Hey all, looking for some help.

I have '99 F150 "scab" that I've owned for about a year. This past weekend it started popping the #6 fuse in the power distribution box located in the engine bay. After going through a series of fuses I've been able to determine that the with the light switch in the off position, then starting the truck, the fuse pops. Test number 2 was to not start the engine, install a new fuse and then turn on the headlights. This second test proved fruitful for about 5 seconds. I had a chime with the door open and all lights. Then nothing other than what I've discovered were DRL (who knew I had those).
The switch has **** for pull out fogs and the 2 rheostat dials. One for panel dim and the other for Auto Lamp delay. (similar switch shown below).

My first inclination was to just go buy a new switch, but, $200 for what could be of no help and non-returnable doesn't seem like a very solid value proposition to me. I've checked several scrap yards with no luck and found a few on eBay for less but not sure what I could end up with there either, and once again, once I pay, it's mine with no option to return it.

I'm sure I'm not the first one to come across this issue and hoping someone can lend some insight before I have to hurl myself to the mercy of a local shop to find a needle in the haystack.

Any and all help would be appreciated!

 

Is fuse #6 for the headlamps only? If the DRL's are coming on and popping the fuse then i'd think the short you're looking for is maybe not in the switch itself since it's blown the fuse in the "ON" and "OFF" positions. Not saying it isn't possible but maybe checking your wiring first would be a good start. Have you disconnected the headlights and checked that the plugs aren't melted or broken?

 

Let's get some more details and clarification before buying anything. The headlight switch itself would be very low on the list of suspects.

Any pertinent history (repairs, mods, vermin)?
Did fuse F06 blow in test #2? Your description is vague.
Does the fuse blow with the ignition switch in the OFF position? How about in the RUN position without starting the engine?
Have you tried disconnecting the headlight switch connector(s) and see if the fuse still blows when the engine is started?

FWIW, short circuits are among the more vexing electrical faults to diagnose. You'll need either a good measure of luck or persistence to find and fix the issue.

 

Thanks for the replies!
@ProjectSHO89 The most recent work was installing new upper control arms and front shocks. One thing during that process that I haven't had a chance to look at was the harness that comes out of the driver's front wheel well. There was a harness that was disconnected from the frame sort of flopping around. Not wanting it to get damaged or hung up on something in the suspension I pushed the plastic plug back into the corresponding hole in the frame. I took a cursory look at it before doing so as the previous owner wasn't great on preventative maintenance. His motto was "there wasn't anything that couldn't be fixed with super glue or a number 8 self-tapping screw" (that's a whole other story). I'll obviously take a closer look at that harness once the rain stops here.

Regarding test # 2. The fuse did indeed blow after about 5 seconds with the lights turned on and the vehicle not running. I'm going to try it again today using just the parking lights and see if I get the same result. I'll also try to test with the ignition in the run position as well as pull the headlight switch.

I've not done any sort of mods to the truck other than to get rid of most of the previously mentioned screws and work to try and get it back to as stock as possible. No signs of the mice or chipmunks under the hood or anyplace else.

I know that electrical gremlins can be the worst to try and figure out, but, the truck is super clean and reasonably sound mechanically so I'd hate to give up on it for lack of patience and what could be just one stupid wire.

Any further suggestions are always welcome. Thanks again to those that have offered up help so far.

 

You need to get your hands on a wiring diagram that shows circuit with #6 fuse. Then you will know what to start checking.

 

Turn both rheostat/potentiometers down all the way and see what happens. Might at least tell you what circuit the problem is on.

You might get a resettable fuse for your troubleshooting work. Then you can do things like unplug everything at the switch and plug them back in one at a time until it pops without spending money on fuses. Plus it's easier to tell when it goes. Turn the rheostats up, wiggle wires, etc. Your local auto parts store or sound shop might have them.

https://www.waytekwire.com/products/...cuit-Breakers/

https://www.waytekwire.com/products/...cuit-Breakers/

 

Just a little confused here, on the diagrams for a 1999 F150 fuse location, fuse #6 in the Power distribution block under the hood, shows that it is for the Radio and related electronics.

My wiring schematic shows that fuse #7 in the Power Distribution box is for the Main Light Switch and fuse #9 is for the DRL relay and the 12V feed for the Fog lights via the fog light relay which is controlled by the fog light switch.

This is from my Chilton Ford manual for 97-03 pickups/expedition and navigator models.

 

I have the Ford diagrams in front of me and are presumed to be correct (at least,most of the time). The description provided of the Chilton diagrams is not correct.

Neither of the "rheostats" should have any connection to the fuse F06 circuit unless their is an internal fault in the switch itself.

Let's wait until the requested tests are done and the feedback is posted before randomly throwing any more stuff against the wall.

 

The fuses are shown in the Owner Manual also. Not the actual circuits though.

Page 154 - 155

6 15A* Parklamps/Autolamps

https://www.fordservicecontent.com/F.../99f12og1e.pdf

 

Hi, Any luck figuring out what was going on? I am having the same problem with my 99 F150