Failure of the Flex Disc on a 12

Failure of the Flex Disc on a 126 Body Mercedes

and how to repair the damage.

All parts used in these pictorials are available at the diesel parts for sale page

Failure of the Flex Disc on a 126 Body Mercedes

Safety and security tips:

Please remember to recycle all your used fluids at an appropriate recycling center. Be mindful to not spill or splash fluids on yourself, others or the ground. Also as a safety tip please remember anytime you are working on, around or under your car, to wear safety glasses and secure the car with wheel stops and approved jack stands!

One part on the Mercedes diesel autos that really get less than a second thought is the flex disc. In short the flex disc is the cousin to the u joint. It is the attachment point between the transmission and the drive shaft and the rear differential and the drive shaft. They are usually very robust, but like everything else they do and WILL fail, often at the most inconvenient time.

They are not hard to replace but since they are under the car, most people rarely check them. I always recommend checking them along with other items like CV boots, brake lines etc., at every oil change. If you see ANY cracking or wear areas on either the rubber or the metal potion, then I highly suggest replacing it. What happens if one fails while driving?

Glad you asked. Depending on your current speed, whether your are accelerating or not and whether the front or back flex disc fails will determine the damage.

The below pics are when the driver had just gotten on the highway and was going about 60mph. He was in a 1986 300sdl and unfortunately the front flex disc failed. Now the special note about the SDL models is that the airbag sensor is under the center console and your guessed it, the driveshaft hitting the transmission tunnel caused an air bag deployment to the unsuspecting driver. If it was not enough for the flex disc to fail, the air bag just added insult to the whole situation.

This is what a nice new flex disc looks like. The whole piece is basically a big rubber disc with 6 metal holes. They usually are very strong, but can and do fail. Check for cracking in between where the bolts go with a very strong light. Any cracking at all and it needs to be replaced.

Here is the font driveshaft where it use to connect to the flex disc. You can still see 2 of the bolts and collars still on the yoke. All three arms of the yoke are twisted like taffy. Huge moment forces were at work here to do this damage. The rear driveshaft was fine and only the front section will need to be replaced.

This is what the driveshaft will do to your car. It sliced a hole in the transmission tunnel, twisted the transmission side yoke and even torn one of the holes apart. Also it disintegrated the flex disc. destroyed the transmission mount, broke the aluminum mount that is ON the transmission and twisted the transmission cross member. Also do you see any transmission linkage??

Another shot showing the same damage. Look at the damage to the transmission mount.

Ahh here is that darn shift linkage. Yes that use to be straight and have clips and bushings. The linkage was twisted around the yoke. All this damage just from a little flex disc failing. Yes. The worst part is the mount that is made on the transmission was snapped off. So out comes the transmission? Well not really. Fortunately the rear of the MB transmission will come off without removing the whole thing. These transmissions have a bolted plate on the rear that contains the mount ear.

This is our donor transmisison that we will use to procure the rear plate from.

We want this rear plate because it has an un damaged transmission mounting ear.

We will need to remove the securing nut with the special Mercedes socket. Don't forget to remove the staking on the nut.

This socket cost about $39 and is the only way to remove the special nut holding the yolk on.

With the yolk gone we can remove the rear plate from our donor transmission.

With the plate removed we have a little cleaning to do. We will install a new output shaft seal before we bolt the plate on our transmission.

Front portion of the drive shaft was donated by a 560sel. I hope the diesel won't mind spinning a driveshaft that has been in a gasoline ride.

Here is a good example of a disc that was near failure. This disc actually came out of the donor car and was close to failure. Multiple small cracks spell impending disaster.

New yoke, seal and end plate installed. This yoke in the picture is not the one that will be used on the car since its the smaller 300d yoke.

Since the tranny came from a 300d the linkage was a little too short. Hand made extensions and bushing solves that problem.

Sheet metal patch has been secured and caulked in place. After the caulk dries the whole area will be undercoated.