I know this might be hard to pinpoint, but I am hoping someone smarter than me with the electrical system in my truck might be able to at least help direct me in the right direction. I am working on a 2004 F150 with the 5.4 Triton engine. I am using the power probe II to test the wires. The left headlight was out. I replaced the bulbs. Still didn't work. I replaced the connector and the hi-beam worked but the low beam doesn't work. So I started out assuming the low beam wire must be broken somewhere. The right headlight works perfect. The probe shows that there is continuity on that wire, however when I put power to it from the probe, it breaks the circuit breaker in the tool. I have worked my way back to the relay box on the left fender well under the hood and each place I probe it does the same thing. I don't think it is a ground issue because the hi-beam works.

Now a couple days later I had another electrical issue develop and I doubt it is related but I am sort of at a loss. The dash lights, and the taillights have also stopped working. The brake lights and the turn signals work. Using the probe, the wires showed continuity but when I put power to them to try to turn them on it broke the circuit breaker on the probe. I tend to believe that this is maybe a ground issue, but I am not sure where to start looking as is had both the dash lights and the tail lights involved. There is one other issue that developed at the same time. The fog lights only work if I turn the high beam lights on. They worked correctly up to this point, but now they only work when the high beams are turned on. If I pull out the fog light part of the switch nothing happens. When I tested the switch with the lights one, I had continuity when the fog light portion was pushed in, And when the high beams were on, but when the switch was pulled out and only the low beams on I got a lower than 12 volt voltage reading but again the lights weren't on.

I am at a loss and hoping maybe someone can at least point me in the right direction. Thank you very much in advance.

 

The Driving lights do not 'normally' power up unless the low beams are on and go off when high beams are on.
Wired that way from the factory, by Law since the early 80s on all Ford vehicles.
I would look behind the driver kick panel where a lot of harness comes and a main ground point..
It may be rusted.
Good luck.

 

In this issue I would definitely use a wire tracer. Best tool to use in this type of issue. You can locate bad grounds and follow single wires from beginning to end. Find weak spots in the system. Btw have you tested the relays. I worked on a 04 with a melted fuse box. Couldn't tell from the outside but everything led to the fuse box. After installing the new box. I couldn't let my mind always wonder IF that was actually the problem. So I did what I've been doing since I was a little boy. I cut it open to find out what it looked like. And indeed melted plastic between the plates causing circuits to cross and get power from other sources and voltage spikes.