No headlights, no taillights, no clue.

I'm trying to get my '91 plow truck ready to go on the road, and now that I need them the headlights and taillights have stopped working. They were working last winter when I was plowing.

Here's what I know:
Bulbs are good.
I think all the fuses are good - at least all the ones I've tested.
I was able to get the headlight switch out of the dash, and I can see that it and its connector have been replaced fairly recently. No signs of heating or anything.
Turn signals work, dash lights work, brake lights work, even the marker lights on top of the cab work. Headlights and tail lights don't.
I tried switching the dimmer switch, that doesn't make any difference.

Specific questions:
1) What is the pinout of the headlight switch connector, as we look at the backside of it? I want to try back probing it.
2) Is fuse #11, 15A, the one that sends power to the headlight switch? That's what the fuse panel layout in the truck says.That fuse is good.
3) What in the world could cause both the headlights and taillights to go out at once?
4) If there is a fusible link to check, where is it located?
5) What else should I check?

It's odd the dash lights work in the tail lights don't because the tail light turns on the dash light in the top markers so check the specific tail light fuse with a test light make sure it has power on both sides. The fusible links to feed the headlight switch and there is two should be out by the starter solenoid they'll be on I believe yellow wires cuz that's battery hot and just give him a give the black lynx a little bit of a tug and you'll know right away if one is burnt

Thanks, Russ. I'll check the tail light fuse and tug on those fusible links tomorrow.

I checked the wires coming off the starter solenoid. There is one that is obviously a fusible link: a short bit that joins to a black and a yellow. All the wires are good, stand the tug test, no signs that the copper has failed inside the insulation.

I found a picture of the back of the headlight switch connector at https://www.broncocorral.com/tech_library/ford-bronco-ii-headlight-switch-wiring-testing/
I do have battery voltage at the B1 and B2. That means that the fusible links really are good, right?

I'm not sure what fuse covers the tail lights. Is it that number 11? Can't find any fuse listed which mentions tail lights.

I have to go to work now. I'll check for voltage at H, which goes to the dimmer switch, after work.

Edited to add for future reference, from the link:

Pin Wire Color Description
B1 Black with orange stripe Battery (+)
B2 Lt Brown (tan) with white stripe Battery (+)
D1 Green with yellow stripe Power (+) for interior lamps
D2 Black with violet (pink) stripe To all interior lamps
H Red with yellow stripe To headlight dimmer switch
I Blue with red stripe To instrument panel lamps
IGN Yellow with black stripe Battery (+)
R Brown To exterior lamps
DN Orange with black stripe To radio illumination lamps

Are they LEDs? My tails went out after some time and were fixed by putting incandescent bulbs back in. Can check the plugs for power too after checking everything else.

Scotty4 said:

Are they LEDs?.... Can check the plugs for power too after checking everything else.

Click to expand...

No, not LEDs. I can't find a flasher that works with LEDs, so I'm sticking to incandescent.

No power at the tail light bulb sockets.

I had a similar issue with my tail lights a couple months ago, it ended up being a bad hot wire going to the fusible link for the lights. Couldn’t find the hot wire in the jumbled mess so I just spliced it with a short wire into the hot side of an empty fuse slot and works like a charm now.

So I’d recommend taking your taillight and headlight fuses out and check for power between the the fuse ports, the hot fuse port and the battery, and the output side and switches and lights.

Im thinking it’s either that, or you have just this problem with the headlights and the taillights aren’t working because of a bad brake switch which is pretty common.

It’d be nice to know what rig you got too, put it down in your signature so we can see.

Also, nice to see another proud Alaskan idi owner on here!

stick_witch said:

It’d be nice to know what rig you got too, put it down in your signature so we can see.

Click to expand...

