Question: I have a 2004 Ford F-150. The A/C blower only works on speeds 3 and 4, but not 1 and 2. I replaced the switch, and it still does the same thing.
Answer: Going by your replacement of the blower switch, I'm going to assume you have the manual HVAC system, and not automatic climate control. As you've discovered, we must test before replacing parts to make the correct repair the first time. You know it's not the switch (since you already bought one), and it's not the blower motor because it works at two of the four speeds. Here's a description of the blower control system to give you an idea of what we have left. The switch is mounted at the climate control assembly (possible failure #1). To change speeds, the control assembly supplies different grounds paths for the blower motor through the blower motor resistor (possible failure #2). The higher the resistance in the ground circuit to the motor, the slower the motor will operate.
To diagnose the condition accurately, you'll need some basic DC circuit skills, a digital multi-meter, and the appropriate wiring schematic. Begin at the blower resistor assembly and check for the appropriate ground paths from the controller. If all is well there, check the resistance through each circuit of the resistor. An open circuit or circuits in the resistor where there should be a specified amount of resistance (ohms) will condemn the resistor assembly. The blower resistor is probably the most common failure of the system, but there could be a poor connection or open circuit in the wiring harness (possible failure #3). Either get the information to test, or get the truck to a technician, but please don't guess.
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