* Headlight Motor Issues: The most common culprit. The motor itself might be failing, experiencing intermittent electrical connections within its windings, or have worn gears causing inconsistent operation. The motor might be binding due to age and lack of lubrication.
* Wiring Problems: Look for:
* Loose or corroded connections: Check all wiring connections related to the headlight motors, including the switch, relay (if applicable), and the motors themselves. Corrosion is especially common in older cars. Pay close attention to connections near the headlight assembly as these are subject to weather.
* Broken or frayed wires: Inspect the wiring harness for any damage, especially where it bends or flexes. A chafed wire can intermittently break contact.
* Relay Failure (if applicable): Some systems use a relay to control the headlight motors. A failing relay can cause intermittent operation.
* Headlight Switch: The switch itself could be malfunctioning, causing inconsistent signals to the motors. Internal contacts may be worn or dirty.
* Vacuum System (if applicable): Some older cars used vacuum to actuate the headlights, though less common on this model year. If your Celica uses a vacuum system, a leak in the vacuum lines, a failing vacuum pump, or a problem with the vacuum reservoir can all lead to erratic operation.
* Fuse: Though less likely to cause intermittent operation, a failing fuse could show as inconsistent function. Check the fuse related to the headlight motors.
* Grounding Issues: A poor ground connection can result in intermittent electrical functions. Check the ground points for the headlight system.
Troubleshooting Steps:
1. Visual Inspection: Start by visually inspecting all wiring, connectors, and the headlight motors themselves for any obvious damage, corrosion, or loose connections.
2. Test the Headlight Switch: If possible, try bypassing the switch to see if the motors work directly with power. Caution: Always disconnect the negative battery terminal before doing electrical work.
3. Check for Power and Ground: With a multimeter, test for power and ground at the headlight motor connectors when the switch is activated. This will help determine if the problem is with the switch, wiring, or the motor itself.
4. Test the Headlight Motors: If you can safely access the motors, you might be able to test them directly with a 12V power source (again, with the battery disconnected first!). This will determine if the motors are the problem.
5. Inspect Vacuum Lines (If Applicable): If your system uses a vacuum system, check the lines for leaks and the vacuum pump for proper operation.
If you are not comfortable working with car electrical systems, it's best to take your Celica to a qualified mechanic. Working with car electrical systems can be dangerous if you don't know what you're doing.
