SLK Cruise - temperamental

[Tim Green]

I have recently purchased a 1999 SLK to go with the W210 E320 CDI estate.
The cruise has a most odd problem. It works fine up to about 75 mph, but at speeds above that it will not engage at all. Any ideas? Also, anyone know what the W number is for that car - is it W170?

 

[bigasotonuk]

The number will be the first 3 numbers on your vin number.
Check that your tyres are the correct profile as similar faults have been reported when incorrect tyres have been fitted.

 

[Tim Green]

That's a very intersting response! I can't see any connection between cruise and the sort of tyres that are fitted - please explain? (However, I think the tyres are standard but I'll check).

 

[brandwooddixon]

The cruise control uses readings from the ABS, which in turn use sensors counting the rate of rotation of a toothed ring fitted to the hub.

The system requires all wheels to rotate at the same speed, allowing for a small difference.

If there is a sufficiuently large difference in rotational speed, say due to a different tyre/wheel size, then as a safelt measure it will trip out (you could be loosing traction due to ice for all it knows).

 

[JEZ.S320L]

That's a very intersting response! I can't see any connection between cruise and the sort of tyres that are fitted - please explain? (However, I think the tyres are standard but I'll check).

I had EXACTLY the same problem on my ML - turned out that the rears (295's) and the fronts (285's) had a different rolling diameter. The c/c 'thought' the tyres were loosing grip and self-cancelled.

To err on the side of caution - I replaced all 4 tyres with the same size. Result? C/c has worked perfectly ever since.

 

[Tim Green]

Well that's very interesting because the fronts are new (had been replaced by the dealer) and the rears are not new.
The tyres are: Front: 205/55 R16 91V Rear: 225/50 R16 92V
I think this shows that the profiles are different: 55 and 50 which could cause the problem I suppose.
Can't say I'm very keen to throw away 2 perfectly good Dunlop SP sports!

 

[JEZ.S320L]

Well that's very interesting because the fronts are new (had been replaced by the dealer) and the rears are not new.
The tyres are: Front: 205/55 R16 91V Rear: 225/50 R16 92V
I think this shows that the profiles are different: 55 and 50 which could cause the problem I suppose.
Can't say I'm very keen to throw away 2 perfectly good Dunlop SP sports!

How do you think that I felt - getting rid of 4 almost new 20" tyres?

 

[jibcl500]

Tyre size is corrct for the size of wheel. Check the air flow meter is genuine Bosch, it should have the bosch and MB symbol and the the two part numbers on it, if it hasnt then its a cheap china made maf that will possibly destroy your engine in the next 1000 miles or so.

I found the CC to not work about 80mph but there again that was all it could do anyway with the dodgy maf fitted.

jib

 

[darrenb1707]

Well that's very interesting because the fronts are new (had been replaced by the dealer) and the rears are not new.
The tyres are: Front: 205/55 R16 91V Rear: 225/50 R16 92V
I think this shows that the profiles are different: 55 and 50 which could cause the problem I suppose.
Can't say I'm very keen to throw away 2 perfectly good Dunlop SP sports!

Your tyres are fine and are not the problem....fronts have a rolling radius of 315.95mm and the rears are 315.7mm. Definitely not enough difference to cause a problem. I have a staggered fitment on my CLK and have NO problems with my cruise control at any speed.

I agree with Jib's post.....look at your MAF sensor. In the past, I have run with both pattern MAFs and genuine ones (and the pattern MAFs DO NOT blow up your engine btw!!!). However they dont last very long and can cause some intemittent problems due to them being more prone to running out of an acceptable range for the Engine ECU. Some of the symptoms DO include cruise control malfunctions. This can happen when your ECU logs a pending fault code. This happens when the ECU detects the MAF being slightly out of range and logs it. If it runs out of range again it will trigger the Check Engine warning light and log a Trouble code "P101 MAF Out of Range".

I ran with pattern MAFs for some time whilst I was repairing an ongoing (and common) issue with the 230K engine where oil contaminates the MAF. I have experienced cruise control malfunctions intermittently that generally coincided with the pattern MAF running out of range. Incidently, what engine is your SLK? If it is a 230K your problem may be a secondary symptom of MAF oil contimination which is very common on this engine. Search the DIY section for more info on this.

 

[Tim Green]

UPDATE: The dealer was very decent and arranged to get Dunlop tyres fitted to the front to replace the ones of indeterminate parentage which he had had fitted. (Existing Dunlops on the rear). Meantime I checked and the sizes are correct. So with four Dunlop tyres - cruise works fine. So the rumours are correct. I think the tyres that had been fitted must have had a somewhat larger circumference than that of the Dunlops. Anyway, all is cured now. Thanks for your help everyone. I was suspicious of MAF idea because the car has only done 50,000 and has been serviced by MB from day 1.