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Well, I took apart the cruise control today and learned a little about how it works. Cut to the chase: I need to send it in for repair, or get a new circuit board.
There are three solenoid/check valves to the unit's vacuum chamber which contains the diaphragm / throttle cable.
The first thing I did was ensure the diaphragm & chamber seals are good, connected directly to engine vacuum.
Next I verified all 3 solenoids work when energized directly with 12v.
One labelled "vac" is normally closed and sits between the vacuum line and the chamber. When energized, it permits vacuum to the diaphragm/cable. The other two are normally open release valves, one labelled "vent" (fine throttle control), and the other "dump" (brake press, probably).
With the engine on & the unit engaged, the board is properly energizing (closing) the vent & dump valves, ensuring the reservoir is sealed. But when I press the "set" button, the vac solenoid only energizes weakly, on & off randomly, and doesn't permit enough vacuum through to the reservoir/diaphragm. The DMM shows only low voltage (less than 2v) on the vac solenoid, where the dump/vent solenoids are energized at 8v+.
The circuit board looks OK, but there's obviously something wrong with it. I guess the next step is to call Audiovox and find out if they can fix it, or send me a new circuit board.
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