72 H/O Blower Fuse

Old December 6th, 2018, 07:18 PM
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72 H/O Blower Fuse

The firewall fuse blows every time the AC blower switch is moved to the high position on my 72 H/O. Car has a new wiring harness and blower controlling the original, rebuilt OEM AC system. Any thoughts about what may be blowing the fuse? The only part not new or rebuilt is the AC control panel itself. Thanks for any assistance!
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Old December 6th, 2018, 09:26 PM
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Does it blow instantly as in a direct short to ground or does the blower run for a few seconds and then the fuse blows? Look closely inside the glass at the ends of the blown fuse, do the ends at the separations look hot/melted with a glob on each end or does it look like a sudden snap with the ends possibly blown in opposite directions?

Burned globs indicate a slow overload such as a worn blower motor pulling excessive amperage or a fuse of too low rating. A separation of the fuse strip without globs or melting indicates a sudden high amperage draw from a direct short to ground. If it appears to be a direct short unplug the high blow relay and see if the short goes away. Assuming the new wire harness is done correctly it's likely in the blower switch/control or the high blow relay.

Unplug the wire from the positive on the blower, put a test light from the positive on the battery to the end of the fuse holder plug going to the blower motor, it should not light with the firewall fuse out and the blower disconnected. This is to see if the wire at the fuse holder is shorted to ground. Unplug the wire from the blower switch and the relay, then test the wire that goes to the high blow relay to see if it is shorted to ground.

Good luck!!!
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