* Failed Blower Motor Resistor: This is the most common culprit. The resistor controls the different fan speeds. If it fails, it might short out, sending full power to the motor all the time, regardless of the switch position. This is particularly likely if the fan *used* to work on various speeds, but now only runs at full blast or not at all.
* Faulty Blower Motor Switch: The switch itself could be malfunctioning. A short in the switch can send a constant signal to the motor, even when the switch is in the "off" position.
* Wiring Issues: A short circuit in the wiring harness leading to the blower motor or the resistor can also cause the motor to run continuously. This could be from damaged insulation, frayed wires, or a connection that's come loose and is making unintended contact. Rodents chewing on wires are a sometimes overlooked cause.
* Control Module (Less Likely): While less common in older cars like the '89 Oldsmobile, a failing control module could also contribute. This is more likely if other electrical components related to the HVAC system are also malfunctioning.
* Vacuum Leak (Very Unlikely but Possible): Some older cars used vacuum systems to control various aspects of the HVAC. A severe leak in the vacuum lines controlling the blower could lead to unexpected behaviour, but this is less probable.
Troubleshooting Steps (in order of likelihood):
1. Check the Blower Motor Resistor: This is usually located somewhere behind the glovebox or under the dash. Accessing it may require removing some trim panels. Inspect it for signs of burning or damage. Replacing it is relatively inexpensive and easy.
2. Inspect the Blower Motor Switch: This is usually located on the HVAC control panel. Disassembling the panel may be needed for inspection. This is a little more involved than replacing the resistor.
3. Check the Wiring: Carefully examine the wiring harness leading to the blower motor and the resistor. Look for any signs of damage, fraying, or loose connections.
4. Test the Blower Motor Switch (Advanced): If the resistor and wiring look good, testing the switch with a multimeter can determine if it's sending a signal even when off. This requires some electrical knowledge.
If you're not comfortable working with car electronics, it's best to take the Oldsmobile to a qualified mechanic. The diagnosis is usually straightforward, but getting access to the components can be tricky.
