W123 240D clutch doesn't disengage - PeachParts Mercedes-Benz Forum

spdrun · #1 06-22-2011, 09:13 PM

So I'm parked on a NYC street (need to move the car tomorrow for street sweeping!), I go to move the car. Press the clutch and the pedal DROPS TO THE FLOOR and stays there. Pull it back up, it springs back, but if I press it, it drops back down.

The clutch doesn't disengage at all, and it was completely fine when I drove the car a few days ago.

What could be going on and how do I troubleshoot this thing? Seems to be a hydraulic clutch, but where exactly is the master cyl located? I've tried pumping the thing -- worked on a Saab I had with a dicey master -- but no joy here.

leathermang · #2 06-22-2011, 09:17 PM

I think the brake master cylinder and that clutch master cylinder share fluid at some place.. Have read they must be bled together or something... there is stuff in the archives on this... I have never had to mess with mine...

junqueyardjim · #3 06-22-2011, 09:28 PM

That is correct, it is that screw topped reservoir in front of the vacuum power brake, that big black turkey on the firewall, drivers side. Anyway, get yourself a quart of DOT 3 brake fluid and fill it up. It might be so thirsty it will work without bleeding it. Try it. If it needs to be bled, on the clutch you can do it by yourself but you have to get under the center of the car. If you can get under the car and be safe, you need a small box end wrench, I think it is a 7mm. Anyway fill the reservoir to the top, get under there and open that screw on the bleed valve just outside the transmission on the passenger side. Open that bleed screw, let the brake or clutch fluid run for a minute or so, close the bleed screw and you should be good to go for a little while, because your clutch slave cylinder is leaking and it will soon all leak out again.

charmalu · #4 06-22-2011, 09:40 PM

The clutch master cylinder is under the dash bolted to the clutch/brake assembly. the reservor for the brake MC is the supply for the clutch MC.

If you look at the reservor, the side next to the fender, there is a nipple with a hose attached to it. the hose goes through the big grommet into the cabin area to the clutch MC.

The bottom of the clutch MC has a hydraulic line that comes out the lowerside to the floor, makes a right bend towards the go faster peddle. then exits through a hole into the engine compartment.
looking down under the brake MC you will see the metal line connected to a short rubber hose, which is part of the hydraulic line that runs over the top of the trans to the Slave Cylinder.

The right side of the transmission is the clutch slave cylinder.

To bleed the whole thing. the FSM states, to connect a hose from the R/F Brake Caliper, to the nipple on the Slave cylinder. then pump the brake peddle to cycle the fluid from the Brake reservor, through the Caliper, to the Slave Cyl. and up to the Clutch MC and to the Reservor.

You need to force the fluid from the bottom to the top to push out the air.

Now you could have a Clutch MC that suddenly started leaking or
the Slave Cyl. lift up the carpet on the drivers side to see if you have an oily mess under it. an/or check under the trans on the right side for a leak at the Slave Cyl.

Take a look at the rear of the Reservor to see if there is fluid in it. It is divided with a wall between the front half and the rear. the rear half if for the Brakes as well as for the Clutch.

This is a You Tube video explaining how it all works.
http://www.youtube.com/watch?v=wdf--suwqw0&playnext=1&list=PLE8DCFB0AB9C7498F
Charlie

spdrun · #5 06-22-2011, 11:07 PM

Update:

Fluid in the back of the tank was low, slightly below the level of the nipple where the clutch fluid hose connects. No joy on the clutch working after I topped it up -- feels like there's air in the system. At least this appears to be a hydraulic problem, not an issue with the clutch itself.

I moved it (clutchlessly!) to a parking spot across the street, which buys me a day to deal with it. I'll try to bleed it tomorrow.

A few questions:
(1) What size (inside diameter) Tygon hose should I grab to cross-connect the brake bleeder to the clutch slave nipple?
(2) Are all of the bleeder nipples 7mm (brake, clutch master, clutch slave) or do I need some other size of wrench as well?
(3) Should I use a one-way speed bleeding valve on the clutch master nipple?
(4) Any gotch'yas in removing the under-dash panel to get to the master cylinder?

Thanks!

babyjames · #6 06-23-2011, 01:00 AM

on the brake bleeder. Do some searching. Best of luck!

Jay.

Stevo · #7 06-23-2011, 10:35 AM

Quote:

Originally Posted by spdrun

Update:

Fluid in the back of the tank was low, slightly below the level of the nipple where the clutch fluid hose connects. No joy on the clutch working after I topped it up -- feels like there's air in the system. At least this appears to be a hydraulic problem, not an issue with the clutch itself.

