* Worn-out blower motor brushes: This is the most likely cause. The brushes are small carbon components that make contact with the motor's commutator. As they wear down, the contact becomes intermittent, leading to the fan running sometimes and not others. The tapping often helps temporarily re-establish contact.
* Loose or corroded connections: Check all wiring connections at the blower motor, the resistor pack (if your Astro has one – these control fan speeds), and the switch. Corrosion can interrupt the flow of electricity, causing intermittent operation. Loose connections can vibrate loose, further exacerbating the issue.
* Failing blower motor resistor: The resistor pack reduces voltage to the motor to control fan speeds. A faulty resistor can cause problems on certain speeds or even completely fail, resulting in intermittent or no fan operation. This is especially relevant since you mention the problem occurring after the fan was running only on high. High speed usually bypasses the resistor.
* Internal blower motor failure: While less likely given the 12V reading, there could be an internal short or other problem within the motor itself that is causing intermittent operation. This often involves damaged windings or a failing capacitor within the motor.
* Switch failure: Though less common, the blower motor switch itself could be faulty. It may be intermittently making or breaking contact.
Troubleshooting Steps:
1. Check all connections: Carefully inspect all wiring connections from the blower motor switch to the blower motor, paying close attention to the resistor pack (if present). Look for corrosion, loose wires, or broken wires. Clean any corrosion and secure any loose connections.
2. Test the blower motor resistor (if applicable): If you have a multimeter, you can test the resistor for continuity and correct resistance values according to its specifications. A faulty resistor will often show open circuits or incorrect resistance values.
3. Inspect the blower motor: If you're comfortable working with electrical components, you might try to carefully disassemble the blower motor to inspect the brushes. Worn brushes are usually visibly short. Replacing the brushes is often a relatively inexpensive repair.
4. Test the blower motor directly: If you can safely disconnect the blower motor from the vehicle’s wiring and apply 12V directly to it (with appropriate safety precautions – use alligator clips and protect yourself from spinning blades!), this can isolate whether the problem is with the motor itself or the wiring/switch/resistor. If it works consistently on direct power, the problem lies elsewhere in the circuit.
If you're not comfortable working with car electrical systems, it's best to take your Astro to a qualified mechanic. Diagnosing and repairing these issues can be tricky, and working with the blower motor requires some careful handling.
