When these transmissions came out, Mercedes marketed them as sealed for life. Turns out they need transmission oil change after all. Of course this is the case when it is vented with the atmosphere and components have wear over time. If you have passed 100.000 km which is very likely for cars of this age, then go ahead of changing the transmission fluid ASAP. Also if your transmission starts feeling more jerky than usual, then change your transmission fluid ASAP.
If you notice late shifting or if it hangs excessively on gears during revs, or even slipping, then it is possible something else that is the issue. There are so many stories around on the internet that changing gear fluid will ruin your car. This is utter bullshit since changing transmission fluid must be done as preventative maintenance. There are some cases where changing fluid will not help save your transmission, because excessive wear and damage have already occurred. Do you see the difference?
Weird shifting and limp mode can often be the result of a failing conductor plate in the 722.6 transmission. This a known failure item and can be changed by yourself. The conductor plate is the electrical connection inside the transmission, which connects the signals from the computer with solenoids that steers the valve body shifting mechanism. Replacing the conductor plate is not covered in this article.
Note on slipping gears
Worst scenario of all is when the transmission is slipping on the gears, which is most likely due to worn clutches. If that is the case the transmission would need a rebuild with new clutch plates. Changing transmission fluid will not fix this issue, and the slipping most likely will get worse afterwards, since the new oil will not contain transmission debris and the clutches will slip easier. Keep in mind the oil change did not ruin the transmission, the transmission was worn out in the first place and total transmission failure is just around the corner.
When changing fluid, you will need more stuff than just the oil. Mercedes used a special automatic transmission oil in blend in the 90s, which was supposed to be sealed for life. Turned out that this was not the case and you cannot get this even now, a modern special AFT blend is the alternative and has perhaps better qualities than the original blend. Make sure you get an ATF which is supported by the 722.6 5-speed, which is an ATF with MB236.14 quality. Do not use any other ATF. There are several other manufacturers other than Mercedes for this type, so look around for a good offer. The modern ATF is usually red in color and the old Benz oil was green/yellow. So it is easy to spot the difference if there is still the original oil in your transmission.
The required quantity for the S600 is 9.1L which is ridiculous! Everything is ridiculous with the flagship Mercedes. It will be less if you do not drain the torque converter, but I highly recommend you drain the entire system when doin an oil change. The smaller engines have less transmission oil capacity, so make sure you check your specific model before hoarding ATF.
You will need additionally a new filter, a new oil pan gasket, new drain plugs and copper washers, new electrical plug and gaskets and bolts. So it adds up!
Therefore I recommend to get a transmission fluid kit which contains all these things. Meyle has one great such kit, which is not too expensive either. I highly recommend it! https://www.meyle.com/en/passenger-cars/drive-components/oil-change-kits/ It contains 7L of ATF MB236.14, filter, drain plugs and washers, gaskets, connector plug, a drain pan magnet, fluid cap tab, drain pan bolts. If you have the S600, remember to get another 3 extra bottles of ATF when ordering the supplies. The S400 also requires more than 7L, it needs 7,7L. All other models require less.
Since Mercedes made this as “sealed for life” transmission, there is no dipstick in the engine bay. And the dipstick tube is sealed with a red locking tab. This tab breaks when you try to remove it. These can be bought as replacement or it comes with most of the transmission filter kits. So you will need to buy a dipstick in order to be certain of the fluid level after refilling. Get a cheap disptick on Ebay, there are loads of options, but make sure it is long enough. It has to be at least 1200mm long for the 722.6 if you have the M120 engine.
Aftermarket versions are totally fine here, do not throw your money away on a Mercedes one when you can get literally the same function for a fraction of the price.
You can try to skip the whole dipstick tube and just fill in the amount you drained, but it should only be done if you are 100% certain that the level is correct. Usually these transmissions tend to have leaked a bit over time through old gaskets after 20+ years, so I would not recommend it.
- Dipstick, must be 1200 mm in length at mininum
- Hex socket, 5mm
- 27mm socket for M120 engine crank pulley
- Torx socket TX30, TX25
- Torque wrench with low scale (5-25Nm)
- Large drain pan
- Narrow funnel
- Rags/lint free paper
- Brake cleaner
Changing the transmission oil for the 722.6 is a multi-step procedure and is a bit more advanced than changing the motor oil. The procedure is divided into a few sections below.
Warming up the transmission
Start with warming up the transmission to operating temperature, this is done by driving it before you start the job. So make sure you prepare everything before so you can jump straight into the job. 5-10 minutes driving at high speeds will be enough. Then with the car running and on a flat surface and standing still, go through all the gear levels with the gear shifter 3-5 times leaving the stick in each level a few seconds. Then put it in park still with the engine running, open the hood and pry off the red plastic tab on the dipstick to open it.
