The resistor is a crucial component in the blower motor circuit. It steps down the voltage to the motor, allowing for different speeds. When speeds 1-3 fail, it strongly suggests the resistor is failing – the higher speeds (4 and 5) often bypass part or all of the resistor, hence why they still work. The resistor might be:
* Burnt out: Over time, the resistor can overheat and fail, especially if there's a short circuit somewhere else in the system.
* Corroded: Moisture can corrode the resistor's connections, leading to poor contact and speed failure.
* Loose connection: A loose connection to the resistor itself, or in the wiring leading to it, can also result in intermittent or complete speed failure on the lower settings.
While less likely, other possibilities include:
* Blower motor itself failing: While less probable given that higher speeds work, a failing motor could exhibit this behavior. The higher voltage of speeds 4 and 5 might be enough to overcome some internal motor issues.
* Wiring issues: A break or short in the wiring supplying power to the lower speed terminals of the resistor is possible, but less likely than a resistor problem.
* Switch malfunction: Although less common, a faulty blower motor switch could cause this issue. However, this is usually associated with no speeds working, not just some.
Troubleshooting Steps:
1. Locate the blower motor resistor: This is usually located behind the glove compartment or under the dash, often near the blower motor itself. Consult your repair manual for the precise location.
2. Inspect the resistor visually: Look for any signs of burning, corrosion, or loose connections. A burnt resistor will often have discoloration or even melted parts.
3. Test the resistor (optional, requires a multimeter): If you're comfortable with basic electrical testing, you can check the resistance values of each speed terminal on the resistor to see if they are within the specified range (found in a repair manual).
4. Check the wiring: Inspect the wires leading to and from the resistor for any damage or loose connections.
Repair:
Replacing the blower motor resistor is usually a straightforward repair. You can purchase a replacement at most auto parts stores. Remember to disconnect the negative battery terminal before starting any electrical work. Always consult your vehicle's repair manual for specific instructions and diagrams.
