Troubleshooting the AC, wasn't getting current to compressor clutch. Working backwards with the wiring diagram, found the relay hidden behind glove box wasn't getting current. Found a the floating fuse holder referenced in the wiring diagram above the center AC duct work & below the radio. Diagram said fuse should have been 5 amp but was 10 and blown. Replacing fuse got current to relay but, with a cold engine, it shouldn't have. Wiring diagram showed the compressor relay has a switched ground, a normally open coolant temp switch threaded into the thermostat housing. The switch had failed closed and presumably had caused the fuse to blow.
Replacing the stock switch should have been easy--it was made by a major supplier, Nippon Denso, and the part number, "ND 1136 C06" was stamped on it. But Nippon Denso (now just "Denso") lists no such part & Denso has no listing for a "water temperature switch" for my 1984 Mitsubishi. The old water temperature switch was threaded 10 mm and fine threads--I counted 28 TPI, and so Summit Racing and others' possible substitutes were disqualified.
Searching Rockauto, NAPA, OReilly, Advance, Beck/Arnley and Autozone was useless because none had a record of a water temperature switch for my truck, just for the (separate) water temperature sensor of the dash temperature gauge.
Today I found a replacement for the switch. The thread count's the same, the thread diameter's the same, it's normally open, so the compressor won't operate when the engine's cold, it closes when the engine gets warm (exact temperature unknown) and stays closed until the temp drops again.
The official part description has none of that info, I ordered on a whim and, by the grace of God, it worked

Part replacement for ND 1136 C06 is Beck/Arnley 201-1453

AC_wiring_diagram.jpgND1136C06_BA201-1453.jpg