My well worn 2005 Super Duty with a 5.4 just lost turn signals, brake lights and flashers. Other lights work fine. I checked all fuses (mini fuses) that can be visually be checked and are fine. I don't have a manual but replaced fuse 18 anyway, to no avail.

Any ideas what to check next? Thanks!

 

Did you actually check for power on both sides of F18 (under dash, not under hood) with a meter or test lamp?

Do that.

 

Yes, did that. 12.82 volts on either side of the fuse.

 

Does the high mount (3rd) stop light work or not?

4-way flashers, front and/or rear?

 

No, none of them work. I have replaced the Brake Pedal Position switch, with a spare that a friend had, and the multi-function switch, for with I had a spare.

I am about to delve into the circuitry. I borrowed a ford service disc from a friend. I can bring up diagnostics for brake lights out or flashers/turn signals out, but not both, so I am looking at common items from both first. The only common item that I can see is the Multi-function switch harness.

This assumes that there is only one thing wrong. Everything worked fine until just before it stopped. No warning.

 

The only thing that is common to everything is F18 and its circuit feed to the first splice. After that, it would take two faults to equal your set of symptoms. The 3rd brake light does not go through the MFS, for example, so a burned out bulb there plus an MFS issue could give you your set of symptoms.

Check for power from F18 at the brake pedal switch first. If good, push the pedal and test the output. If good, move up to both sides of the MFS. You'll need to look up the wire colors and pinouts on the disc you now have, I don't have mine handy at the moment.