Flex disk replacement questions
 

Just completed a valve job and the car has a lot more power now. This has highlighted another issue - a hard "thunK" when shifting, especially into high gear. The result on shift is distinctly different depending on the situation:

1) Under moderate or heavy acceleration the shift is crisp but no big thunk is apparent.
2) Under very light acceleration, or if you are easing out of the throttle as the trans shifts (like in traffic) you get a BIG thunk from the rear area.
3) similar noise when trans is shifting down but thunk is not as apparent.
4) The noise does not seem to eminate from the front of the driveshaft (tail of the trans), it seems to be in the back near the differential.

A few have advised me this is an indication of the need to replace the Flex disk(s). Do you usually need to replace front, rear, or both ?

Another thing that has been suggested is differential mounts. If these are worn, would there be any other characteristics to look for? Car seems fine other than this hard "thunk" on shifting.

Sure sounds like a tell tale indication of the rear flex disk being bad. I would get under the car and take a look at the rear disk, It is likely showing cracks or at least some movement or slack. Sometimes they look ok from the outside but the rubber is weaka nd too flexible allowing the movement that causes the "clunk" noise.
I would just start out doing the rear disk, it is easy and quick. The front disk is a lot more involved and almost requires a hoist, it can be done without, but not fun at all. The rear is usually the culprit and for $60 and a few hours you can find out. If you really want to be sure and you are going to pay an indy shop to do the work anyhow, have both flex disks and the center drive shaft bearing and support replaced, it will eliminate all noise and you will onlyhave to have it at the shop once. While they are under there, replace all the rubber mounts holding the exhaust up, they get neglected and rot. For all of them you are looking at maybe $20 and they can cause vibration noise and rattles when they go bad. Make sure to have paint marks made on the drive shaft sections prior to seperating the sections so that it can be put back the same way and this will eliminate future vibrations, the drive shaft was balanced at the factory and is intended to be mated up in a certain way as to keep the balance, if it is put back too far off from the original position, you will get vibrations and not be happy.
Good luck

Get the flex disks checked ASAP, and f they are bad replace them immediately. You do not want these to fail while driving.
Ed

a good mechanic can tell which one or both are bad, a failed flex disk will let the drive shaft whip around under the car = bad.

Christopher, have you replaced a front flex disk yourself?

If so, please post more details on the procedure.

I couldn't get the bolts loose and decided to try again another day. No room!

Thanks!

I have replaced a front flex disk and I never want to do it again. I did it with no lift or hoist and it was a poor idea. Very difficult to get leverage and space to work. Must pull the rear section of the drive shaft and the center bearing and support to really get at it very well. That is why it is the likely choice to do both flex disks and the center bearig and support at the same time. The rear disk is the one most likey to go bad and is easiest to change, not too bad. the front is a different story, not fun, and not easy. If you can stomach it, have a shop do it for you, I dont like paying them either, but it is not a fun job at all and like usual, I was not even satisfied with myself when I was done, it was just a pain. Sorry no better guidance

I do need to check mine on the W126 (a 420SEL), but while servicing the trans on the W107 (a 300SL), I noticed the front disk was in bad shape, so I got on that ASAP while under there. I did not feel it was a too awful of a job. Of course it's a different chassis and I have no first hand expericence on a W126 flex disk replacement, but I would think they would be similar.

No hoist or lift in my shop either, but a good quality floor jack (or two) and HD jackstands are a must for any underside work. And NEVER use just a jack alone.....always use the jackstands supporting the weight.

Cliff

Front flex disk is not that hard in the car in question, maybe 2-3 hours at a leisurely pace. The front is far more likely to go bad than the rear. Rears rarely go bad - maybe twice the life of the front.

Thank you everyone. That's what I needed to know. Interesting that there are two different opinions as to which disk is the more likely to go bad. I do believe that, in my case, it is most likely the rear one. I will probably get under there and get the rear changed right away, and take a look at the front and the bearing etc.

The front one is smaller and is subjected to a lot more heat. It will fail sooner.

I helped someone replace the transmission mount once. We had to disconnect the driveshaft and the car was on a lift in a heated garage with every tool imaginable at hand. I wouldn't want to attempt this without a lift!

It's also subject to a lot more torque.

You could probably make this into a difficult job if you were looking for a big challenge but it isn't a very difficult job on the car in question - 1990 300E. The left rear wheel needs to be off the ground, e-brake released, so you can spin the driveshaft and get at each bolt. And you don't need to disassemble the driveshaft - lowering the exhaust a bit allows you to pull down the driveshaft and wiggle the disk out at a 45 degree angle. A lift is always nice but not necessary.

You know, there is something about this situation that keeps bugging me. The car shifted fine,no thunk, before we pulled and refreshed the head. Also, I had the trans serviced the day before we pulled the head and it got a clean bill of health, and was shifting fine after that also. I first noticed the "thunk" when shifting on my way home after we reinstalled the head. The head needed a valve job because #3 cyl had a chipped valve and #4 and #6 were blowing by the intake valve a bit. This made me wonder "What changed because of the head revamp that might affect the shift?"

The answer I keep coming up with is vacuum. I am wondering if the PO had someone adjust the modulator to "fix" a bad shift that was being caused by a poor vacuum condition caused by the valve situation in the head. If this has been done, I believe that now that the head has been rebuilt, vacuum has been restored and the modulator valve would now be set too high, causing the overly hard shift.

A new flex disk is on the way, but I keep thinking this might be a modulator valve adjustment. Anyone with experience or knowledge of this system care to comment ?

I don't think the valve problems would have created a significant vacuum deficit.

At any rate, the modulator is easy to adjust. Give it a try.