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gtjunior 06-10-2011 12:14 AM

Aggressive clutch engagement A160 ACS
I have a 1999 A160 5 spd with ACS & it intermittently has very aggressive clutch engagement, generally in 1st & 2nd. Doesn't happen all the time & nothing specific induces it (although if you put the car in 1st, foot on the brake & give some throttle it will usually do it). Another forum member is experiencing similar issues in this tread.

The car has done 180,000km but the clutch doesn't slip, MB put it on there diagnostics and mentioned the vacuum hoses needed replacing, apparently the ACS is linked to throttle position, fuel load etc etc didn't seam like a real plausible reason to me.

Could it be the brushes on the electric clutch actuator? are there any sensors in the top of the gear box that can be checked, or could it part of the self adjusting clutch/pressure plate system.

lofty 06-10-2011 07:52 AM

your gearbox is basical;ly a manual box and as far as I know no differences accept that the ACS actuator does the job of the clutch on a fully manual version , I thiunk if you had actuator problems you would get an 'F' display so I would discount that one, you could check the sump breather tube that connecting the the ECU induction line and MAY have some bearing on the tick-over which could be causing the problem , but I've not hjeard of thios problem before.
There is information on my page 59b that will explain the system every little helps.
I can't think why you would want to pree the accelorator and keep your foot on the foot brake, normally the car is help on the hand brake, see also the instructions for driving ACS which I have taken direct from the owners manual, which makes mention of holding the car with the revs.
Bert Rowe's-Mercedes-Benz 'A'-class info. Mercedes- Benz 'A' Class (ACS) Auto clutch-less transmission

gtjunior 06-11-2011 04:29 AM

thanks for the reply, I will check the sump breather tube/ ECU induction line ( not sure where this is, but I will have a good look).

although the ACS essentially takes the place of the clutch pedal there must be some sensors/switches that tell the clutch actuator when to operate

evilscotsman 06-13-2011 11:32 AM

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 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.

gtjunior 06-14-2011 11:26 PM

thanks for your detailed reply, all plausible explanations however the problem is not consistent, ie it doesn't happen all the time.

as a side note, if I am stopped, in gear with foot on the brake after a short while (say 15 sec or more) you can feel the clutch start to engage ??? possibly pointing to an issue with the CSC

BTW I have tried the ACS reset procedure many times (ie .... brake, select 1st then neutral, brake off brake on select reverse etc) however it never seems to make any noticeable difference nor effect the clutch take up point.

Can a dealer calibrate the clutch take up point with their Star Diagnostics?

evilscotsman 06-16-2011 03:28 PM

My ACS clutch used to creep in when I was stationary in gear just like yours. Its a pointless system with flaws, just like most MB "innovations" - I bought a manual pedal box, master cylinder and had a copper brake pipe made up and converted mine to full manual. Job done, light clutch, and no nonsense.

ACS is a crock mate, just like the whole "engine under the floor" crap.

I also tried the reset procedure, I think its an urban myth.

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