother!

Cheers, Robs out!