Ur MB hydraulic clutch adjustment

[AdrianH]

Ok thanks. I think the first thing to do is measure the slave rod, then go from there. Pointless trying to adjust that out if it's wrong.

[BackintheFold]

Originally Posted by AdrianH

Ok thanks. I think the first thing to do is measure the slave rod, then go from there. Pointless trying to adjust that out if it's wrong.

Yes, that was the conclusion I came to when thinking mine through. Start with checking the slave rod length, to make sure the slave cylinder can fit in to the gearbox housing without pushing the clutch release lever in the box, then adjust Dave and Stuart's rods until the pedal sits at rest on the rubber bung on the pedal box without activating the master cylinder.

I think that's all you can do with the MB-from what i can tell the biting point is determined by the wear of the clutch. The adjustment is so that the parts all line up-so the slave rod sits in the slave cup and against the release bearing but not exerting pressure on it (EDIT -Not true. There is a spring inside the slave that exerts a considerable force on the piston, such that is can't be removed and replaced easily), and the master rod sits in the master cup with the clutch pedal resting on the rubber bung, so the re is no pressure activating the master cylinder.


Happy to be corrected, I will be doing this when it is warmer...

[BackintheFold]

Originally Posted by BackintheFold

Of course, adjusting the slave rod length (should be 87.5mm) and

Can someone confirm this length? Somebody previously let me know the length but i can't find the message now.
I recall that it was 87 or 87.5mm, but confirmation would be good-and apologies to the person who let me know before if they are on this site!

EDIT: I'm pretty sure the length is 87 or 87.5mm
In practice, that's not going to make a difference and the alternatives are probably 50mm/65mm/105mm so easily distinguishable.

[BackintheFold]

I’ll be pulling the slave out on Friday to check the rod’s length. If anyone has a genuine slave to hand, please can you measure the distance from the centre of the mounting hole to the tip of the rod when the rod is in the resting/extended position? I’m not intending to remove the hydraulics, just remove the slave to check the rod length.
Thanks

[BackintheFold]

Can anyone confirm from actual experience how the clutch wears on an MB in terms of pedal travel?
Logically, I would think that when everything is new, the biting point will be at the top end of travel-that is, you’d hardly have to press the pedal at all for it to work.
Then, as the clutch wears, you’d need to press further and further on the pedal to separate the clutch plates, so when the pedal can’t be pushed any further down to the floor to separate the plates, THAT is when you’d need to change the clutch.

The alternative is the opposite; a new clutch needs you to press the pedal almost to the floor to separate the plates and as it wears the clutch can be separated with the pedal almost at rest.

In either case, a slipping clutch can occur with the pedal not being pressed at all.

Thoughts?

[BackintheFold]

If anyone has a clutch slave rod, can you confirm that if mine is too long I will be able to cut it down with an angle grinder, and use grinder to smooth off one end, then simply replace it?
If it has a plastic end cover, I suppose I don’t have to worry too much about that end being smooth!

Alternatively,Anyone with an 87-88mm long spare slave rod please let me know if you want to sell it. If I’m taking the slave cylinder out it may be easier to have spare ready to drop in.

[BackintheFold]

Took slave out today. Took 5 minutes. Measured total length of rod and plastic end. 88mm so that is now discounted as the source of my one off slip.

Trying to get it back in I found the slave piston was now out against the circlip holding it in the cylinder. I could not force it back in and then rain stopped play.

Tomorrow I shall be opening the bleed valve and trying to refit it without the hydraulic back-pressure. Failing that I’ll take it off the hose and fit it, then refit the hose.

What joy!

The next port of call will be trying to adjust the pedal bite point. If that is unreachable I’m going to drive and see if it slips again. If not, I’ll shelve further work.

[BackintheFold]

Took slave out in 5 minutes.
Measured cylinder and rod and many thanks to Dave for providing dimensions of the Sachs slave.
The length of the rod can be different-it depends on the size of the slave and the cup where the rod sits inside the cylinder.

Having found nothing wrong with the slave I tried to put it back. With the spring inside it pushing the piston out, coupled with the fluid in the system and the awkward positioning this proved impossible.

Rather than drain the system and replace the slave empty, I’ve taken Dave’s advice to fit a new sachs slave. This should remove the slave from my issue. Also, with new fluid and bleeding it will remove that possible cause.


Out of interest, this is my understanding of the MB hydraulic clutch:

The slave has a spring in it that pushes the rod out to make contact with the clutch lever inside the bellhousing. The length of the rod is immaterial: you have to take account of the dimensions of the slave cylinder too (specifically, their length and the depth of the cup the rod sits in), and aftermarket ones can vary.

Tractor Dave has used sachs slaves (ref 6283 001 031) and reports no problems. I had what was receipted to be a Pagid, but there were no markings on it and it could easily have been another brand.) The AUDI part number is 4a0 721 261. I got one for £63 off ebay. Autodoc have been cheaper in the past.

The slave rod is pressed against the clutch release lever. This pushes the piston in the slave back to "a position". the fluid is then stable in the hoses back up to the master.