Now measure the fluid level with the dipstick. The way to read the dipstick is dependant on the dipstick itself, however there are two levels, one for cold and one for warm transmissions. Use the upper warm level and see how much oil is left in the transmission. Mine was quite low, certainly above the cold level but lower than the minimum warm level. Measuring before can make an indication on how much oil you need to add and if it has leaked any oil.
The main reason for warming up the transmission before the oil change is for the oil to drain much easier, since it has to pass through the labyrinth in the valve body gallery. Now raise the front of your vehicle, either with jack stands, or drive it up onto ramps. Shut off the car and put it in Neutral and make sure the handbrake is on. You want the gear in Neutral position, so we later can turn over the engine to locate the torque converter drain plug.
Draining the old fluid
Have a clean drip pan available. Keep it clean so you can spot debris or transmission material in the old oil after you have drained. This will tell you about the condition of your transmission. Have the drip pan completely empty so you can later measure the drained quantity. This will make it much easier know how much oil you must add later. Draining the oil is done in two steps, first the oil drain plug from the oil pan, then draining the torque converter.
Place the drain pan under the transmission oil pan. Use a hex socket to open the drain plug. Here you might have issues actually loosening the drain plug, if it has never been opened before. This bolt can seize incredibly hard. Mine got rounded off completely and I needed a bolt extractor tool to get it out. Thankfully a new plug comes with the transmission oil change kit.
Be ready for the oil to drain out with immense speed. Be careful not to burn your hands on the warm oil. Already now you can see if the oil has been changed before and within reasonable time. My oil was very black and brown and for sure never changed since the car was new. The evidence for that will come later below.
The oil will drain for quite a long time so take a 20 minutes coffee break. When you come back the oil should only slightly drip out. Reinsert the plug so the dripping will stop for now.
draining the Torque converter
This is a bit more tricky to drain. It is certain that you will not find the drain plug lined up for draining. So to locate it, you will need to turn over the engine by hand. Therefore the transmission must be placed in Neutral position! The torque convert bolt should be accessible at the front end of the transmission through a small access hole.
The engine can be turned over by hand by inserting a socket into the crank pulley and rotating it clockwise (looking towards the engine from the front of the car, all cars have the rotation of the engine clockwise). First you need to take of the lower front plastic panel for access, the screws are easily found.
If you have the M120 engine, grab a 27mm socket and a large socket wrench for leverage. The engine can be a bit hard to turn over due to the vacuum in the cylinders. See clockwise arrow direction of the lever movement on the picture below.
Get a helper to look for the drain plug, this is preferable. If you are doing it by yourself , you need to constantly be checking between turning the engine slightly. This might take a while if you are unlucky with the position of the drain plug. The procedure can be sweaty, but a longer lever will make it easier. Do this procedure until the torque convert bolt is showing up and lined up straight over the access hole.
Now comes a tricky part. You need to loosen this plug without loosing it inside the transmission bell housing and without showering yourself in transmission fluid. There is a lot of fluid here and it will shoot out with great force. Make sure you have the drain pan lined up under the transmission. Carefully loosen the bolt and when its becoming loose, be ready to lower the bolt straight down and out of the way like a ninja. If you end up loosening the bolt into the drip pan it is better than loosing it inside the bell housing.
There is a lot of oil in the converter and you will have to wait a bit until the dripping stops. When the dripping stops. Replace the old plug with a new one and a new copper washer in the transmission filter kit. This will minimize chances for leaks and is recommended. The plug must not be over torqued since the torque converter is fragile. Torque this plug to 10nm, which is quite a lot for this size bolts. Much more force and it will strip for sure.
Removing oil pan
Now it is time to remove the transmission oil pan. This can be a bit messy, so do not wear your nicest clothes. It is held in place with six Torx bolts. You also have to remove the bolt securing the wire on the right front corner. Undo all of the bolts then leave a couple in the corners. Now use one hand to support the pan while you undo the last two bolts and gently lower the pan to the floor. The pan will contain a lot of transmission fluid trust me.
With the transmission pan on the ground, you can now inspect it for particles, shavings etc in the remaining oil. This really shows the health of the transmission. My pan looked alright, but there was noticeable sludge at the bottom. There were no magnet either. Probably the factory oil. The next clue in the next section.
Replacing oil filter
Now it is time to replace the oil filter. Just pull the old one off straight down and notice the orientation, there is a plastic tab which goes into a slot. The old oil filter might have a date on it. My old filter was from 1995, meaning it never have been changed since the car was sold new in 1996. So this explains the gunky oil and harsher shifting of the transmission after 100.000 km and 25 years. Unbelievable that it never was changed earlier, probably due to the low mileage.
The little plastic tab on the filter goes in the slot shown below.
