CRUISE CONTROL FAILS AT 55MPH & OVER - DEALERS DISAGREE ON CAUSE - ANY INSIGHTS?

Asked by May 29, 2009 at 05:13 PM about the 2000 Mercedes-Benz CLK-Class CLK 320 Convertible

Question type: Maintenance & Repair

Last year, I bought my 2000 CLK 320 convertible with 48K miles from the original owner (California) & didn't discover until I was driving it home to Illinois that the cruise control won't stay set at speeds between 55 mph & 100 MPH (works fine at less than 55 & over 100).

The Mercedes Dealer Service rep in Hoffman Estates, Il. said that it was due to the wrong size TIRES (no mention of wheel size) & once I put the recommended tire size on, it will correct itself.

Currently, my car has 18" OEM chrome AMG wheels & tire sizes:
FRONT = 245/40R 18 93H
REAR  = 265/35Z R 18 93w

Needless to say, it will cost me a pretty penny to change all 4 wheels & all 4 tires, so I got a second opinion.

The Mercedes Dealer Service rep in Naperville, Il. said that he sees Mercedes cars will all sorts of oversized wheels come into his shop all the time with no cruise control issues, & that the cruise control is "self-adjusting", so tire size shouldn't affect it  -  but the only way to find out for sure if the other service rep was right or wrong is to have them switch my car's wheels with another car that has the 205 55 R16 tires & 16" wheels (probably off a loaner car)& take it for a test drive & test the cruise control.

That "test" will cost me about $300.00.

I'd like to think that modern automotive computers & technology could do a better job pinpointing problems than what I've gotten from local Mercedes dealership service departments.

Has anyone else experienced a problem like this with their Mercedes cruise control? If so, do you know what caused the problem or how to fix it?

Does anyone know if or how such a 2000 Mercedes CLK 320 convertible cruise control issue could be more precisely and directly diagnosed & corrected?

I'm open to any & all ideas that a diminutive woman with no automotive skills (me) can research or try that won't cost me hundreds or thousands of dollars for experimental work that has no guaranteed results.

Thanks

2 Answers

5

I have a 1997 E280 and had the same problem. Turned out to be a leakage in one of the pipes. Mine was the air pipe that lowered and lifted the headlights. Hope this helps.

2,435

In one respect your 1st mechanic was right, your tyre sizes are wrong! When your changing the size of your wheels it's important to select a tyre size that matches the rolling radius of your old tyre as closely as possible. With the wheel and tyre combination you have now your speedometer will be giving you a false speed reading which means that you could think your doing 30mph when in fact your actually doing closer to between 33 - 38mph, and the faster you go the more inaccurate it's likely to be. It's recommended that the MAXIMUM difference in diameter should be +/- 2.5% (UK law). So, lets use your current tyre size for the examples. The 2nd mechanic suggested you had the wheels swapped for 205/55/R16, so we'll take this size as the ones originally on the car from the factory. Your current tyre sizes are, Front: 245/40/18 which is 3.32% bigger than the original, where as your Rear: 265/35/18 is 1.74% bigger than originally fitted. I recommend doing some shopping and down scaling your tyres to 215/40/18 front and 245/35/18 rear. Both of these sizes are exactly 0.47% smaller over all which you wouldn't even notice as in actual measurement it's only 3mm. As for your issue with the cruise control, take the car for a drive, recreate the fault, then go to a garage and ask them to check for fault codes on the ecu. It's important to recreate the fault first so that the error code will be present when it's tested.

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