Heater problem
1997 GMC C1500 V8 Two Wheel Drive Automatic 130,00 miles
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I have replaced the black BOSCH relay behind the glovebox, the blower RESISTOR and the blower MOTOR and still cannot get the blower to work on HIGH. I remember in older model years of GMC's and Chevys that their was a high speed blower relay that caused this issue.

If there is a HIGH SPEED relay, I can't find it. Any suggestions?

Thanks,
Jeff

See below

I've already replaced the relay you refer to and the blower still does not work on high.

Thanks,

Jeff

Check the connector for signs of melting, could be the switch or the circuit to the blower relay.

Turn control switch to Hi and look for power at the hi speed relay orange wire-No power either you have an open wire or the blower switch itself within the heater/ac control module-

Also check AC maxi fuse 50amp-it powers the hi speed relay

I've replaced the blower, relay, blower switch, and blower resistor and still have no high fan speed.

Must be wiring between the switch and relay, maybe the ground.

Here's the electrical wiring diagram-try tracking it from the HVAC module to the blower motor-