1. Power Supply:
* Fuses: Check the fuse box for any blown fuses related to the power seats. Your owner's manual will show you the fuse layout. Even if a fuse isn't completely blown, it might be weak and intermittently failing.
* Wiring: The wiring harness for the seats is old and may have deteriorated. Look for broken, frayed, or corroded wires, particularly where the wires connect to the seat motors and the main harness. Check connections at the seat itself and under the carpet near the seat.
* Switch: The power seat switch itself could be faulty. Try cleaning the switch contacts with contact cleaner (electronic cleaner, not general purpose cleaner). If the problem persists, the switch likely needs replacing.
* Ground Connection: A poor ground connection can interrupt the power flow. Make sure the seat and its frame have good ground connections to the car's chassis. Clean any corroded areas.
2. Seat Motor(s):
* Motor Failure: The motor(s) themselves might be burned out, seized, or worn out from age. This is a common failure point in older cars. Testing the motors requires some electrical knowledge and a multimeter.
* Gearbox Problems: The gears within the motor's gearbox can strip or become damaged, preventing movement.
3. Other Components:
* Seat Control Module (if equipped): Some higher-end vehicles of that era might have a dedicated control module. If your DeVille has one, it could be at fault. This is less likely, but possible.
* Relay (if applicable): There might be a relay controlling power to the seat motors. A bad relay could cause intermittent or complete failure.
Troubleshooting Steps:
1. Start with the simplest: Check the fuses first.
2. Visual Inspection: Carefully inspect all wiring related to the seats. Look for obvious damage.
3. Test the Switch: See if you can get any movement with the switch or if it's completely unresponsive.
4. Check Grounds: Make sure all ground connections are clean and tight.
5. Test the Motor(s) (advanced): If you have some electrical knowledge, you can test the motors for continuity and power using a multimeter.
6. Consult a Wiring Diagram: A wiring diagram for your specific year and model will be invaluable in tracing the circuit. You might find one online or in a repair manual.
Important Note: Working with car electronics can be risky if you're not familiar with them. If you're not comfortable working with electricity, it's best to take your Cadillac to a qualified mechanic specializing in classic cars. They have the experience and tools to diagnose the problem accurately and safely.
