I successfully repaired the cruise control on our '90 300TE yesterday. Following the good advice of many others here, I removed the cruise control from under the dash and resoldered the printed circuit board (PCB). I must admit, I was reluctant to do this at first as even a very close inspection failed to reveal any suspect looking solder joints.

At first I confirmed everything else associated with the cruise control was in order. With the wiring diagram for the car and a multimeter, it is possible to check all the wiring and the correct functioning of the control stalk and hall-effect speed sensor. The continuity of the motor, solenoid and potentiometer in the actuator can also be checked. All these checks can be performed at the connector for the cruise control unit. To check the hall-effect sensor roll the car back and forth with the ignition on and the multimeter monitoring the output voltage of the sensor. The voltage should swing between approximately 0 and 11 volts.

With everything else appearing OK, I resoldered the PCB. Whilst I don't consider it an ideal method, I did this by soldering through the protective coating over the PCB. The final result does not look as nice as the original (and will probably also be of suspect reliability) but at least the cruise control now works.

Has anyone found a solvent that will remove the coating on the PCB without doing any damage? If I had to do this job again I would be much happier if I could remove the coating on the PCB first. Many modern "conformal coatings" for this application can be removed by a suitable solvent or are designed to be soldered through (which this one appears not to be).

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107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.