How to change automatic transmission oil and filter in W123

automatic transmission mercedes 280ce

Now to the post you all have been waiting for. Time to defy old myths and the car companies themselves! Mercedes usually warns about changing the transmission oil yourself and in newer models they are so called sealed where you cannot even measure the fluid level without buying an accessory dipstick. I do not agree on this level of manufacturer control and you can totally do this yourself even on a Mercedes. Sometimes I really think that car companies have a bad attitude towards their customers. Just take for example the VW diesel scandal which they tried to cheat everyone…

Thoughts

Fluid changes are the number one maintenance priority of making your car perform optimally and mitigating excessive wear and damage. Fluid changes of the transmission oil should always be your concern. Some idiot say that changing automatic transmission oil in an old car will cause it to slip and thus you should not change oil. This is only true if the transmission is already very worn/broken and the only thing keeping the transmission from slipping is the high friction of metal particles and the old smudgy oil which is totally broken down and provides little friction and cooling. Then of course it might starting slipping when fresh oil is added which sole purpose is to give less friction and cool the internals. However not changing the oil will only delay the inevitable of total transmission failure.

A good transmission will always benefit from an oil change since this reduces friction and wear!

Mercedes does not recommend this job and it should be left to the “professionals”, all my respect to the mechanics, but they work on a hectic schedule and the quality of the oil change might not be perfect 100% of the time. Not saying they usually make mistakes, because they usually don’t when talking about good workshops. Anyway I think changing automatic transmission fluid is something you can easily do yourself on a Mercedes as any other car. It is not as easy as changing the engine oil, but not very difficult either. There are some fall pits so just be prepared IN ADVANCE.

I cover here the procedure for the M110 engine with the 722.xxx transmission. The procedure is basically the same for all older Mercs. Changing the automatic oil will give you a good indication about the condition of your transmission unit so you can take preventative measures or just continue like usual.

Preparation

You will need to get some new stuff

  • 6.6L ATF Dex3 on earlier models
  • 6.1L ATF Dex3 on later models (usually 81-85)
  • New filter and oil pan seal (get the right one from Mercedes)
  • Narrow Funnel
  • Brake cleaner
  • Shop towels (lint free)
  • Jack stands and jack
  • Dripping pan
  • Tools – sockets and hex bit
  • Torque wrench
Draining

Unless it is a very warm summer day, go for a drive (recommended is 24km to really warm up the transmission to operating temperature, but sure you can go less if it’s highway speeds) , it will drain easier and faster when warm. Try planning to do this work when coming back from work or after a trip to the store to avoid unnecessary driving.

There are two choices when draining the fluid. Either just from the oil pan or both from the oil pan and the torque converter. I highly recommend to drain from both so you can get rid of as much of the old oil as possible.

Torque converter mercedes 280ce
Torque converter, but no drain plug in sight. The engine has to be turned.

Get the car up on jack stands, at least in the front. Block the wheels and apply the parking brake. Put the Transmission in Neutral (N). Start by locating the torque converter at the front of the transmission. In 99% of the cases you will have to turn the engine so the torque converter drain plug is pointing straight down so you can access it. In order to turn the torque converter you will have to have the car in Neutral! Use a socket wrench to turn the crankshaft pulley bolt in front of the engine and rotate the engine CLOCKWISE. If you have a helper he or she can tell you when the torque converter drain plug is coming into sight. Otherwise you have to crank the engine a bit at a time.

Torque converter drain plug mercedes 280ce
Torque converter drain plug in position for draining.

When you turned the engine so you can see the drain plug, don’t drain the torque converter before you have started draining the oil pan.

Open the oil pan drain plug with a hex socket, be careful to avoid burning yourself on the warm oil suddenly coming out very fast! Do the last unscrewing by hand to catch the bolt so it doesn’t fall into the drain pan.

automatic transmission oil
Draining the transmission oil pan

When no more oil is coming out reinstall the drain plug so you won’t loose it. Now you can remove the torque converter bolt with the same procedure as with the pan. Reinstall when no oil is coming out of the torque converter.

Now it is time to remove the oil pan completely for installing the new filter. The pan is secured with 6 bolts. be careful to hold it horizontally when loosing the last bolts because the pan contains a lot of oil still. After the pan is removed, let the fluid drip for a while from the valve body before removing the filter. Cleanliness at this point is of importance, do not wipe off the oil coming out of the valve body or you might get dirt on the valve body which is quite bad. It is pretty hard to clean the valve body since oil is constantly coming slowly out of it underneath for days.

The filter is held by three Phillips screws, be careful to not loose these screws! Also be careful of even more fluid coming fast at you when removing the filter from the valve body.

With the filter gone, the transmission will keep on dripping slowly, I mean for days if left like this. So have the drain pan under at all times.

The quantity in the drain pain will determine how much oil you need to add later. Overfilling the automatic transmission is not good due to overpressure which may lead to excessive fluid leaks and foaming of the oil.

