Using this forum - and other troubleshooting resources - I am still unable to get my cruise control to work. Here's where I'm at:

1989 260e. All the bulbs replaced with new ones straight from the local Mercedes Dealer.

I pulled the underside dash cover. With my multi-meter, I tested the switch. All looks good.

I checked continuity between amp plug and actuator plug. All looks good.

I checked acuator motor. The solenoid sounds good. The motor I'm not sure about. It draws about 170mA from 12v, both directions. I heard a slight whirring (with a tint of grinding) noise from its general vicinity- but didn't see any movement. Should I see the arm move on the acutaor? If so - then I know the actuator is bad (or jammed or stuck).

I resoldered amp and reinstalled. Got the cuise to work on first test drive (at about 41mph)- but after shutting the car down, then starting up again - the cruise didn't work. It has worked twice since (both at under 50mph). Seems to be on cooler days after it has sat for a while (12+ hours).

I have a hunch (or false lead?)- that the temeprature has something to do with it. Seems that a warm engine (or warm ambient) may be creating some sort of thermal expansion that seizes something mechanically or shorts something electrically. But that's just a guess. Any thoughts? Others experience?

Also, I didn't mess with the Hall Sensor. Anyone have repair or troubleshooting experience with it? How often do they go bad?
Thanks.