Over the past month, the instrument cluster lights on my 1986 F-150 302 EFI with 187,000 miles have been getting dimmer and finally went out completely about three days ago. Yesterday I was stopped by a police officer for having no tail lights. Upon inspection, I find the front parking lamps, front fender marker lights, and license plate lights also do not work. All other lights (headlights hi/low beam, emergency flashers, turn signals front/rear, cargo light, and interior lights) work. Coincidentally, the front parking lamps, front fender marker lights, and license plate lights are all on the same fuse per the owner's manual. I removed and visually inspected all fuses and tested each with a DVOM - all fuses tested good. I checked the ground wires to the non-working lights and the mounting points are clean and tight. If I run a wire from battery positive to just ahead of where the hot wire enters the receptacle of any non-working light, the light works so it seems this would confirm that it is a power problem and not a bad ground. Since multiple lights are not working, it seems more of a control issue rather than a wire-to-the-light issue. I am not experienced at reading wiring diagrams but wonder if there may be a relay involved and that the relay is stuck open (or closed). Sorry for the long post but as you can tell I am struggling. Any helpful comments are welcome and appreciated.

 

You are a leg up on most people asking these questions because you understand some basic diagnostic procedures. First though it would make it a lot easier to test most of the hot side with a test light and not a volt meter. It is pretty basic though to test a power path. I don't think (not 100% sure) that there is a relay involved. The first thing I would suspect is the headlight switch. If the harness will allow, pull the switch out and down where you can get at it to back probe for power. At that point you can also make a jumper wire, find the hot lead in the switch harness connector, plug the test lead into and see if they light up if they do, switch may be bad. If not, next thing to check would be the bulkhead connector. With the headlight connector un hooked you can use your ohm meter and hook it to the + on any of the bulb sockets and probe each of the wires in the connector to see if you have continuity in the circuit.

 

Thanks for the kind words - at least I know I don't know much. I did have a test light and have now tested each fuse with the test light and each fuse tested good, but I replaced the "tail light, parking light, marker and license plate" fuse just to be sure. As you say, there seems to be no relay involved though I did replace the trailer light relay "just in case". I replaced the headlight switch with a Napa part but did have to re-use the old switch push/pull rod as the Napa switch did not come with a push/pull rod. Sorry I do not understand the last two sentences of your post. Where is the bulkhead connector? Is the headlight connector you mention located where the wires enter the headlight? Sorry for my limited knowledge. Thank you.

 

Not sure but I think a bulkhead connector he's talking about is a whole-harness connection between the cab & engine bay... the 90s trucks have those, we don't.

It seems you have a problem in that brown-wire circuit that feeds all that stuff.

My own suspicion is melted wiring at the headlight switch, there's a tan-colored wire that can overheat and the wires at the switch area melt together and cause all kinds of problems.

Another potential problem is a (partial) short circuit someplace in that wire and most power goes THERE instead of to where it's supposed to go. Such shorts aren't always bad enough to blow a fuse but will provide a ground path for the electricity (which takes the path of least resistance).

If you have overhead cab marker lights, they're also fed by that same brown wire, the connector is near the parking brake pedal.

Another common place for problems is user-added trailer wiring connectors at the back of the truck.

 

I will look for the bulkhead connector as you suggest. Yes sir, I think you are right on target regarding the brown wire as we are able to run a jumper wire from battery positive to a push pin inserted just before where the wire enters the light socket on any of the non-working lights and the light works. This would seem to confirm that the issue is a power issue a not a ground issue. I did replace the headlight switch with a switch from Napa but did not see any melted wires in the area - the wires actually look newish as shown in the photo of the instrument cluster connector in my earlier post. I will check again for melted wires in the headlight switch area. The (partial) short circuit is more of a challenge with my limited knowledge. Is there any way to test for a (partial) short circuit? No issue with cab marker lights or user-added trailer wiring as I have neither. Thank you for your help.

 

Here is a picture of the instrument cluster connector.

 

Hmmm...

The EVTM can help you, Gary's site has the entire thing but the Exterior Lamps stuff is right here:

Exterior Lamps - ???Gary's Garagemahal

As for finding WHERE the actual problem is, let's ask @kr98664 for help, he can be pretty entertaining to read.

 

Take the headlight switch out again, plug it up into the wiring, and then pull the switch out and check for voltage on the tan/white wire. That is the wire that feeds the lights, goes through the switch, and then out on the brown wire that runs all over the truck. That fuse you looked at feeds the tan/white wire.

And no, this does not feed the headlights, that is a different part of the same switch.

Make sure you keep everything plugged in and hooked up when testing, to keep a load on the circuit and make your tests valid.

 

Mr. Franklin2 -
Sorry to write back before proceeding but I do not understand your instructions. I can certainly take the headlight switch out again but how do I "plug it up to the wiring" without reinserting it into the connector from the fuse panel? And if I "pull the switch out" to check for voltage on the tan/white wire, am I "keeping everything plugged in and hooked up while testing"? Not trying to be a smart-aleck but I want to follow your instructions correctly. Thank you.

 

I think he's saying to physically remove the headlight switch, route the electrical connector so it comes out below the dash near the column, and reconnect the two down there and do your testing there. I don't remember how physically possible this would be to do but it's what comes to my mind.

"Pull the switch out" sounds to me like pulling the **** out, just as if the assembly was attached to the dash in its normal position.

 

Thats it. Sorry i wasnt more clear.

 

ctubutis - thanks for the explanation.
franklin2 - no apology necessary - you are helping me.
Last night I did remove the new headlight switch then touched a test light's probe to the female connector where the tan/white wire inserts into the plug that plugs into the headlight switch and the test light lit up. Seems that would indicate we are getting power from the fuse block to the headlight switch. Is that the test we need to do?

Edit: The wire I am testing is (best I can see) is black with an orange or pink or red tracer. I found this wire going into the back of the fuse block (it's the wire above and to the right of the brown/white wire in the block photo), and also going into the back of the plug that plugs into the headlamp switch (it's the bottom wire in the plug photo). I could not see it in the truck but in the photo of the back of the plug it looks as though the brown/white wire (the white is about faded away), is next to the black/orange wire in the back of the plug. Sorry for the poor photos but this is a very tight space.

 

http://www.garysgaragemahal.com/component-testing1.html

 

You need to make sure you are testing the correct tan wire with the white stripe. There are several hot circuits to that switch, and they serve different things. The headlights work, so that is a hot wire going to the switch that is good. But if the tan/white is dead, you will have nothing on the running lights.

It's best to do the testing with the switch plugged in. If you can't for some good reason, then probe it like you are doing, but that can lead to false positives. With the switch plugged in, you can also activate the switch, probe the tan/white, and then probe the brown wire. The switch transfers the power from the tan/white to the brown, so you will be able to test it all right there.

If you want to approach this differently, you can get a small battery charger, hook the negative lead to a good body ground, and then hook the + lead to some sort of probe like a clean nail or even the unplugged probe on a meter. Then with the charger on 12v, you can probe the brown wire, when you do all the lights outside should come on. You could plug the switch in the connector, pull the switch **** out, and then probe the tan/white wire, when you touch that the lights should come on again. If they come on and then go out when pull away from the tan/white, then you know the tan/white is dead and is probably burnt somewhere up in the harness, or there is a problem with the fuse socket or the fuse box. You could leave the switch on, and go probe the fuse with 12v and see if the lights come on.

 

Hard to tell, but it looks like the Tan/White wire is missing off the connector.....