PeachParts Mercedes-Benz Forum - View Single Post - I was hoping one of the MB techs had experienced this

Greg in Oz · #5 11-17-2003, 06:24 PM

On the weekend I investigated my 190E clutch problem further.

Firstly, using the measuring gauge tool as depicted in various workshop manuals, I again checked for clutch plate wear. With the tool inserted in the slot at the slave cylinder it pushed in such that the notches on the tool were not visible. According to my manuals this suggests that the clutch is NOT worn out.

Next I connected a "one man" bleeder (ie. bleeder hose with one way valve) to the slave cylinder bleed nipple. I then opened the bleeder nipple. Some residual pressure escaped immediately. My trusty assistant (aka wife) then pushed the clutch pedal ("is that the one on the left?") while I observed. What came out could best be described as resembling well used diesel engine oil! It was black. After much pumping of the clutch pedal and topping up of the reservoir, we eventually had clean fluid flowing and all the rubbish out of the system. Whilst the car had always been serviced at either authorised MB dealers or reputable independants, it was obviously all too difficult to flush the clutch hydraulics while doing the brakes. This had proved that it was at least possible to draw fresh fluid from the reservoir into the master cylinder, ruling out any blockage in the hose between the two. Again, I checked for clutch wear with the measuring gauge (since I now knew there was no residual pressure possibly holding the slave cylinder piston rod partialy extended). Again, the tool indicated the clutch to be OK.

At this point I decided to try a test drive. Immediately I noticed the clutch engagement point was much lower in the pedal travel, where I would expect it to be. After driving for a short time the clutch engagement point was getting higher in the pedal travel. After around half an hour with everything hot (it was a hot day at around 35C) it was possible to get clutch slip by barely touching the pedal.

With the car home again, I opened the slave cylinder bleed nipple to find the system under pressure. Obviously, the fluid could not return via the master cylinder to the reservoir to relieve pressure build-up due to expansion with temperature. This pressure is what is causing the slave cylinder to hold the clutch partially released.

The following morning, with everything cool again, I decided to try pushing fluid back to the reservoir via the master cylinder. I connected a snug-fitting vinyl hose to the right front brake calliper, and had my wife press the brake pedal ("that's the one in the middle, right?") while I opened the bleed nipple to fill the hose with fluid. I then attached the other end of the hose to the bleed nipple on the clutch slave cylinder. With both bleed nipples open, I asked my wife to start pressing the brake pedal. The hose blew off the nipple at the slave cylinder with great force. Again we attempted the same procedure while I held the hose firmly at each end. Fluid escaped around the bleed nipples but it was not possible to get fluid to flow back to the reservoir via the master cylinder. At this point I decided I could not do much more short of dismantling things. I again bled the clutch hydraulics and also did all the brakes after a brake fluid change.

My conclusion from all of this is that fluid can be drawn from the reservoir as required, but cannot return as is necessary due to expansion. Earlier systems with an adjustment at the master cylinder push rod would allow the master cylinder piston to be adjusted so as to allow the return of fluid to the reservoir with the pedal in the up position. In the absence of any such adjustment on my later model car (as postings by others have verified), I can only assume a fault with the master cylinder.

My next step will be to remove the master cylinder and dismantle it to try to find the cause. I have checked the price and availability of one should I need to replace it. This may be some weeks away yet as my father's '93 E320 coupe (M104) water pump is becoming more urgent and will have to take priority over my 190E which is at least still driveable. I am mindful though, that with pressure remaining in the clutch hydraulics that the clutch release bearing will be doing a lot of work it was not intended to do. I must also take care not to allow the clutch to slip when things get hot either.

Thanks to all who have posted replies. I will keep you informed of any developments as they occur.