Originally Posted by jonboll
hehehe, I knew I read something about this before, forgot to check the techinfo before posting... Bad rheostat in the dimmer switch. I'll go check that out
Agreed. As for the high-speed relay, it'll be somewhere under the hood near the blower.
I have to do this from memory since this was the usual fix for the problem on older GM cars, and my car has the electronic system so I can't look at that one for specifics, but here goes (and maybe we should have a TechIno article on this):
On older GM cars, if the high speed blower setting quits, but the other (lower) speeds still work, the problem is most likely in the circuit for the high-speed bypass relay under the hood. This relay allows full power to the blower motor without running the risk of melting the dashboard switch. However, the massive current flow, especially as the blower motor ages, tends to melt an in-line wiring harness connector, and the high-speed relay eventually won't get enough power any more. It will click just fine when you move the dash switch, and you can sometimes even measure voltage on the fat red wire leading to it, but it no longer provides enough current flow to drive the blower.
The fix in such cases is to follow the fat (10-gauge) red supply wire back from the relay towards the engine. (The relay itself has a four-conductor plug, two fat wires and two thin ones, and is mounted on the firewall or a bracket hung off the evaporator housing, in the vicinity of the right (passenger'*) side hood hinge.) You will find a plastic in-line wiring harness connector, possibly with two or three other wires included. Pry open the connector and you may find that the 10-gauge contacts have melted. The other wire connectors are probably fine, so reassemble the connector, solder in a bypass connection around it for the 10-gauge circuit, and you should have your high-speed blower back again.