14-Pin Servo Cruise Control Diagnostics
For 1982-1995 Mercedes-Benz
For 1989-1998 Porsche
Updated 07 February 2015
Please note that not all cars had this type of cruise control. Around 1991 Mercedes launched the ASR system, combining ABS with cruise and idle control to give birth to their first traction control system. If your car has ASR then it does not have this 14-pin type cruise control system.
STEP 1 - Preliminary Checks
Is your Speedometer working OK? If you've got no speedometer then the cruise control is likely not getting a speed signal either.
On the 124/201/123 chassis cars:
Your speedometer is cable driven. A small shaft spins inside of a cable and that drives your speedometer needle. On the back of the speedometer there is a sensor to pick this signal up and convert it to electrical signals for the cruise and other components. If your speedometer needle bounces a bit it CAN cause problems for the cruise control. This was the case on my 300D 2.5, I had a 5 mph bounce in the speedometer needle at times, this made the cruise control buck and surge a lot! A new cable is $60-$100. The later model 124 (93+) have electronic speedometers and are not effected by this.
All Brake Light bulbs intact? The cruise system gets a critical signal by tapping into the brake light circuit. If you've applied the brakes, this will disable the cruise system. If you're brake lights are not working or you have a bulb out that might effect that signal and cause the cruse system to malfunction.
If you've check out all of the above, go to step 2.
STEP 2 - Road Test
Test the cruise first on the road. Once above 30 mph, hit the cruise control stalk in the 'accel set' direction. If it tries to work, engages momentarily, then your problem is most likely the cruise amplifier, see #3. If it does not work at all see #4.
STEP 3 - Possible Amplifier Repair Needed
You can send me the amplifier to repair if you'd like.
If I have a unit in stock for your car, I would like to have your old unit as so that I may continue to provide this service to others. So after you've received the replacement amplifier, you can put your old unit in the box and ship it back to me.
If I don't have a unit for your car, send in the one you have now, I'll repair it and have it back to you within a weeks time.
As a precautionary measure, you should check out the health of your cruise control actuator, See Step 5.
STEP 4 - Signal Testing
Test the signals being sent to the cruise control unit. The cruise control stalk might not be working properly or your brake disengage might not be working.
Cruise Control Signal Testing
If everything checks out go to #5, if not, you probably have to replace the cruise control stalk. I've not really seen many broken/bad ones but its always possible. If the brake disenage pin does not pass then you'll need to investigate the brake switch at the brake pedal assembly.
STEP 5 - Actuator Testing
Test the cruise control actuator and make sure the linkage is connected! As the cruise control actuator ages, it will tend to draw more current from the cruise control amplifier, there is a limit on what the amplifier can provide. Should the actuator draw past this limit then there is a good chance that eventually it will kill the amplifier.
Testing the VDO Cruise Control Actuator
If this checks out, go to #6, otherwise replace the actuator with one that tests out OK and start back at #1.
Well, if you've tested everything that I've said the only thing left to be faulty is the cruise control amplifier. Like I said earlier, I'd be happy to repair it for you. Feel free to drop me an email if you've got questions or are interested!