Howdy!

I have this crazy situation going on with my 2001 F150. The driver side headlight went out, so I replaced it with a new one. It did not light up, so I changed it with another lamp. Still, no light up. I checked both fuses in the inside fuse box and the fuse under the hood - all were good. I took out my voltmeter and removed the bulb from the driver side headlamp connector. I measured 10.38 volts on the regular headlight side, and 12.3 volts on the high beam side. When I put the bulb back in, once again no light up on regular headlight, but the high beam worked ok.

Next, I removed the lamp from the passenger/right side. I measured the voltage and both high beam and regular headlight side read 12.3 volts. I went back to the driver side, and this time the socket read 12.3 volts on the driver/left side! I plugged the lamp bulb back in the driver side only and it shined brightly. I turned the switch off, and put the passenger side bulb back in also, then turned the switch back on. This time the driver side lit up, but it is very dim....not bright like it was previously. I removed the driver side bulb and once again the voltage read 10.38 on the volt meter. I'm going nuts! Could it be a bad ground wire somewhere? If so, where is it? Thanks!

 

I could be wrong but i think both filiments share the same ground wire, i think its only a 3 pin connector. How does the connector itself look?

 

Yes, both sockets accept 3 pins each. The center pin is the ground, The pin hole on the left is for high beam, the pin hole on the left is for the regular headlight, as you hold the socket with the connector tab on the bottom.

 

I think I figured it out. Very bizarre. I removed both light bulbs again and measured the voltage. The passenger side was fine at 12.3 volts, but once again, the driver side was 10.5 volts. While the switch was still turned on, I used a flattened piece of copper wire to ream out each of the pin holes in the connectors. They were pretty clean to begin with, and I had previously cleaned it with electrical contact cleaner spray. But I figured something must be partially insulating the connection. Once done, I measured the electrical output again and this time the voltage was at 12.3 volts. I put everything back together and all seems to be well now.

I figured one of two things occurred: 1. Either it was a dirty connection, or 2. While cleaning the connection, I pulled the wire sufficiently away from an area it was shorting against metal. But all is working now and only time will tell if it holds up! Thanks for all the help!

 

Had a similar issue a few years back. Driver’s side headlight wasn’t working. Wiggled the plug and it lit up.

Spliced a new plug in and been good ever since. I’m sure the plug warps just enough to not make secure contact over time.

 

Right,
Test the power to the socket with the lights on
Plan on replacing the socket