Running lights
FTE Legend
Joined: Nov 2015
Posts: 30,935
Likes: 4,124
From: Angier, NC
I can only think of 2 reasons:
The running lights I believe are on a fuse, it could be bad. Also because you have the glass type the ends & holders sometimes get a film and dont make a good connection so pull and clean fuse & holders.
The other would be a bad head light switch. If it is OEM to the truck you may want to change it out for a new one.
The last thing is to add a head light relay kit. It will make the head lights brighter and it takes the load off the head light switch and that will keep it from tripping the breaker built into the switch.
You can get the relay kit from LMC, DC, JBGY and think Ebay.
It is the best thing I have done to my 81 F100
Dave ----
The running lights I believe are on a fuse, it could be bad. Also because you have the glass type the ends & holders sometimes get a film and dont make a good connection so pull and clean fuse & holders.
The other would be a bad head light switch. If it is OEM to the truck you may want to change it out for a new one.
The last thing is to add a head light relay kit. It will make the head lights brighter and it takes the load off the head light switch and that will keep it from tripping the breaker built into the switch.
You can get the relay kit from LMC, DC, JBGY and think Ebay.
It is the best thing I have done to my 81 F100
Dave ----
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Trailering
Joined: Oct 2024
Posts: 20
Likes: 2
I can only think of 2 reasons:
The running lights I believe are on a fuse, it could be bad. Also because you have the glass type the ends & holders sometimes get a film and dont make a good connection so pull and clean fuse & holders.
The other would be a bad head light switch. If it is OEM to the truck you may want to change it out for a new one.
The last thing is to add a head light relay kit. It will make the head lights brighter and it takes the load off the head light switch and that will keep it from tripping the breaker built into the switch.
You can get the relay kit from LMC, DC, JBGY and think Ebay.
It is the best thing I have done to my 81 F100
Dave ----
The running lights I believe are on a fuse, it could be bad. Also because you have the glass type the ends & holders sometimes get a film and dont make a good connection so pull and clean fuse & holders.
The other would be a bad head light switch. If it is OEM to the truck you may want to change it out for a new one.
The last thing is to add a head light relay kit. It will make the head lights brighter and it takes the load off the head light switch and that will keep it from tripping the breaker built into the switch.
You can get the relay kit from LMC, DC, JBGY and think Ebay.
It is the best thing I have done to my 81 F100
Dave ----
thanks I cleaned the fuses like you said and got a new switch. Nothing changed. Is there anything else you think it could be
Hotshot
Joined: Feb 2004
Posts: 10,836
Likes: 2,612
From: The Shenandoah Valley
I'm guessing that by "either", you mean in park or headlight positions?
Moderator & parts slinger
Joined: Feb 2007
Posts: 50,247
Likes: 5,813
From: S/W Missouri, Polk county
If all are out, I like already suggested suspect fuse, switch, wiring, ground. Maybe, but probably not, you had a elec spike and blew all 4 bulbs.
Yes power to them should be turned on by pulling the headlight switch to the 1st notch. Do you have power to the side marker receptacles when the switch is pulled out ?
Yes power to them should be turned on by pulling the headlight switch to the 1st notch. Do you have power to the side marker receptacles when the switch is pulled out ?
Thread Starter
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Trailering
Joined: Oct 2024
Posts: 20
Likes: 2
If all are out, I like already suggested suspect fuse, switch, wiring, ground. Maybe, but probably not, you had a elec spike and blew all 4 bulbs.
Yes power to them should be turned on by pulling the headlight switch to the 1st notch. Do you have power to the side marker receptacles when the switch is pulled out ?
Yes power to them should be turned on by pulling the headlight switch to the 1st notch. Do you have power to the side marker receptacles when the switch is pulled out ?
Hotshot
Joined: Feb 2004
Posts: 10,836
Likes: 2,612
From: The Shenandoah Valley
Yes I figured it out there wasn’t a ground wire connected there were 2 wires going into it from the running light power instead of 1 power and 1 ground. But when I press the brakes now the headlights get super dim and almost down to nothing. Could this be cause of the the ground?
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Trailering
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Moderator & parts slinger
Joined: Feb 2007
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From: S/W Missouri, Polk county
You have a elec/wiring issue going on for sure, especially if the brake light activation affects your headlights. Is this a NEW issue.... after you repaired the tail light issue?
So this is a tail light issue, not a side marker light issue. I miss read in the 1st post earlier, sorry. So the new headlight switch did not help any? Has any one wired/tapped in a trailer plug to the tail light harness back there?
