How to keep on cruising

25 September 2011 - 21:40 By Gerrit Burger
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I have a year 2000 Mercedes Elegance A/T which has 130000km on the odometer and is in tip-top condition.

My problem is the cruise control. It generally functions without any problems, but sometimes on the open road it just cancels itself for no apparent reason and, of course, the drive reverts back to the accelerator.

When the ignition is switched off, the computer resets itself immediately. This could happen after 50km or sometimes 100km or more. It is just frustrating and I'm wondering if you have any suggestions to overcome this glitch - Eddie

Eddie, I discussed the puzzling behaviour of the cruise control with a workshop manager of a Mercedes dealer near me, and by a great stroke of luck he had come across the same thing before.

He recalled that, in the previous instance, it had been caused by an over-sensitive stoplight switch. As you know, the cruise control is automatically cancelled when the driver touches the brake pedal.

If the stoplight switch, which sends the signal to the computer, has been adjusted too sensitively, the slightest movement of the pedal (such as when the car goes over a sharp bump in the road) could cause the cruise control to disengage.

On later models the function of the brake light switch has been taken over by a brake pressure sensor, but this sensor can also develop an intermittent fault.

Replacing the switch/sensor or re-adjusting the switch is not a difficult job in itself, but getting to these components can take several hours of work, which makes it expensive.

You would therefore want to be sure the problem is linked to inadvertent brake-light activation before delving into the innards of the brake system.

One way to make sure would be to get an auto electrician to rig up a small LED, wired in parallel with the brake lights and attached temporarily in the driver's line of sight on the dashboard.

If this LED flashes when the cruise control performs its unappreciated self-cancelling act, there's a pretty good indication the glitch is somewhere in the brake system.

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