High, mine's a 31,000mile SLK230 with combined cruise and limiter lever. When I engage cruise above 45mph, it cuts out, sometimes lower. If I pul to resume it has a pull but then cuts again. Sometimes if I wait until below 30mph and pull, it zooms up to 45-50mph then cuts, often on bumps in the road. On a few occasions when starting and pulling away, I get the feeling that once I pass 40mph or so(not specific), it seems to engage on it's own and keep accelerating, even when I gently apply brakes. On two occasions it took a good stamp on the brakes and pushing the lever away through "off" a few times to get it to go back to sleep. The dealer has had it in and couldn't work it out and needs it back.

Does anyone else know of this fault and what might cause it?

Also, if you have an SLK, I have a rattle behind the dash, high up and behind the centre console. Any ideas what it might be to track it down and fix it. I guess it may be one of the slide out tray's springs loose?

 

Hi,
So youre UK spec. (IOM)
Firstly should you be continuing to drive this car since something is obviously very wrong if the system does not cancel the moment you TOUCH the brake pedal.
This is an important safety feature which you report is not working!! The system should sense ANY movement of the brake pedal thru use of the brake pedal switch. This switch has two circuits. One for the brake lights and one for the ABS/Cruise etc.

Your car is a low mileage 99 so I would be surprised if your column switch had gone bad. These usually last for 100k+ However your intermittent symptoms and your comment about bumps does seem to suggest it could be the cause.

Cruise is covered by diagnostics so what (if any) code is showing? This should help narrow field.
Since I don't like spending any money on parts I would unplug / inspect / replug all connections in the Cruise circuit.
Key points are Fuse holder, brake pedal switch, column connector and switch / module.

ABS / ASR working OK? These systems also use a common speed sensor input so if these are OK it is unlikely speed data problem.

keep us posted, Stuart

 

Well, they had it back in yesterday. Still no fix. The dealer asked me if the problem had started after I fitted the radio (JVC KD-LHX552). I couldn't say because I only used it around town before I fitted the stereo unit (it's amazing BTW- my entire favourite music is on a 1gb SD card which just lots in!). I removed it and took it for a drive and the problem is still there.

The Diagnostics Technician there reckons that the whole Cruise/speedtronic system is embedded in the main ECU for the car, so he will try swapping speed sensors about and finally get a loan ECU from Mercedes to try next. The ASR certainly cuts in when I try my best to kick the back end out!

 

I agree the CC is all part of the main EMS ECU.
If the ASR is working fine then it certainly is a strange one and unlikely to be the speed sensor.
Did they check all the connections as suggested due to link with "bumps"?
Has anbody actually looked at the EMS? It isn't full of oil is it?
Stuart

 

I dunno. They did say they've spent an hour's worth of labour so far, but I get the distinct impression that MB Technical is leading them as they keep mentioning that they are waiting for technical to get back in touch. The fault is not really bumps only, as the IOM has virtually third world roads everywhere (except for the sacred TT course- and that's bumpy as hell in parts..I have to back my bike's suspension damping right off when I go out having fun)

It cuts when engaged above 50, and if you resume, it will accelerate but cut out when approaching anywhere from 48 to 55 mph. Strange indeed. The car does have 7" wide with 205's on the front and 8" with 225s on the rear, but they did say that's normal and the profiles match the manual. They are Firestones though, and I'm not sure if perhaps the profile isn't out enough to cause speed differential at about that speed. I'll suggest that to them next week. The tyres are crap anyhow. The Tech thinks one of the speed sensors is possibly not right and that it is not out far enough to produce a fault code but might trigger the Safety function in the Cruise software. Lost me.

If they do get an answer I'll post it up, meanwhile, I'll just enjoy the rain and wind and keep the roof up for a bit! Bummer.


Is there a known problem with Oil getting in the EMS then?

 

Stuart,

I've just trawled back and found the oil in the ECU thread so no need to answer on that one, thanks.

 

Mark,
Thanks for the feed back.

I should have asked whether wheel and tyres were OE.
If they have a different rolling circumference to standard then iy is quite possibly the tyres are the cause. I thought 230 qnd 320 had the same tyres. 205/225 doesn't sound OE.

I had a Ford that "auto" accelerated to full full speed after 2 hrs on Cruise without intervention. (I like trying to judge my position etc so that I don't have to cancel cruise even on UK motorways.) Problem was that the software had a line of code which resulted in a non plausible result and for some reason this resulted in "full speed". The cause was that the expected revs/mile vs the actual was within a given range per minute but exceeded a finite cumulative value after 2 hrs!
Ford of course said this was impossible but later discovered that the cause was incorrect coding of the tyres for the vehicle. Somebody had fitted Option take tyres in order to meet my order wothout recoding the ECU.

Good luck let us know how you get on.

 

My take was the differing rotation of the wheels, as it cut out at a lower speed (30!) the other day whilst cornering. Bugger! I've just been out in the rain, and despite the dealer saying the car has the correct tyres...they are wrong!

The manual states that for mixed tyres, I should have a 7Jx16" front wheel with 205/55ZR16 tyres and at the bac kI should have 8Jx16" wheels with 225/50 tyres. Well, after going outside in the rain with a torch I found I have Firstone Firehawk ZS80 rears 225/50 and at the front Firestone 700s with 205/50 on! Aha!

So, as I go faster, the front wheels will speed up more than the rear and aventually I get a speed difference enough to trigger the ASR which cuts out the cruise control as part of it's logic. If I corner, the fronts go faster again, so the speed it cuts will be lower.

The dealer has a used SLK230 in right now so I'm going to ask if they can swap the wheels and tyres over run it to see. Any bets?

I calculate it to be a 3% error with this profile fitted. Shouldn't be enough but it does follow logic as to when it trips out, cornering and all that. I'll let you know when I find out.

 

Sorry I didn't provide the tyre sizes for you. I didn't want to get WET. LOL.

Of course before going to the trouble of doing a finite test a diagnostic test should be carried out to give some lead as to what is happening. This can only be done by an MB dealer. Is your dealer an MB dealer. If so get them to do one FREE.

If this doesn't show anything up...eg. faulty wheel sensor...then the tyres maywell be the answer.

JFI. Are you aware some manufacturers actually use the rolling circumference for tyre pressure monitoring.
Zero the system to allow for wear with known correct tyre pressures. Then monitor difference from the datum when the steering is straight using the steering angle sensor. Simple really!

It all depends on the logic that is used as to wether the tyre size will trip the system. I suspect that the logic WILL be used as a means of checking sensor performance.
Strange however that your ABS/ASR-ESP remains OK since these systems are all linked and presumably you are not getting the system malfunction lights displaying. I assume the system malfunction lights work on startup?. Mmmm?

Stuart

 

I spoke to the technician, and yes it is the only MB dealer on the Island (and busy too, at least a week and a half for bookings and the Island is crawling with mercs of all ages). No fault codes of any description, and my '99 model doesn't have any tyre pressure monitioring as far as I'm aware.

It's back in on Thursday, and they'll swap wheels with a used car in stock to see if the tyres are to blame. I do hope so.

Thanks for all your tips and help Stuart, I'll post again after Thursday when I have news.