I moved it (clutchlessly!) to a parking spot across the street, which buys me a day to deal with it. I'll try to bleed it tomorrow.

A few questions:
(1) What size (inside diameter) Tygon hose should I grab to cross-connect the brake bleeder to the clutch slave nipple?
(2) Are all of the bleeder nipples 7mm (brake, clutch master, clutch slave) or do I need some other size of wrench as well?
(3) Should I use a one-way speed bleeding valve on the clutch master nipple?
(4) Any gotch'yas in removing the under-dash panel to get to the master cylinder?

Thanks!

The bleed nipple on the clutch slave is 7mm and 9mm on the brake slave. If you want to save some grief get hose that fits over both nipples and use small hose clamps. put a box end wrench of the appropriate size on the nipples, then the hose and clamps. Open them and pump the clutch like crazy by hand, kneeling next too the open driver's door, crawl under, close the nipples. If you dont have clutch yet, repeat the operation. Sometimes you need to use some Teflon tape on the threads of the nipples. You dont need to get to the clutch master at this point, report back. good luck, you shouldn't have a problem.

charmalu · #8 06-23-2011, 11:28 AM

There is nothing to do with the clutch master with the bleeding process. no bleed nipples on it.

The fluid had to go some where. have you looked for a leak? did you look under the floor mat? No since bleeding the system if there is a leak, you will be right back to square one.

Did you look at the Video I posted? he uses a pump oil can in the demo. some have used it with success.

As Steve said to do with pumping the peddle and clamps on the hose to the nipples. you want an air tight seal. you don`t want to reintroduce air around the nipple threads.

One thing to think about. If the brake system and or the clutch system hasn`t been flushed out in many years, you will be just loosening up gunk and what ever and just circulating it through the clutch lines and the line to the R/F caliper.

Also if there isn`s a rubber cap on the bleed nipples. there will most likely be dirt and grit in the hole to get pushed into the system. make sure they are clean.

There is not a lot of room between the tunnel and the Slave cyl to work. can be a pita to work on. once you get through the frustrations of clutch bleeding, you will get a nice certificate from the Forum, "I Survived Clutch Bleeding 101".

Charlie

rs899 · #9 06-23-2011, 11:56 AM

Quote:

good luck, you shouldn't have a problem.

charmalu · #10 06-23-2011, 12:06 PM

Quote:

Originally Posted by rs899

Charlie

Stevo · #11 06-23-2011, 12:12 PM

I think most of the problems with clutch bleeding seem to be with the hose coming off, solved with the clamps (I use a 1/4 drive socket and extension to make adjusting the clamps easy, not a screw driver). Also leaking around the nipples, solved with teflon tape. I only needed to use tape once.

I believe Dot 4 is a more expensive but the recommended choice over Dot 3

rs899 · #12 06-23-2011, 12:21 PM

Quote:

I believe Dot 4 is a more expensive but the recommended choice over Dot 3

I would suggest buying a larger size , like a 55 gallon drum.

spdrun · #13 06-23-2011, 03:37 PM

Quote:

Originally Posted by Stevo

The bleed nipple on the clutch slave is 7mm and 9mm on the brake slave. If you want to save some grief get hose that fits over both nipples and use small hose clamps. put a box end wrench of the appropriate size on the nipples, then the hose and clamps. Open them and pump the clutch like crazy by hand, kneeling next too the open driver's door, crawl under, close the nipples. If you dont have clutch yet, repeat the operation. Sometimes you need to use some Teflon tape on the threads of the nipples. You dont need to get to the clutch master at this point, report back. good luck, you shouldn't have a problem.

Should I pump the CLUTCH or the BRAKE? I'd think that the point of the procedure was to use the brake system's pressure to force hydraulic liquid into the clutch system.

kerry · #14 06-23-2011, 03:56 PM

The clutch. If pumping the brakes put hydraulic pressure into the clutch system, you'd have a hell of a time keeping the clutch engaged when coming to a stop

Apart from the fluid reservoir the two systems are completely separate.

Stevo · #15 06-23-2011, 04:35 PM

Quote:

Originally Posted by spdrun

Should I pump the CLUTCH or the BRAKE? I'd think that the point of the procedure was to use the brake system's pressure to force hydraulic liquid into the clutch system.

Yes, pump the bejusus outa the BRAKES to purge the clutch system. you dont have to pump and do the opening and closing thing. Just pump, fast and furious then go under and close both nipples. Make sure the clutch pedal stays UP.