Inspection

Now it is time to inspect the oil pan for metal particles, and fluid quality. If there are no visible particles and the fluid has a fresh red color (slightly darker than new oil is normal) and there are no oil sludge at the bottom of your pan, your transmission is just fine and you have changed the fluid in time. Also use your smell, the fluid should have a sweet smell and not smell like excessive burnt oil. If  you find a very tiny amount of metal particles and no sludge in the pan, your transmission is probably fine too, but the mileage of the car is starting to show and you should consider finding a replacement transmission. If there are both particles and the oil is very sludgy and of dark color, then you probably have noticed some shifting problems already. Also the fluid change have been long overdue and the clutches are starting to wear out. Decide if you should change or service your transmission!

transmission oil pan w123
Inspection for metal particles and sludge in the oil pan. The oil here had a sweet smell and was only a bit darker red than new oil which is perfectly normal. In the picture it might look that it had some metal particles but it is simply some dirt that fell off from beneath the car and my sloppy handling before I managed to take the picture.

In my car, the pan was empty of particles and sludge, the oil also was very red and clear still after many years as seen in the picture, however I did not drive the car a lot either the last years since it has been in a “project state”. The shifting is super smooth and I have not encountered any shifting problems. These transmissions are usually rock solid and will outlast the car itself, only if however if the fluid have been changed at regular intervals.

Cleanliness

Clean the pan very throughly with some brake cleaner, wipe it completely dry with no fluffs from paper hanging inside it. Use your palm to feel it is super smooth and no dirt traces are left. Now you can also clean the surface of the transmission where it meets the seal, so you can expect a leak free gasket around the pan. Do not clean the valve body though, just leave it wet with fluid.

Cleaned transmission oil pan w123
Cleaned transmission oil pan and new gasket
Installation and Refill

Fit the new filter and apply the new gasket to the pan.

Install the oil pan and do not over tighten the bolts to not crush the gasket, make the rubber do the sealing and not brute force. Use a crisscrossing pattern like tightening the lug bolts to ensure evenly distributed gasket. If you are OCD then you should use a torque wrench.

Now time is due to measure the quantity of oil you extracted. A leak free automatic transmission will not use any oil and this is the quantity you should use. Pour in exactly this amount into the transmission through the filler tube, which also happens to be the fluid level checking tube. You have to use a narrow funnel to avoid spilling oil over your (hot) manifold which will make a lot of smoke and can cause fires.

Filling transmissio oil 280CE
Fill transmission oil through the filler/measurement tube. Use a funnel

Inspect under the transmission to see if there are any leaks. Then it is time to start the engine. Let it go for a few minutes before you start engaging all the gears one by one. Do this gear cycling  3-4 times with a minute in between so the fluid can distribute itself within the transmission.

gear lever mercedes w123 280CE

gear level mercedes w123 280ce
Cycle through the gears 3-4 times with staying in each gear for about a minute
Level measurement

After this is done, check the fluid level again with the car running in idle. It should be some below the maximum mark, also it can also be slightly under the minimum when the transmission is not warm. The level will usually be below minimum when the oil is cold even when the transmission is filled with exact fluid quantity. The transmission oil heats up slower and you need to go for a drive to get the right operating temperature, and the fluid will start expanding reaching to the recommended level. Check under the car again now while car is running to be sure there are no leaks. Then go out for a spin, drive at least 24km at normal cruising speeds. Do not race or do any high RPM of the car. When coming back after your drive the oil should have reached the right operating temperature. Remember to keep the car idling when checking the fluid level again. If the fluid level is still under the minimum mark, fill more ATF slowly and re check until it reaches between min and max level on the dipstick. If it is already this level, then you have filled the right amount and your car has probably not leaked a single drop between oil changes. Normally you have to refill some after this test drive so the level can reach the desired level between minimum and maximum, older cars have a habit of sweating slightly some oil through old gaskets.

If you over fill, so that the level is above maximum, do not panic! Then you have to suck out some fluid from the filler tube with some brake bleeder tool or similar oil extractor hose. It is critical that the level is between the max and the min mark, since little fluid can make gears slip and wear out the clutch material, and too much fluid can make to much pressure and your transmission might start leaking from gaskets and the oil can start foaming.

The only big difference here between engine oil and automatic transmission fluid, is that the transmission fluid has to be warm and the engine has to be running while checking the level. The engine oil can just be checked while parked and also when the engine is cold. If you happen to check your transmission fluid and forgot to turn on the engine, you would swear and rip your hair out since it looks like there is too much oil, then you turn on your engine, and while the fluid is still cold it looks like it is too little.

-> Correct automatic transmission oil measurement is when fluid is warm with engine at idle 🙂

Cheers, Robs out!

Author: robs

Passionate about older Mercedes Benz where driving, maintaining and learning new things is what keeps me doing this. Enjoying writing about cars and teaching others on the black arts of car maintenance. Have a major in space technology engineering and currently doing IT consulting for a living.

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