Headlamps not working?

Paul Steinberg

PCS Life Member President
Staff member
Super Site Supporter
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.
 
It also takes tons of time to learn how to use it. I am active on another forum and someone asked the members to post their least used tools, and the power probe was one of them. I have a power probe but never took the time to learn how to use it. Probably be a lot easier if I was to work with someone that already knew the tool and could teach me. I will look for a youtube video after posting this.
 
Let's see, has a button that you can apply a ground or voltage. The test lead lights red when you hit power and green when you hit a ground. There are some other models with more features but the basic one has one more moving part then a test light. If cannot use a test light, you should step away from repairing cars.
 
I have one also. Finding a lower source was my problem with it. But test light a small battery charger have stood me will for years. Don't forget to test the headlights also. I had a time tracing a power problem on one of the trucks. Till i figured out that both low beams had burnt out at the same time.
 
Power probe has 2 alligators clamps and comes with leads about 15 ft long, that attaches to the battery posts. Does not make much sense to be testing electrical components without a fully changed battery in place with a good connection of pos and neg connections at the battery.
 
Back
Top