* Stuck Blower Motor Resistor: This is the most common cause. The resistor controls the blower motor's speed. If it fails, the motor may run at full speed constantly or not at all, depending on the nature of the failure. The resistor often overheats and fails, sometimes even melting its plastic housing.
* Faulty Blower Motor Switch: The switch itself might be malfunctioning, sending a constant signal to the motor even when the switch is off. This is less common than a bad resistor, but possible.
* Short Circuit in the Wiring: A short circuit in the wiring harness leading to the blower motor can bypass the switch and resistor, causing the motor to run continuously. This could be due to damaged insulation, chafing wires, or corrosion.
* Faulty Control Module (Less Likely): In some vehicles, a control module manages various functions including the blower motor. While less probable in a 1992 model, a faulty module could cause this issue.
* Direct Power to Blower Motor: A very rare possibility is that there's been some modification or damage that's directly supplying power to the blower motor, bypassing all the normal controls.
Troubleshooting Steps:
1. Check the Blower Motor Resistor: This is the easiest and most likely place to start. Locate the resistor (usually under the dash, often near the blower motor itself). Inspect it visually for signs of burning, melting, or damage. If you see damage, it needs replacement.
2. Check the Blower Motor Switch: Try turning the switch on and off repeatedly. If it feels loose, sticky, or unresponsive, it might be faulty. Replacing the switch is more involved than the resistor.
3. Inspect the Wiring: Carefully examine the wiring harness leading to the blower motor, the resistor, and the switch for any signs of damage, such as frayed wires, bare spots, or corrosion. Repair or replace any damaged wiring.
4. Disconnect the Blower Motor: As a test, disconnect the power connector to the blower motor itself. If the problem persists, the fault lies somewhere in the circuit before the motor. If the motor stops, the problem is within the motor or its wiring.
If you're not comfortable working with car electronics, it's best to take your 1992 Oldsmobile 98 to a qualified mechanic. Incorrectly diagnosing and repairing electrical problems can lead to further damage or even fire hazards.
