The W126 flex disks. Usually forgotten about until it's too late. And when it's too late, it's time to call a tow truck and hope much damage is not done.

The flex disk is a hard rubber composite part that connects the driveshaft to the tranny (the front flex disk) and the differential (rear flex disk). The rear one is easy to see especially. Just look where the drive shaft connect to the differentia. The front one will require you to get under the car directly and then it's easy to spot.

Now dont worry usually a good flex disk will last from 150,000-200,000 miles before even starting to wear out but every now and then it's best to take a peek at them. If they are cracked, missing bolts, falling apart or otherwise not looking right, then replace them when as soon as you can. Now, keep in mind that the life of the flex disk (especially the front one) depends on mainly a few factors. One, is the alignment of the drivetrain itself which is depended on the condition of the engine mounts and tranny mount If they collapse, then the drivetrain will become misaligned and basically "flex" the disk too much and cause premature wear and failure. Stress cracks start to delevlop, especially around the bolt holes, which is not good in the long run. Usually the first flex disk to be replaced and most often replaced is the front ones. Luckily the front one is not especially hard to replace as there is good amount of room to work with and it can be done without removing the whole driveshaft, which is a PITA job in itself. The tricky part of the job is getting the flex disk on properly, like most mercedes jobs the trick is not getting the part off but getting it back on. This is one of those jobs, the disk is so easy to remove that many people dont pay attention to the way the old disk was orientated or the bolt positions and stuff. Take heed, pay attention to that as you take off the old disk as it will save time and trouble when you put the new one on. The disk can only go on one way and only one proper way. In german, the part has the direction orientation written on the disk, as I saidn in German. If you can read German then it's simple, but if not, then it will be of little help. Luckily I spent three years in Germany and picked up just enough to get around. So basically the German words says "this side to the rear". And thus that is how you will orientate the disk. That's half he battle. The next is to make sure the holes are lined up properly as the bolts are different lengths, and remember to use all six holes and not just three or you'll be sorry. Common sense I know but seen it happen. The rear flex disk is same procedure but a little less room as you'l have the differential in your face. Same procedure though. Get under the car, unblt the three bolts to the driveshaft. The driveshaft will drop a little bit, then undo the bolts that secure the disk to the differential. and place the new disk in it's place in the same direction manner and bolt holes the old one came off with. Reconnect the bolts and you are done. Not hard at all but many shops will charge arm leg and if a dealership a couple of ears also to do that job.

Flex disks are not exspensive alone, usually around $45-$65 a piece from non-dealer source for OEM, which is what you want to aim for, OEM. The labor involved is straight forward BUT remember to pay attention to the bolt holes and orientation of the disk. It goes on only one way so it's really hard to get it wrong or things wont fit right.

What happens when a flex disk disenegrates on you. Well usually as it goes, the signs are there, especially for the front one. When you get in your car and start it up, it will start up normal. Then if you put the car in reverve which is usually the first gear people tend to go into when leaving an area, if will feel like a very rough shift. It will almost feel like the tranny that is shifting rough. Then it will do it again, when you shift into drive sometimes, usually not as harsh. Then when you get going at low speeds especially you feel a great deal of vibration as if the drive shaft is not balanced usually accomplied by a clunking sound from the undercarriage center console area. Yet, the tranny shifts through gears smoothly, but the vibration is there. Probably the flex disk on it's last leg. If ignored, it will eventually fail. what happens when it fails? Well depending on the speed, it basically breaks into pieces, causeing the driveshaft to become disconnected from the tranny. Now the force of this breaking can range from a small boom sound to the sound of an explosive device under yoiu car. At 80 mph a failing flex disk will usually cause a hole in the floor around the tranny bell housing area and since the airbag cpu module is right above that area in the center console it will probably rattle that thing and cause the airbags to deploy. Now at this point, you can kiss your drive shaft, at least that section, goodby as the the prongs on the driveshaft that connects to the flex disk is probably mangled, and the part of the tranny that connects to the flex disk might be mangled or not. So in worse case scenario of letting the front flex disk disenegrate is, the cost to weld the hole in the floor shut, airbag replacement, driveshaft replacement, flex disk replacement of course, and possible tranny parts replacement. $thousands of dollars in damage possible over a $50 part and some DIY time. Dont make that mistake. take it from me. Never heard about that flex disk until it was too late. Dont be that guy.

 

whew lucky.