Andy_M

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.

White_Knuckles

Your flex discs can be easily inspected. The round rubber devils are where U-joints are used at each end of the driveline. The rear F-disc is exposed where you can look for cracking between the 8-9 mounting holes. Cracks are a sign it's going but stretched holes or splits would be - don't drive it.

The engine end may be harder to inspect as often covered by heat shields. Replacement is tricky and sometimes requires the exhaust pipe dropped. There have been reports that the front can let go and basically shred the tunnel and other $$ parts with it. Not good.

You really can't tell by sound. If one end is shot it can "telegraph" the thud along the shafts entire length.

neanderthal

if the front guibo grenades, it will activate your airbag. definitely get it checked out and replaced as necessary.

squablow

I did this on my w115 diesel, it was a really simple swap. Just had to do the front one. Mine was tore in half and was barely holding on, I'm glad I caught it when I did. Pretty unsafe if it breaks. I think I paid $50 for the part.

Andy_M

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 ?

hineywineries

I really doubt that the flex disc is your problem. I would have looked at the glycol filled engine mounts or the tranny mount until you sad it wasn't there before. The modulator may need adjustment or you are lacking a vacuum signal to the tranny. Check all your vacuum connections again.
Peter

TerryA

1st, Check the flex discs and change if necessary. It is an easy job. Just make sure you put them in the same way they come out. Don't rotate anything while the driveshaft is disconnected.

2nd, It sounds like the transmission is not shifting correctly. I would have competent MB transmission guy check it out. A sticky valve body will cause the clunk you are talking about.

3rd, When you changed the head you most likly disconnected the cable that is connected to the throttle. It affects the shift points. Check to see if it was replaced and adjusted correctly. A transmission guy would know this.

Good Luck

Andy_M

No, the intake and all throttle cable connections were left in place when the head was removed/replaced.

hineywineries

Any Final word???

Andy_M

Flex disks were not the problem here.

When I got underneath to adjust the modulator, I noticed that the vacuum line was not firmly plugged in to the modulator. It was resting against the connection and not moving at all - it looked like it was plugged in, but it wasn't. Fixed that and no more hard shifts.

CharlestonN

The bushing can also cause that, i think thats whats up with mine