1. Power Supply:
* Fuse: Check the fuse for the power window circuit. The owner's manual will tell you which fuse to check. A blown fuse indicates a short somewhere in the system. Replace the fuse, *but don't run the window until you've found the short circuit*. If the fuse blows immediately upon replacement, you have a serious short.
* Wiring: Inspect the wiring harness from the switch to the motor. Look for:
* Broken wires: Look for any exposed wires, cuts, or frays, especially near the door hinges where wires flex.
* Corrosion: Check for corrosion on the wire connectors and terminals. Clean them with a wire brush and electrical contact cleaner.
* Poor connections: Make sure all connectors are securely plugged in. A loose connection can cause intermittent power.
* Ground: A poor ground connection can also cause this problem. Trace the ground wire from the motor and make sure it's securely connected to a clean metal surface. A loose or corroded ground will restrict current flow.
2. Regulator:
* The window regulator is the mechanism that moves the window. A faulty regulator can cause the motor to strain and stop. If the motor is running but the window isn't moving smoothly or at all, the regulator is a strong suspect. This often manifests as slow movement and stopping. You'll need to visually inspect the regulator for binding or damage. This often requires partially disassembling the door panel.
3. Motor:
* While you've replaced the motor, it's possible the *new* motor is faulty (though less likely).
* Even with a new motor, if the regulator is binding, it could still overload and stall the motor.
4. Switch:
* Although you replaced the switch, double check the wiring and connection at the new switch to ensure it's properly installed and making good contact.
Troubleshooting Steps:
1. Start with the simplest: Check the fuse first.
2. Visual inspection: Carefully examine all wiring for damage.
3. Test for power: Use a multimeter to check for power at the motor connector when the switch is activated. You should have 12V present if the switch and wiring are good.
4. Inspect the regulator: This often requires removing the door panel. Look for binding, broken parts, or anything obstructing its movement.
Important Considerations:
* Safety: Disconnect the battery negative terminal before working on the electrical system.
* Door Panel Removal: Removing the door panel is usually necessary to access the regulator and motor. There are many online videos demonstrating this process for a 1990 GMC Vandura.
* Professional Help: If you're not comfortable working on car electrical systems, it's best to take it to a qualified mechanic.
By systematically checking these areas, you should be able to pinpoint the cause of the problem and get your power window working again. Remember to replace the fuse *only* after you have ruled out a short circuit. Otherwise, you may damage the wiring or other components.
