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I have an after market cruise control that quit working and it turned out that it had a transistor burned out that activates one of the vacuum motors. I replaced that transistor and it is still working ten years later.
What I had to do to trouble shoot the device was to put it on the bench and devise a speed pulse signal, similar to the one produced by the car, with a signal generator. This allowed me to activate the unit and then examine what functioned and what did not. As soon as I could see that the throttle ON vacuum motor received no electrical signal to its terminals it was fairly easy to trace the circuit to find the burnt out transistor.
If you are not able to do a bench test of the unit, another worthwhile thing I have found to do, regarding malfunctioning electronics, is to visually examine the circuit board very carefully, and with a magnifying glass. Way more often than you would guess the culprit will turn out to be a cracked solder joint. This recently turned out to be the case with the windshield wiper control module of my GMC. It had 5 cracked solder joints on the plug connector. If you take the vehicle to a dealer to get the module repaired (very common problem on this model GMC) it costs about $240 as they just replace the module. Or you can discover the cracked solder and repair it yourself for $0.
I have found cracked solder joints to be the cause of the malfunction on a lot of electronic devices, from computers to radio receivers and various other electronic gadgets. Very few people even think to consider this. Most stuff just gets tossed away and replaced, of course at a much greater expense.
Good luck to you however you decide to resolve it.
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