I have recently bought a 1976 f250 with the 460 in it as a project to fix up. I have gotten to the point where I need to get all the lights working so I can get an inspection done. I suck at electrical work, I can re build engines but just can’t get the hang of electrical work lol. The rear signal lights work as well as brake lights and one of the front headlights ( assuming the other one is just burned out, but have not confirmed yet) and all dash lights work. the reverse lights, the four small lights on the sides, and both amber lights at the front do not work. All the bulbs look fine and all the fuses look fine as well. Any ideas? And if anyone knows what fuse to check that would be good to know. Can’t read much on the box so not 100% I was looking at the right fuse but they all looked good to me

 

Welcome to FTE

Electrical is not that hard.
It just makes a big loop from start all the way back to finish.
A test light, do not get a LED test light as they can throw you off on testing and a cheap volt / ohm meter. HF meter & test light will work.

First dont just "look" at the bulbs and fuses test them or replace them. Now just because they are new dose not mean they are good also but more they are if new.
I would replace the bulbs of the lights that do not work. Head lights replace both in pairs and they dim over time and most burn out at the around the same times.
I would also go over the grounds starting at the battery.
Battery> motor
Motor> frame yes frame needs to be grounded
Motor (intake bolt)> cab (firewall) Yes the cab needs to be grounded 10 gauge wire works.
It would not hurt to run a 10 gauge wire from frame to bed some where.
I dont know about your year truck but my 81 F100 had ground wires at each corner of the radiator support and fenders to ground the head & turn lights up front.
Note on older car / trucks the light housings need to be grounded through the screws that hold the housing in place.

With all the grounds in place and clean and bulbs & fuses replaced if the lights still dont work we will need to do a little testing.
You will need to get to the wires going to the light sockets and with the test light, can use meter but the light you can see right away.
See if you have power on any of the wires. If not you will need to trace back the wire looking to see if it is broken and where you get power.

That is what you will need to do for any lights not working, trace back till you get power or find a broken wire or bad switch.
Dave ----

 

Thank you for the response, I will give all of that a try when I get home tonight

 

Just take your time and if you need more help we are here for you.
Dave ----

 

So I installed new headlights, the side that worked is still working great, the side that didn’t is working but the light is really dim compared to the other side. I checked all the grounds and cleaned the sockets for the lights with baking soda and water, but the one is still very dim. As for the front signal lights, I replaced them too and they still don’t work. I checked the sockets with a volt meter but I don’t know if I did it right. I put the positive point from the meter in the socket and touched the negative to the bumper. It showed around 270 mv and when I turned the signal on I could see the voltage dropping and rising as it should. But still no light when I would install the light

 

There is a connector inside the engine bay, both sides, on the inner fenders. If I recall correctly it has 4 bullet connectors. I would disconnect that connector and check for corrosion there. While you have it apart check continuity of each wire from that connector to the head light, turn signal and side marker sockets. You should have close to zero resistance in that section. Not a bad idea to peel the old tape off that covers those wires and check for any cracks in the insulation, if everything looks and tests okay, tape it back up. If you find high resistance or any bad insulation, replace the wire and/or the connector that had the terminal with high resistance. If you do not find any issues in that portion of the wiring harness and you have unscrewed, cleaned screw, terminal and mounting location with a wire brush, sand paper or similar abrasive and re-seated all the grounds you see, you will know your issues are further back in the wiring harness.

 

That dim head light is a bad ground to that light.
It should have 3 wires in the plug. 1 is low other is high and 3rd is the ground.
With the test light and head lights on you should be able to find what wires do what and then follow the ground wire to where it is grounded at.
My 81 is at the fender / radiator support with a screw.
Pull the screw and sand the area to metal and the wire end and screw it back and see if it works now.

If working with a meter I would only check with volts. Should be 12 volts at the socket when on.
MV can give wrong readings. If you have 12 volts on & off at the socket then it is either the bulb or no ground at the socket.
Dave ----

 

Ok, thank you, will give all that a try later tonight

 

Ok, so I did more work on it tonight and I got the one head light to be as bright as the other (it was a disconnected wire that I didnt see) but still no luck with the signal lights. I took the connections apart that are on each side. There are 5 wires but 4 connections (2 female and 2 male) on the two connectors. I did some testing with a volt meter and the two that go to the head light are around 12 volts the one that goes to the little rail light is at 12 volts ( I still need to replace the bulbs on the side lights so I don’t know if they are working properly). The 4th connection goes to the signal light and only has a couple hundred mv .including grounds there are 3 wires that go to each signal light. I did some testing and and on the one side each wire had about 100 mv going through it. On the other side one of the wires had 12 volts and the other two had about 100 mv. But I didn’t do much work on this side because I ran out of time so I don’t know all the details about this side. Does the fact that the connection for the signal light is only getting a couple hundred mv mean the problem is further back?

 

Generally, yes. But it could be right at the connection as well.
Perhaps it has failed internally over the years. Would be a little unusual, perhaps, but not unheard of.
Although from your descriptions, I don’t recognize the connections. Maybe I haven’t worked on your year truck enough, but it sounds like an odd number of wires.
You are testing the running lamps with the headlight switch both at the first position, AND the second position, correct? The older design switches only had the front marker/running lights on with the light switch in the first position. Then when you pulled the headlight switch out to its full headlight on position, the marker/running lights went out.
And you are checking the turn signals with the key on. Also correct?

 

I forgot to mention. While it’s not as common with our trucks, as with some others, it’s not unusual for some wires to be loose, burned, or corroded, right at the headlight switch connector under the dash.

 

No I was only testing in the second position… I’m going to feel really smart if I go home today and it works just fine because I only had it in the second possession lol. But yes I was doing the signal light with the key on. If it is still getting low voltage once I test in the first position, I don’t think it’s the connection because I tested just behind the connection and it was still only a couple hundred mv. I thought it was a weird kind of connection too just because it has 5 wires and 4 connections but it looks good to me.

 

If it’s still acting up tonight I will follow the wires until I hit the switch and just keep testing them till I get proper voltage

 

Sounds good. But if you can post up some pictures of the connections you’re dealing with, that would help us out at this end as well.
Your truck should have a switch that activates the parking lights in both positions. I think that one position function ended somewhere back in 68 or so. But it doesn’t preclude someone putting in an old switch, or a new switch going bad, or even more common, a brand new switch being screwed up right out of the box! :-(

You might check back at the turn signal flasher/relay as one step. In this wire checking process.
There is a separate flasher unit for hazards and turn signals. They are usually interchangeable, but the four-way flasher unit is heavier duty.
Worth disconnecting the turn signal one and checking for voltage on one wire. Although these wouldn’t affect front marker lights, it would certainly affect turn signals.
Some of that bundle of wires are for turn signals in the front, while others are for the marker lights.
I thought side marker light wires were brown, so perhaps the front marker light wires are brown also.

 

Did some more work on it. I was able to get one front signal light to work. But it only works as a signal it does not light up at any other point other than when I am signaling. ( is that how it’s supposed to be? I know on my 87 the signal lights work as running lights and signal lights, is it the same on this truck or are they strictly signal lights?) On the other side it’s not working but I’m getting around 10 volts to the socket. I think tomorrow I will go to town and pick up bulbs for the little maker lights so I can see if they are working and I might get a new socket for the signal light to see if that fixes it. If that does not work I will post some pictures and slowly make my way further up the electrical lines.