Why does only the “high” fan/blower setting work?
The high setting is wired differently than the lower ones. The coil- resistors (look like springs) mounted in the evaporator box, are wired directly to the switch in the car. For the high setting, GM wants maximum current, with no loss due to the long wiring run and the switch. So, they put a relay in close to the blower motor. They probably didn’t want the hot resistors behind the dash (fire hazard) and the long wire-runs to the switch don’t matter if you’re slowing the motor down anyway. So they only use the relay for the high position. If a blower works on high, but none of the lows work, than that half of the circuit has the problem. If it works on low, but not on high, then it is probably the relay or the wiring to/from the relay.
