Why would the highest setting on a heater-AC controller unit in the dash of a 1994 G-20 van not work - unit was replaced but problem remains - all other settings work - bad wire maybe - how to repair?

Answer 1

I am answering my own question. After reviewing several websites, including this one, I suspected that the relay/resistor might be the culprit. This is a small box under the hood, over on the left hand side, screwed into the heater core box cover(???) to the immediate right of the blower motor. It's the only relay over there, and is above everything else, so it's easy enough to figure out. There are five wires leading to it, heavily wrapped with black tape. It is easily separated via a two part housing that "clicks" together. In my case, when I took it apart, it was instantly obvious that both the relay resistor half and the plug-in wiring harness half were both cooked (lucky me!) Note that there was no hint of this looking at the outside of the housing; it was a stealth burn-out. The relay resistor can be bought at any good auto parts store, but the combination is apparently a dealer item and pretty pricey, so I hit a local junkyard and got two of them for basically nothing (I took each one apart first to make sure it wasn't fried, before I clipped them out). After that it was just a standard wiring splice job. Be sure to disconnect the battery before you start, as the red wire stays hot even if the van is completely off. If you have the part and proper tools, it's about a 45 minute job.