The master has a piston too. This is connected by way of a pull rod on a lever fitted to the top of the clutch pedal. This can be adjusted for length.
The lever then attaches to a push rod that goes to the master. This should be adjusted so that the rod sits making contact with the cup in the cylinder. The other end is attached to the lever. This push rod can be adjusted.
These two rods must be adjusted in sync. They will alter the position where the clutch pedal starts to have effect on the system. However, the clutch pedal rests against a rubber stop. Thus correct adjustment is such that the clutch pedal will start to have affect immediately that it leaves the stop. This gives the greatest opportunity for the full travel of the pedal and master to push the fluid through the system and operate the clutch. With old, compressible fluid and worn cylinders this is best.

The alternative is that the master push rod is loose in the cup. Too loose and it may come out of the master cylinder. With the pedal at rest, you could adjust the rod right out of the master cylinder. That would also mean the pedal could travel (say) 2 cm without having any effect on the fluid.

As the clutch wears, the rod in the slave will move as the spring keeps it in contact with the cup and release lever. The fluid should be at rest with no clutch pedal application. As the clutch wears, a small amount of fluid will enter the system to take up slack.

I can only think that either
(1) the slave on mine was incorrect-either the spring pressure or the position it placed the rod in.
or
(2) the fluid is bad. It was only two years old at the time of the single slipping that took place on the hottest day that I drove it last year. It may have expanded in the heat.

(3) EDIT-Or somehow extra fluid got in to the circuit, resulting in the slave cylinder being pushed out, despite being supposedly "at rest".

Adjusting the master rods looks like it will require a lot of swearing. I am fitting a new slave and bleeding, then test driving it. Then I may try adjusting the push rod. There is probably 1 or 2 mm of play in it, but it is hard to tell.

There is no adjustment of a properly operating system, apart from using the master rods to get the pedal to operate correctly, which is with the pedal at rest on the stop not exerting any pressure in the system, and starting to operate the system as soon as pressure is applied to the clutch pedal.

[BackintheFold]

Slave replaced today. What an ass of a job. Gave up at one point!
Got it in by good luck in the end.
Quick drive and is working but need longer drive to test properly.

[BackintheFold]

Tips-
You need a lot of clearance under the car, you can get both arms up in the space but you can't get any sort of direct mechanical pressure with your arms there.

I used a lever in the form of a philips screwdriver to get the slave to rest on the cut out at the back. I put it through the slave mounting hole and lever against the back stop, then levered using the mounting hole CAREFUL! Don't screw up the thread.
If you have to lever like this then run the bolt in the hole to ensure the threads stay "good".

Once the slave is located against the backstop, the mounting bolt will fit---provided it is lined up straight--- This is impossible to see when you are fitting it. Someone looking from above would help!! If the hole is at an angle the bolt won't line up with the hole, and getting the mounting to rest on the backstop would be next to impossible.
I used a small socket drive so I could push the bolt head in as I rotated it. ALWAYS ensure you have the bolt to hand before starting the fitting process-if by luck you get it lined up on the first attempt, do NOT assume it is easy-you were lucky...So don't miss the opportunity by having to locate the bolt on the floor-and of course the socket to tighten it up.

If you can have someone else help DO THAT! They can line the thing up from above while you force the thing in from below.

The slave has to be straight to fit in, there is a TINY bit of angular freeplay but the last bit is very tight-the slave will be rubbing along the backstop.

Lubricate the rubber bellows/gaiter. You are jamming rubber into metal, so this will help. I used a tiny bit of washing up liquid to avoid any chance of contaminating the clutch plates with grease/WD40, goose lard etc

If you are replacing an old slave, leave the old one connected to the fluid line and fit the new slave "dry" (just a little fluid in to lubricate the internals and internal seal). This may take a while, and you don't want fluid pi$$ing in your eyes while you do it.

Swapping the supply line is easy-make sure you have the right sized spanners (mine were 11mm for the bleed nipple, 12mm for the supply line (a HEL braided type, and 13mm for the mounting bolt.)

I've heard bleeding can be tricky. I've been fortunate it seems with MB engined cars/slaves. The pedal got stuck down after each press-and-hold, but could be brought back easily and with the bleed nipple locked off the pedal worked fine each time.

There is a method where you can cut off the supply hose, eject fluid by forcing the cylinder back in it, and then crimping the supply line closed. The resulting "vacuum" assists in holding the slave rod back. This helped, but the difficulty for me was the last few mm of fitting. Nothing can really make this easy. The vacuum doesn't hold well, so you have to re-do it after a couple of minutes. Otherwise you are trying to compress the fluid while fitting...not good.

If you are refitting an old slave you've taken out to measure the rod...you can probably get away with opening the bleed nipple, compressing the slave rod back in to the cylinder and clamping the "old style" rubber supply hose. That is not an option with the HEL line I had.
This rod measurement is also pointless. See my post above. The size of copy parts-specifically the piston and cup-affect how long the rod should be. Compare old and new to give you an idea. Sachs is OEM. Mine had the VW/AUDI symbol ground off but the VAG part number remained.
TractorDave recommended the Sachs slave; so do I. Stick with OEM. It was £40 more than a pattern part but that's nothing compared to having to do this twice.

This was one of the hardest jobs I've had to do. Lack of space, car on ramps on floor and force needed to force the cylinder into position.
My review is therefore:

1 star-would not recommend to a friend.