So this is a tail light issue, not a side marker light issue. I miss read in the 1st post earlier, sorry. So the new headlight switch did not help any? Has any one wired/tapped in a trailer plug to the tail light harness back there?
Hotshot
Joined: Feb 2004
Posts: 10,836
Likes: 2,612
From: The Shenandoah Valley
Your '78 pickup came with 3 wire tail light sockets. One wire is the positive for brake light filament, one wire is for much smaller running light filament, and the third is the ground for both, If you have only two wires per socket and your tail / stop lights are the glued together plastic assemblies where the sockets twist in, you are missing wires. Example, Dorman light socket 85898 has a red wire, a white wire, and a black wire. As best as I recall .... black is the ground .... but I forget if red or white is brake or tail. In a dual filament light bulb for brake and tail, one filament is fat, that is the brake / signal filament. The skinny filament is for tail and it burns less brightly. If your sockets do not have three wires, you have the wrong sockets. Lights will work very dimly almost as if not working because there is a weak path to ground through the running lights which include the marker lights, but it's not the path you want. It would be that weak ground I described.
Get new sockets and make good grounds. You may have another issue, but until obtain and install the correct 3 wire sockets, you'll not likely find out.
Get new sockets and make good grounds. You may have another issue, but until obtain and install the correct 3 wire sockets, you'll not likely find out.
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Trailering
Joined: Oct 2024
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Likes: 2
Your '78 pickup came with 3 wire tail light sockets. One wire is the positive for brake light filament, one wire is for much smaller running light filament, and the third is the ground for both, If you have only two wires per socket and your tail / stop lights are the glued together plastic assemblies where the sockets twist in, you are missing wires. Example, Dorman light socket 85898 has a red wire, a white wire, and a black wire. As best as I recall .... black is the ground .... but I forget if red or white is brake or tail. In a dual filament light bulb for brake and tail, one filament is fat, that is the brake / signal filament. The skinny filament is for tail and it burns less brightly. If your sockets do not have three wires, you have the wrong sockets. Lights will work very dimly almost as if not working because there is a weak path to ground through the running lights which include the marker lights, but it's not the path you want. It would be that weak ground I described.
Get new sockets and make good grounds. You may have another issue, but until obtain and install the correct 3 wire sockets, you'll not likely find out.
Get new sockets and make good grounds. You may have another issue, but until obtain and install the correct 3 wire sockets, you'll not likely find out.
Moderator & parts slinger
Joined: Feb 2007
Posts: 50,247
Likes: 5,813
From: S/W Missouri, Polk county
Some wiring info I collected over the years from fellow FTE members....
Here's what the factory wiring diagram says for rear lights:
Blk/rd= backup lights
Black= side marker, tag lights (on style side)
Brown=tail lights, tag light (on flareside)
green= RH stop and brake
Yellow/blk= LH stop and brake
Rear light wiring:
BROWN: running light circuit
YELLOW with BLACK stripe: driver-side stop/turn
GREEN: passenger-side stop/turn
BLACK with RED stripe: backup lamps
73 & 79 headlight-marker lights use same color codes
Brown= side marker and parking lights
White-blue trace= turn signal
Red-black trace= Low beams
Green-black trace= Hi beams
Black=ground
Flat bed wiring…
Brown= Run Light
Green= Brake light- L
Yellow= Brake Light- R
Black= Reverse Light??
For the front lights....The brown wire is the marker light (side and turn) it is also the top wire on the turn signal socket (three wires in a triangle). The black is ground of course. The white/blue it the turn signal.
Here's what the factory wiring diagram says for rear lights:
Blk/rd= backup lights
Black= side marker, tag lights (on style side)
Brown=tail lights, tag light (on flareside)
green= RH stop and brake
Yellow/blk= LH stop and brake
Rear light wiring:
BROWN: running light circuit
YELLOW with BLACK stripe: driver-side stop/turn
GREEN: passenger-side stop/turn
BLACK with RED stripe: backup lamps
73 & 79 headlight-marker lights use same color codes
Brown= side marker and parking lights
White-blue trace= turn signal
Red-black trace= Low beams
Green-black trace= Hi beams
Black=ground
Flat bed wiring…
Brown= Run Light
Green= Brake light- L
Yellow= Brake Light- R
Black= Reverse Light??
For the front lights....The brown wire is the marker light (side and turn) it is also the top wire on the turn signal socket (three wires in a triangle). The black is ground of course. The white/blue it the turn signal.
Hotshot
Joined: Feb 2004
Posts: 10,836
Likes: 2,612
From: The Shenandoah Valley