Questions and Answers

Mercedes Benz 380SL Not Cruising Q. Hi Vincent: Thanks for all the work on your site. I wonder if you could suggest how best to proceed with repair of my cruise control? This is a 1985 Mercedes Benz 380SL with about 110,000 miles (but who knows for sure, the odometer has a stripped gear and no longer counts). Car has power steering and air conditioning. The cruise works fine up to about 60 mph.

At faster speeds, it will tend to surge, as if it is over correcting in both directions as it tries to speed up or slow down to hold the correct speed. It will still hold speed, but the throttle is opening and closing by great amounts, way too much acceleration and deceleration.

The wild throttle response could almost be described as violent, if the cruise is left on and the condition is allowed to continue. If it starts to surge and you disconnect the cruise and reconnect it carefully (hold speed steady before engaging), it may work okay for a while, but it will start to surge again shortly.

Thanks for your thoughts.
Connel

A. Mercedes Benz cruise control systems have been fraught with problems for a long time. The only way to test that system is with the Mercedes Benz Cruise Control tester. If other testing and measuring instruments are used, the control unit may be destroyed.

A couple of other things that can cause this problem is having the wrong bulbs in the brake lights. No, I'm not on drugs. The type and wattage of bulb in the brake lights will have an effect on the cruise control system. I have repaired many Mercedes Benz cruise controls just by putting Mercedes Benz bulbs in the brake lights.

Another common cause is play in the rear differential. Mercedes Benz differentials get very sloppy over time and this extra play will disrupt proper cruise control operation.