Date registered: Dec 2008
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hi mate, I got your pm but better to discuss here to help you and others.
The ACS has 2 engagement stages, when the brake is pressed, and a gear selected, the clutch is disengaged. When you select a gear with either hand or foot brake on, the clutch stays disengaged, when the brake is released, the clutch is moved to the biting point, then on throttle application the clutch engages fully depending on how much throttle is applied. Strong throttle pedal action will cause instant full engagement and resulting wheel-spin.
(2 black and white sensor switches on top of the 'box under the corrugated shift cable connections detect the gear position - visible from the engine bay, r/h/s behind the screenwash bottle)
If the brakes are held down in gear, and throttle applied, the system will engage the clutch anyway which would usually result in wear in the same way as riding the clutch on a manual. The clutch will bite when the throttle is applied, no matter what the brake position is.
The CSC or concentric slave cylinder which is a combined hydraulic actuator and thrust bearing in one, may be stiff inside and that may cause harsh engagement when the ACS stops pushing. The clutch is basically always engaged, and the ACS pump applies hydraulic pressure to expand the CSC which pushes on the clutch cover fingers, releasing the clutch's grip on the flywheel. When the clutch needs to engage or grip, the ACS simply stops pumping - the CSC contracts back to its original size - which drops the friction plate back into the flywheel by not pushing on the clutch cover fingers....so one guess is a stiff CSC, or a blockage in the hydraulic pump part of the ACS unit not letting the fluid "relax" or return to the pump chamber - like a kinked flexi hose between the metal line from the ACS pump and the CSC coupling, this flexi is not fully visible and can only be felt with the fingers on top of the gearbox while lying under the passenger side looking up just behind the passenger wheel area of the floor. (only suggestions or possibilities of where to look before anyone pipes up) - other reasons may be corrosion on the gearbox input shaft which can cause the clutch to snatch as it returns, or even a fault with the dual-mass flywheel which can cause rough engagement.
Unfortunately except for a kinked or damaged flexi pipe, all the other suggestions mean the engine would have to come out to look at them. Not a job for the faint hearted.
I hope this helps, sorry its not a simple thing to diagnose, but will likely be some of the above suggestions, however to get to the bottom of it will be expensive in labour terms simply due to the awkward design of the A-class which needs the engine out for most jobs.
So to recap I would want to check the ACS flexi hose, the CSC slave cylinder/release unit for sticking, the gearbox input shaft and clutch plate hub for rust, and the dual-mass flywheel face/hub for wear or damage to the cushioning part.
The throttle is "fly by wire" so the ecu always knows the pedal position, if the engine revs smoothly even on a light pedal then I would not suspect the throttle potentiometer, but even with no throttle input, while moving, and the next gear selected, the clutch engages fully anyway, hence there is always full engine braking available. The clutch only releases when the brake is applied and the car is under 5mph, or stopped in gear, or the selector is moved from a gear to the neutral position and into another gear, otherwise the clutch is engaged constantly.
Last edited by evilscotsman; 06-13-2011 at 11:40 AM.