My 1998 Mercedes SLK 230 Kompressor suddenly started to drop out of cruise control over a 15 mph band between 79mph and 94mph (on autobahns and closed roads of course). It works fine at all speeds up to 78mph and above 95mph. It will engage for moments only between these speeds and then drop out.
It has spent a whole day in the main dealers workshop and a
diagnosis of a faulty engine management unit was made, cost to
replace £1,300.
It went in for this, had a new EMU fitted, but came out unchanged with the old unit as the new unit made no difference.
The workshop appears to have no idea of the cause and suggests booking it in for further extremely expensive diagnostic tests but not really knowing what they are looking for. It has only done 23.000 miles with full servicing and two original tyres still on. They also said Mercedes would make no contribution to the unit wearing out at 23,000 miles as the car was so old.
What is the diagnosis? How is it cured?
John Knox
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If the MB workshop didn't subcontract the work to a car electronics specialist (who knows VDO Tempostat units, which I take it is what yours is), take it to one yourself, at least for a preliminary chat. You could probably have retro-fitted a new cruise-control unit for well under half of what the MB dealer has already charged you. In that the new EMU has failed to cure the fault for which its installation was specifically authorized, I would reject it and claim a full refund, and get ready to go to court.
It seems like a typical dealer rip-off, compounded by ignorance and incompetence, similar to one that caught me out over the back axle in my Capri some years ago: dealer diagnosis was that the axle needed replacing; true diagnosis (discovered later, of course) was that the LSD lubricant needed changing to the correct one, having been wrong from first manufacture. If dealer workshops don't know what they are doing (not uncommon), they will enlarge the solution to the problem in the hope that it will cover all possibilities, much to the benefit of their bottom line.
Finally, if you're not already a member, the £35 to join the MB Owners Club might be further money well spent, especially if your problem persists:
www.mercedes-benzownersclub.co.uk/
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Given that the system functioned at most speeds I would have thought it very unlikely the ECU would be at fault. I wonder what diagnosis they did to arrive at that conclusion!
I would suspect that the ECU is receiving duff information at certain speeds (i.e. speed signals are judged by the ECU to fall out of the limits of plausibility and so it switches the system off). My first suspicion would be tyres. The overall diameter of the tyres must be very closely matched on all four wheels and ideally they should be of the same diameter so that any change with speed is similar on all four wheels. Changing all four wheels for brand new ones might fix the problem (could the dealer 'lend' you a set to try this?).
The other possibility is a speed sensor which is giving a weak signal and dropping out at certain speeds.
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Roger
Many thanks for your advice. I will try to find an independent VDO Tempostat expert. There used to be a genius for all 'difficult' problems in the area but he retired without handing on his expertise.
Thank you also Aprilia, (a superb bike). Tyres were the first thought but they are entirely symmetrical with two relatively new fronts fitted by the dealer. I will try to find how the sensors are tested. I will also broach the question of trying an alternative set of wheels and tyres.
John
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I think you will find that the cruise control on this model is not made by VDO tempostat but is in fact integrated into the engine management control system ,which is a fly-by wire control of engine idle & cruise conditions.
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The question is whether the ECU uses the ABS sensors to determine speed or the speedometer feed off the gearbox, if the former then tyres of different diameters could be a factor.
Is it an auto or manual, if manual try it in a lower gear, yes it will be revving rather high though take it to perhaps 85 and set cruise, if criuse stays in in a lower gear then it could be engine speed related, might point the dealer in the right direction.
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Thank you for the further advice.
Have had a response this afternoon from the MB Dealer who has been repeatedly reminding MB National technical support of its failure to respond to the Dealer's request for advice. They have been advised to check the control stalk for contact damage which seems a total non-starter as the speeds for the failure to hold cruise are consistent.
The second thing they have advised to do is to try replacing the entire speedometer module as they say this is linked to the ECU controls. They would do this on a trial basis. Have confirmed the ECU is made by Bosch.
The dealer has agreed to try with a borrowed set of replacement wheels to see if this could be the cause before stripping the dash.
John
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PS the cruise works from ABS sensors and the car is automatic
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Yes, it does use the ABS sensors. If there is more than a very small discrepancy between wheel speeds it thinks the car is skidding or planing and shuts off the cruise. Hence tyres must be closely matched in diameter.
I heard of a similar case where dealer spent £1000's replacing parts and then noticed one tyre was different to the other three....
Best of luck with it, anyway!
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Thank you all. The M-B Dealer has now tried the SLK with a complete set of wheels from another SLK and tells me the problem is cured. Did not have a pair of new tires to replace the partially worn rear tyres, neither does our main tyre outfit so have taken the result on trust. It does seem to be very similar to the case descibed by Aprilia.
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Thank you all. The M-B Dealer has now tried the SLK with a complete set of wheels from another SLK and tells me the problem is cured. Did not have a pair of new tires to replace the partially worn rear tyres, neither does our main tyre outfit so have taken the result on trust. It does seem to be very similar to the case descibed by Aprilia.
I know of another case where someone spent £5k at an MB dealer, changing ECU and complete transmission unit. Was eventually cured with a set of new tyres!
Hope it is indeed cured now.
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PS - should add that I always recomment anyone with one of these cars to rotate the wheels every 3000 miles - do a corner-corner rotation (but check tyres for directionality first).
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