I was recently asked how to diagnose nonworking headlamps. These instructions apply to headlamps with a floor-operated dimmer switch. If you have questions, feel free to ask.
Most likely is poor ground or poor contact at the back of the headlamp itself where the prongs are subject to corrosion. The best way to troubleshoot headlamps is to remove both headlamps from one side and use a volt meter or test probe and check for voltage at the sockets. On the low beam socket, there are both positive and negative terminals. You can check the grounds with an ohm meter, and the positive wire with the test probe to determine which of the 3 black wires is hot and ground on the 3 terminal bulb. Then do the same for the 2 terminal bulb. If there is no power, then go to the floor switch and check the terminals there. 2 of the 3 should be hot and the one that isn't will be when you depress the switch. The top single wire is 12 volts into the switch and the bottom 2 are split, one going to the low beams and the other going to the high beams depending on the depression of the foot switch. If you don't have power at the foot switch when the headlamp switch is pulled out, then you have a defective headlamp switch or it has become unplugged. To remove the headlamp switch you have to pull the switch staff out, and then reach up and depress the small button on the top of the headlamp switch and continue to pull on the staff. Once it comes out, using a tool unscrew the headlamp switch nut and lower it towards the floor. You can then probe the various terminals to see if there is power coming into the switch. If no power comes into the switch, then the problem lies somewhere within the harness and that is a troubleshooting lesson for another day.
Most likely is poor ground or poor contact at the back of the headlamp itself where the prongs are subject to corrosion. The best way to troubleshoot headlamps is to remove both headlamps from one side and use a volt meter or test probe and check for voltage at the sockets. On the low beam socket, there are both positive and negative terminals. You can check the grounds with an ohm meter, and the positive wire with the test probe to determine which of the 3 black wires is hot and ground on the 3 terminal bulb. Then do the same for the 2 terminal bulb. If there is no power, then go to the floor switch and check the terminals there. 2 of the 3 should be hot and the one that isn't will be when you depress the switch. The top single wire is 12 volts into the switch and the bottom 2 are split, one going to the low beams and the other going to the high beams depending on the depression of the foot switch. If you don't have power at the foot switch when the headlamp switch is pulled out, then you have a defective headlamp switch or it has become unplugged. To remove the headlamp switch you have to pull the switch staff out, and then reach up and depress the small button on the top of the headlamp switch and continue to pull on the staff. Once it comes out, using a tool unscrew the headlamp switch nut and lower it towards the floor. You can then probe the various terminals to see if there is power coming into the switch. If no power comes into the switch, then the problem lies somewhere within the harness and that is a troubleshooting lesson for another day.
