Audi S1 Sachs clutch

[Stagman72]

Hi,

I have fitted an uprated Sachs clutch (flywheel/pressure plate/friction plate/slave cylinder) and it drove well for about 100 miles. The bite was very low though. Without warning (on a hot day?) the clutch went limp preventing gear engagement. Pumping the clutch wasn't enough to recover the pressure. Master cylinder, feed pipe to reservoir and bleed connection down at the bell housing changed as a precaution (slight dampness noted on master cylinder and reservoir feed pipe). Did not fix the issue as the problem persists! No visible leakages anywhere and no fluid level drops. Almost entire clutch system now replaced.

Tests:- After fresh bleed, engine off clutch feels fine. Engage gear clutch down and car can be pushed. Start engine (with clutch down) still ok and gears can be engaged with engine running & clutch held down. Clutch released slowly in neutral (not to point of connection to reservoir) and clutch pressure maintained.

• * As soon as clutch released to highest point (i.e. connected to shared brake/clutch reservoir), clutch pressure collapses and gears cannot be engaged.

Has anyone got any ideas pointers as I don't believe the new clutch itself is at fault but obviously it has been changed. It oddly looks to be the reservoir that is dropping the pressure? Breaks are fine though and I can't see the connection. Many thanks

[DuraAce]

Did you get the friction plate in the right way around?

[GreenV8S]

Is it possible you've got mismatched master/slave cylinders? By the time the m/c piston has retracted far enough to open the port to the reservoir, the hydraulics should already be fully depressurised.

Is it a concentric slave, or the lever type? If the lever type, maybe the lever isn't mounted correctly and has come adrift.

[Stagman72]

Thanks for the responses.

Yes very careful about the friction plate being “gearside” to gearbox. It also ran quite happily for 100 miles or so.

The slave is a SACHS concentric and sold as a suitable replacement to the standard. The MS is OEM. I do wonder if the match is on the limit of compatibility. However, the clutch completely collapses when released to the top so, mismatched or not, why does that happen? Trapped air was our first thought but have pressure bled, manually bled and vacuum drawn. The system recovers pressure then loses it when the pedal is released BUT ONLY when the engine is running and the clutch pedal released to top position!

Edited by Stagman72 on Saturday 25th May 18:54

Edited by Stagman72 on Saturday 25th May 18:55

[GreenV8S]

Is there any form of self adjustment on the clutch?

[Stagman72]

GreenV8S said:

Is there any form of self adjustment on the clutch?

Not on the S1 Sachs clutch. The original was a self adjusting plate though.

[GreenV8S]

Stagman72 said:

the clutch completely collapses when released to the top so

This is puzzling. If the hydraulics are working properly, the system should be unpressurised when the pedal is at rest and it should return to the same state after you pump the pedal. You make it sound as if the hydraulic system is still under pressure as the pedal returns to rest. Is that what you're really seeing?

[Stagman72]

GreenV8S said:

This is puzzling. If the hydraulics are working properly, the system should be unpressurised when the pedal is at rest and it should return to the same state after you pump the pedal. You make it sound as if the hydraulic system is still under pressure as the pedal returns to rest. Is that what you're really seeing?

Sorry I couldn’t respond earlier as the forum was closed to ‘new members’ due to some kind of abusive posts.

I don’t think I have described the behaviour particularly well so will have another go at it. Thanks for your patience by the way.

I can get to a position where the clutch hydraulic system is behaving as expected with the engine off. In this state the clutch pedal feels normal when pressed down and the system pressure is felt underfoot. With any gear selected and with the clutch pedal held down, the car is free to be pushed. This proves the slave is working under these conditions. The pedal then returns when fully released to the rest position (gear now engaged and car difficult to push) and the process is repeatable. Critically the system successfully re-pressurises when the clutch pedal is pressed down again, sufficient enough for the slave to disengage the selected gear and the car is free to be pushed again.

When the engine is running however, it’s a different behaviour and the system fails. Initially with the clutch pedal pressed down and the system pressurised the engine can be started, all gears are selectable (whilst holding the clutch pedal down) which proves the slave is acting as expected again and disconnecting the gearbox. When the pedal is released fully to the rest position (neutral selected on the box at this point) the pressure is released and the pedal will return to the rest position at the top. However, a subsequent pressing of the clutch pedal results in it going flat to the floor or feeling spongy at best suggesting no or little pressure in the system and no gears can be selected. If I don’t release the pedal fully to the rest position and press down again, the clutch pedal will re-pressurise the system and gears can be selected whilst holding the pedal down. It’s the action of releasing it fully to the top rest position (perhaps due to the MS being open to the reservoir) that somehow creates voids and prevents the system from reaching normal pressure again under subsequent pedal presses.

I really hope I’m making sense this time as it’s got me more than puzzled and at a bit of a loss on what to examine next.

[GreenV8S]

I can't think of any reason for the clutch to behave so differently when the engine is running. Given that the change seems to be repeatable and reversible, this suggests you're looking for a hydraulic problem (trapped air, failing seal) rather than a mechanical one.

Any access to use a mirror, endoscope etc to see what's actually happening to the clutch release mechanism?

[Stagman72]

Again I couldn’t respond due to a lock down, what’s the score with the Forum?

Anyway thanks for your comments it certainly is an odd case and looks to be hydraulic. The plan is gearbox off again to check the ‘new’ slave. Using a garage this time as spare time is tight and the S1 is needed. I’ll post the results.

[Stagman72]

The problem looks like the pressure plate as it had loosened off. When torqued up it must have become caught on the locating pins and held against the rather low torque levels.

The morale is to use a clamping tool rather than the 90 degree turn at a time manual method. Still weird how the clutch hydraulics went limp after running the engine.

We live a learn.

[GreenV8S]

Maybe there was some sort of knock-back going on. Glad you got to the bottom of it before any damage was done.