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73-87 Chevy _ GMC Trucks => Interior & Equipment => Heating, Ventilation & Air Conditioning (HVAC) => Topic started by: 78silver on March 25, 2010, 08:24:37 PM
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My heater fan works on low and medium speeds but blows a 20 amp fuse on the panel if switched to high. I've read posts made previously in this forum and have studied the wiring diagram posted by Vile. With a good fuse and the relay removed, I get 12 volts to the heater-AC selector switch, to the battery and blower terminals of the fan switch (when switch in high), and in the heater relay harness at the clip on the orange wire which connects to the coil input terminal of the relay. Plug in the relay and the fan runs in low and medium- the default setting of the relay- move the fan switch to high to put current through the coil in the relay and the fuse blows.
In getting to this point, I've tried replacing the relay, tested the fan switch for continuity and ground faults, tested all the wires the harness with the resistor and switches unplugged for grounding, and tested for grounding between the orange wire and all other wires in the harness. I've unwrapped and inspected wires in the harness as far as the firewall, no nicks or scorching. I have cleaned the grounds from the fan and the relay, and the clips on all wires in the relay plug socket.
At this point, any ideas or suggestions anyone could make would be very welcome.
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You have a bad ground. At least that's what was wrong with mine... same behavior. To test, run a ground jumper to the one of the motor mount bolts and replace the fuse then switch to high.
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Thanks, you nailed it. ;D
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;D glad to hear it!