The transmission in the Mercedes 126 is a very robust design, as one would expect. In its day, it was one of the best units available, which led Porsche to use it in the legendary 928. But as with all other systems on these cars, we should not let the reputation for longevity stop us from performing frequent services.
As a general rule of thumb, transmission fluid should be changed every 30,000 miles. Unlike modern cars, where the trend is to do away with service access ports altogether, these Mercedes transmissions allow us to drain not just the pan but also the torque converter. In other words, we can remove almost the entire fluid charge without resorting to such gimmicks as power flushes.
Whereas engine oil should be changed hot, transmission fluid is best changed when merely warm to protect the mechanic from a very real risk of scalding. The steps are as follows:
Drive the front of the car on to ramps or support on jack stands.
Make sure that the torque converter drain plug is accessible. Unless you have been very lucky, you will usually need to blip the engine on the starter to get the plug to come into view through the access hole. It can be "walked" the rest of the way with a large, flat-head screwdriver. It may also be necessary to remove the cross-member between the two front subframes ("dogbones"). The six retaining bolts have 17mm hex heads are are torqued to 45Nm.
Loosen the transmission pan drain plug with a 5mm hex socket, then allow the pan to drain into a suitable container. If possible, collect all the used fluid in one container and then measure it; this will help us dispense the right quantity when we refill.
When the pan has stopped draining, loosen the plug on the torque converter (same size as on the pan) and allow the torque converter to drain out. There is much more fluid here than in the pan.
When the two main flows have ceased, loosen the six bolts retaining the pan (13mm heads) and carefully remove the pan. Don't spill the remaining fluid all over yourself! Resist the temptation to wipe the innards with a shop towel: we don't want any pieces of lint in here.
Remove and replace the transmission filter, retained by Phillips-head screws.
Thoroughly clean out the pan with a lint-free cloth and replace the rubber gasket, ensuring a proper fit all the way around. Reinstall the pan, torquing the bolts gradually and evenly up to 10Nm. (The manual says 8Nm; a little more is fine. But please don't go overboard: these pans can be warped and they're not cheap.)
Replace both drain plugs, using new copper crush seals. Factory spec. is 14Nm, or about 10lbs/ft. Tight with a 3/8 drive works fine. If you removed the cross-member, replace it, being careful not to cross-thread the bolts.
(Optional) Crack open the trans cooler lines at the radiator (17mm wrench) and drain the small amount of fluid from this area. Replace the lines if their condition is dubious; they are not expensive. Retighten all connections.
Using a fine-mesh filter, add four quarts of fresh Dexron-Mercon transmission fluid (ATF) through the dipstick tube. If you can afford it, use synthetic fluid: it helps keep temperatures down by reducing internal friction, and heat is the number one enemy of automatic transmissions.
Start the engine and slowly add another three quarts.
Back the car off the ramps, run it through all the gears, pausing for a couple of seconds between each gear, then check the fluid level. You do not want to be over the minimum mark at this stage. Transmission fluid expands greatly when hot; the reference marks on the dipstick are calibrated for hot fluid. So it is o.k. to be half-an-inch or so below that mark while the fluid is cold.
Drive the car for about 20 minutes (yes, at least that much) to warm the fluid thoroughly. Transmission fluid takes longer to warm up than the engine oil or coolant and requires the friction of actual driving to get up to temperature. Check the fluid again on level ground. Make sure you don't leave any lint on the dipstick before replacing it in the tube. Any level between the marks is fine, but under no circumstances do we want to exceed the maximum level. If you need to add fluid, do so in very small increments and check again.
If you used synthetic fluid, you may notice that the shifts have become noticeably firmer. Compensate, if needed, by adjusting the modulator valve on the driver's side of the transmission. Turn the little key counter-clockwise until the shift quality is to your liking. Bear in mind that too much slip is bad for the internal clutches.
