Sup y'all!
First post to the forum, glad to be here! I've got something that's been stumping everyone I know including my pretty righteous Google fu skills.

I've got a 2012 F150 XLT with the manual HVAC with blower speed settings 1-4. Speeds 1, 2, and 3 work fine. When I turn it to speed 4, it basically blows at speed equivalent to speed 1. Then knock "down" to 3 and it goes back up to correct level 3 speed. Already replaced blower and resistor. Any ideas?

 

Welcome, what does the connector look like? Is it melted? Did you use Ford parts?

 

No connectors are melted. Funny you mention that, I even noted while working on this that they were surprisingly clean and intact. The actual blower I put in was not a Ford part but I was having the same issue before replacing so I doubt that's the issue. Resistor is Motorcraft.

 

The blower motor has two paths to ground according to the manual.
The blower motor switch ground speeds 1 2 3 through the resistor and the switch itself I guess.
Speed 4 is a direct ground also through the switch.

I read a quick test when I was having trouble and it involves using a test light to confirm the switch is working properly in each position.

 

This is an interesting piece of input. Thank you! I'm going to see what I can do with this.

 

@JMFT1776
Here is the print from a 2013 manual with the explanation.
Notice the two ground paths, G203 going through the resistor and switch.
G202 going through the switch for hi speed, I think.

 

Thanks for this. This is similar to everything I found as well. It's really not a complicated mechanism. From what I can see, levels 1-3 just take a different path through the resistor hence limiting voltage accordingly while level 4 (high) is just wide open, straight path to the blower, closed circuit. Well I pulled the plug off of the blower motor and put a volt meter on it on all 4 levels and all showed 13.74 volts. So I don't know how the speed is being controlled. I would have thought the resistor was reducing resistance (hence increasing voltage) for every level up, but this would certainly prove that theory wrong. Also, I unplugged the resistor and then no levels worked, including high. So somehow level 4 is going through the resistor as well. I have a feeling it's just a straight "no resistance" loop, but still, that's not what the schematics show. Anyway, I have a feeling this is a demon above what I can understand. Took it into my local trusted shop. I'll let y'all know what I find out.