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post #1 of Old 06-26-2017, 05:56 PM Thread Starter
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HVAC blower switch failure mode

The HVAC blower 4-way switch failed in my '11 SL when high current flow through the switch melted its plastic body, which caused the knob shaft to weld itself to the body.

Is this a common failure mode of this switch?

I have a Dorman rebuilt HVAC controller (which contains the switch) ready to install, but I'm trying to figure out why the high current flow occurred; whether it was caused within the switch itself or by a bad resistor pack or blower motor.

The high current flow occurred in pin 13 of the switch, terminal for the highest (#4 ) fan speed position (evidenced by slight melting of the harness connector at that pin only). That terminal connects the blower motor to ground through the switch directly, completely bypassing the resistor pack. The fact that the resistor pack is completely bypassed in the highest fan speed switch position leads me to conclude that a short occurred in the blower motor and caused the high current flow in the switch when in the #4 fan speed position. The only flaw in this conclusion is that neither of the two 15-amp fuses on this circuit blew, which I would think they should have.

Hopefully this is a known issue. Anybody experienced this?

Last edited by farfel; 06-26-2017 at 10:25 PM.
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