It is advised not to touch the valve body too much to get dirt on it, but you can inspect it and wipe it off slightly with a lint free cloth. However the transmission will continue to leak so maybe just leave it alone. If you need to change the conductor plate, then the whole valve body needs to be removed. Utter cleanliness needs to be followed.
Now take the new oil filter and place it under the valve body, make sure the tab aligns in the slot and simply push the oil filter into the hole. Make a hard slap to the oil filter where the oil filter neck goes into the valve body. It will snap in place. The filter might feel a bit wobbly, this is normal, but make sure it is pushed all the way in by smacking it.
Replacing transmission connector housing and gaskets
Next step is to replace the connector sealant inlet, which is located just above the valve body on the right corner of the transmission. The oil change kit comes with a new connector inlet body and new gaskets. The old ones tend to get leaky due to old gaskets and the plastic brittle due to years of experiencing heat.
The way you loosen the connector, is to flip the tab on the outer plastic ring downwards. This will free the connector which will make access to the connector plug. Just pull out on the cable and the cable should come loose.
Study the new connector housing and you see there is a little bolt in the middle of it. The old connector plug housing has to come out by loosening this bolt. Use a long 7mm socket and a flexible extension in order to get it out. It should not sit very tight.
With the bolt loosened you can just pull out the connector housing. It might be a bit stuck if the old gaskets are gripping, but just pull and it should come out. Inspect the connector pins and clean up any dirt and oil in the connector hole before replacing the new housing.
Study the orientation of the connector pins so you will be able to get the connector housing in the right orientation. Pop the new connector housing in and make absolutely sure you are lined up before pushing it in. Bending connector pins will be a lengthy procedure where you will need to replace the conduction plate. Tighten the bolt just enough to be snug, do not overtighten here otherwise you will break the plastic.
Now just plug plug in the transmission wire and tighten the cable with pushing the connector tab upwards.
Replacing oil pan gasket
Clean the transmission oil pan thoroughly and throw away the old gasket.
Place the new gasket over the cleaned transmission and remember to put in the new magnet if your transmission oil pan did not have one already. If the old one is cracked or have shavings in it, just replace it. Also remember to replace the old drain plug and copper washer.
The transmission oil change kit comes with new transmission oil pan bolts. Preferably use these. There are 6 torx bolts to secure the pan. Along with aluminium spacers which you should now clean. Also clean the entire surface where the gasket will sit on the transmission to ensure a proper seal.
Hand tighten all bolts before torquing them down. Use a small low scale torque wrench which you know is good. The drain plug is torqued to 14 Nm and the pan bolts are only torqued to 8 Nm. Go around them a few times, since tightening adjacent bolt will loosen the one next to it.
Adding new transmission fluid
First see how much fluid you have drained, pour the old oil into one used oil container or similar so you can measure the level. This is especially important when you have not drained the torque converter, since you will not get all the oil out. To be precise, you will never get all of the oil out. And fluid level is more important than with engine oil. Keep in mind that you might add more id the fluid was a bit low before you drained the transmission. As was the case with mine.
When you know how much quantity you can add. Start by pouring the required quantity into the dip stick hole, use a small funnel to avoid spills, also pour slowly since it will use some time to get down the long dipstick tube. Avoid spilling since it it will not be good for your manifolds to get oil on them.
In order to check the level the engine must be running. Remember to put the dipstick lid back on, but do not place the locking tab just yet. Let the fluid run down into the tube for a few minutes before doing anything else.
Warm up, test drive, fluid level check and inspecting for leaks
Before going for a drive. Lower the car back to level. Then start the engine and start cycling the transmission through the gears with the leaver slowly. Do this 3-5 times with the lever in the different gears a few seconds. Then with the car still running, place it in park. The transmission is now still cold, but initial measurement must be done before driving off.
To measure the cold level, you will need to insert and reinsert and clean off the dipstick a few times before you can get an accurate reading. Note that the level should be not higher than the cold mark. Also note that measurement while the transmission is cold is more inaccurate than when warm. If the level is way over the cold mark, you must suck up some fluid since it is overfilled. IF OK, then check under the car that no transmission is leaking out. Take with some extra transmission fluid, the funnel and rags. Then go for a spin.
When you have been driving with highway speeds (>80km/h) for about 10 minutes, find a gas station or similar with a level surface. Do not turn off the engine and cycle through the gears again with the lever. Set the gear in Park. Then measure with the dipstick, wipe it clean reinsert and measure until you get a clear reading. If the oil level is now below warm marking, pour in only 100 ml at a time before measuring again. Cycling through the gears is necessary between each re-measurement and always keep the engine on . Repeat the process until the level is in between minimum and maximum at the warm level on the dipstick.
Finally you can install the little red tab so it can secure the dipstick tube cap.
Cheers! Robs out!