My passenger side headlight on my 2005 S-type 3.0 went out and I replaced it with a new H7 bulb, but it is still not working.

I though maybe the bulb was bad and bought another one, but still not working. Checked the voltage at the socket and it is good (around 12.5 volts) when there is no bulb plugged in, but as soon as I plug in the bulb, the voltage drops to zero. The volt meter is hooked up to the back of the socket and even if I simply touch the bulb to the socket conectors the voltage drops to zero and returns to normal as soon a I stop touching it.

Is there a relay that might need to be replaced or is the computer controling the voltage to the socket and shutting it off as soon as it sees a load?

Obviously the fuse is fine or there would be no voltage at all. Any suggestions would be appreciated.
Troy

 

vanatic99, from the sounds of things, you have a wire that has broken inside the insulation and it is causing a high resistance connection and as long as there is no current flowing, it will provide the full voltage. But, as soon as any current is passed through the wire, the break will drop all the voltage, leaving you nothing at the far end. As to where it is broken, all I can say is that it is somewhere between the fuse box and the headlight. If it was further up stream, you would see both headlights affected. But, without a unit like a TDR (Time Domain Reflectometer), you will not be able to find the break unless the wire just happens to make a sharp bend somewhere and you can see the broken copper.

I know this might sound silly, but try swapping the fuses between the two headlights. It is possible that the fuse is developing an internal high resistance. I have seen this a few times and while it is not common, it can be an easy fix. The fuse will look good, but if you put a multimeter to it, you will see the higher resistance. While you are there, also make sure that the fuse fits into the opening snugly. If it is even a little sloppy, it may also lead to issues.

If you need more help, let me know.

 

Thank you Chris. Makes sense, I'll try replacing the socket this weekend and I'll try a run from the fuse box direct to the socket. I'll also try swapping the fuses left to right.
Troy