Stick, are you not seeing the seven lines of vehicle in my sig? Should look like this:
'83 240D
'87 F350 4x4 6.9L T19, 4.1 gears, regular cab with Banks Sidewinder,
'89 F-Superduty 2wd 7.3L 7.3L ZF5, 5.13 gears, regular cab NA,
'89 F350 4x4 7.3L ZF5, 4.5 gears, crewcab NA,
'91 F250 2wd 7.3L E4OD, ?? gears, supercab NA,
'91 F350 4x4 7.3L ZF5, ?? gears, NA regular cab dualie plow truck,
'94 F350 4x4 7.3L E4OD, 4.1 gears, NA crewcab

So, the problem with the headlights was the dimmer switch. It had failed open on both settings, there was lots of green crud, and it fell apart as I tried to pull it off the connector. The headlights are on again.

Now I just need to figure out the taillights.

Stick, you're saying that there is a taillight fuse? Do you know which that is? Where is that brake switch?

Each pedal (ebrake, clutch, brake) should have a switch connected by the linkage under the dash.

https://www.autogenius.info/ford-f350-1987-1991-fuse-box-diagram/

Looks like fuse 1, top right, 15 amp.

I said the rear brake lights work, but they don't work well; they're dim. The tail lights have the two prong bulb, one filament for tail light one filament for brake light. When we push the brake pedal, I read about +7V above ground on the brake light prong in the socket, and about -3V below ground on the other prong. That's not right.

It looks to me as if the wire labeled R in the drawing above sends power to all the exterior lights except the headlights. So, it's sending power to the front parking lights, the rear parking lights (the ones that aren't working), the marker lights on top of the cab, and so on. When I pull the headlight switch out one or two clicks, the R wire does have battery voltage on it, and all the exterior lights come on EXCEPT the rear parking lights and the marker lights along the side of the bed, which seem to be fed from back by the tail light fixtures.

Where does the R wire go once it leaves the headlight switch? I want to follow the voltage down the wire until it's gone, but I don't know where it goes next. I figure that there must be a problem either where the R wire branches out to all the several exterior lights, or somewhere between that branch point and the tail lights.

Does anyone have a schematic that shows the circuit for the parking lights for a '91 F350?

Scotty, I'm at work now, I'll check that fuse 1 this evening, thanks. Since the brake lights do come on, sort of, that probably indicates that the brake switch is OK. The ebrake switch could be bad. I'll press down the ebrake pedal and see if the brake light on the dash comes on, and test the switch further if it doesn't.

I followed that link, and sure enough, it says fuse 1: stoplights. I was fixated on brake lights and totally missed that. Since the brake lights - oops, stoplights - do come on, sort of, that fuse is probably going to be OK.

On my bullnose, with floor switch, the redwhite wire goes from dimmer switch, to the floor, and then exits as two wires, high and low. From there it goes out and splits to each socket L/R.

For the tails and markers it has a double brown going forward and rear. In the rear it goes to a plug just behind the rear tank and from there splits to each side. The reverse, license lamp, brake, and ts wires all go there as well. There should be a ground there for the license lamps, and one on each light. My truck had the trailer hookup spliced there and was all messed up.

I can't confirm wire colors but would assume this is close to the same. Though, you probably have the head light switch on the column?

I cleaned it all up, removed the splice connectors, and cleaned everything up, also pulled LEDs I had put in and it works great now.

My '91 plow truck has the dimmer switch on the floor, and the headlight switch on the dash. That's so much nicer than the OBS mess in the column.

Thanks for the info on the plugs. Mine was probably the same when it came from the factory as a chassis-cab. My truck has a nice steel flatbed, and non-factory lights on the bed of course. If the bed installer plugged his harness into the stock plug, I'll probably find a connector where you are describing, behind the rear tank.

I got to thinking about the funky voltages I found in the taillight socket. I'm wondering if one of the many parking lights around the bed have a short to ground. That should be blowing fuses, shouldn't it? So far I haven't found a blown fuse.

One thing I need to do is make sure that the bed has a good ground to the frame, and that the frame has a good ground to the batteries. None of the lights on the bed that work are bright. The front parking lights are bright.

The taillight problem came from the bed not being grounded. I cut off a bad marker light on the bed, which may have been a fault to ground, then when I clamped one end of a jumper cable onto the frame and the other end onto the bed the lights lit up.

Scotty, I do have the factory connector there behind the rear tank. It looks as if the bed installers did use a Ford kit to hook their flatbed to the Ford wiring.

Everyone, thanks for the help.