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Discussion Starter #1 (Edited)
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?
 

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It is NORMAL for LOTS of electrical of all types to melt under normal use now and way more than one brand. What happens when you put plastic right next to high power requirements in normal use. The switches used to be steel bodied there and now plastic to save weight. Blower motors are some of the biggest power hogs there are. Why no fuses blew. My last 3 Fords all melted blower switches with zero wrong other than the switches are crap quality. New switch and back to like new. If the blower had shorted it would NOT be working right.

DON'T buy a Ford if you don't like that, they will smother you with melted electrical of every type you can possibly name.
 

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Your blower motor is drawing too much. Replace it along with the switch, connector and whatever else is damaged.
 

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It will draw too much over a long period of time which will heat up the switch and eventually melt it, it doesn't have to pop the fuse.
 
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