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Hey all, recently got pulled over for dim taillights on my '93 F250, 4wd 351w ZF5. They do work but they are quite dim. I got a fix it ticket and I'm trying to figure out this issue, I believe it to be a bad ground but can't find what ground ties into the taillights.
All other lights work perfectly. Brake lights are very bright, so are turn signals in the front and rear. Headlights work etc. Everything works EXCEPT the rear running lights. With a multimeter at the socket I am getting 8v. I noticed they don't come on with the switch either, they are always on when the key is turned. I have replaced the multifunction switch and the headlight switch in the past. Previously the lights did not work at all until I replaced both the heaadlight switch and multifunction switch, and they started working but only with the key in run position. I did note that when replacing the multifunction switch, the pin relating to hazards was severely burnt, my hazards do not work but I figured it would not affect taillights. I want to cut out a new harness from a scrapyard truck and replace that to rule it out but was hoping someone on here had any good advice or pointers before I just take this thing to a shop. Thanks in advance.
Last edited by hotrod_lincoln; Mar 10, 2026 at 10:09 PM.
Interesting problem. The ground for that system is called g100 and its located on the radiator support, behind the drivers-side head light.
Are you running LED tail lights?
Going over the schematic, I dont see a smoking gun that would cause the problem your describing. Did the tail lights ever work properly since you've owned the truck?
If I were to guess, I'd say your tail lights are not wired up correctly. Perhaps theyre accidentally wired into the fuel pumps monitoring circuit or the fuel gauge circuit.
if you look under your truck, at the drivers-side frame rail, do you see any obvious repairs to the harness?
For fun, have you tried pulling your engine codes?
Hey all, recently got pulled over for dim taillights on my '93 F250, 4wd 351w ZF5. They do work but they are quite dim. I got a fix it ticket and I'm trying to figure out this issue, I believe it to be a bad ground but can't find what ground ties into the taillights.
All other lights work perfectly. Brake lights are very bright, so are turn signals in the front and rear. Headlights work etc. Everything works EXCEPT the rear running lights. With a multimeter at the socket I am getting 8v. I noticed they don't come on with the switch either, they are always on when the key is turned. I have replaced the multifunction switch and the headlight switch in the past. Previously the lights did not work at all until I replaced both the heaadlight switch and multifunction switch, and they started working but only with the key in run position. I did note that when replacing the multifunction switch, the pin relating to hazards was severely burnt, my hazards do not work but I figured it would not affect taillights. I want to cut out a new harness from a scrapyard truck and replace that to rule it out but was hoping someone on here had any good advice or pointers before I just take this thing to a shop. Thanks in advance.
pull and reseat the connectors on the firewall a few times or clean the contacts if you can
Interesting problem. The ground for that system is called g100 and its located on the radiator support, behind the drivers-side head light.
Are you running LED tail lights?
Going over the schematic, I dont see a smoking gun that would cause the problem your describing. Did the tail lights ever work properly since you've owned the truck?
If I were to guess, I'd say your tail lights are not wired up correctly. Perhaps theyre accidentally wired into the fuel pumps monitoring circuit or the fuel gauge circuit.
if you look under your truck, at the drivers-side frame rail, do you see any obvious repairs to the harness?
For fun, have you tried pulling your engine codes?
The bulbs are LED, but regardless I get 8v at the socket with no bulb in it.
The taillights did not work at all when I bought the truck. Replacing the headlight switch and multifunction switch fixed this issue but they are still dim. Not dim enough to be pulled over for it but here we are I guess.
I can see splicing done to the harness, someone tapped into the factory wires to make a trailer plug and the wires they added are dangling and not connected to anything, but going further back everything looks proper. Do you know the color coordination of the wires so I can use a multimeter and test if I'm getting 8v before the connector?
The bulbs are LED, but regardless I get 8v at the socket with no bulb in it.
The taillights did not work at all when I bought the truck. Replacing the headlight switch and multifunction switch fixed this issue but they are still dim. Not dim enough to be pulled over for it but here we are I guess.
I can see splicing done to the harness, someone tapped into the factory wires to make a trailer plug and the wires they added are dangling and not connected to anything, but going further back everything looks proper. Do you know the color coordination of the wires so I can use a multimeter and test if I'm getting 8v before the connector?
LED tail light bulbs on these trucks are the worst. Get rid of them and swap back to incandescents. That'll solve 1/2 of your problems.
This is gunna blow your mind, but stick with me here. How do you know your running lights are very dim, and not a trickle current barely activating your brake lights?
If you find the 8v terminals on your tail light bulbs socket, have someone step on the brakes; does the 8v jump up to 12v? If so, then thats your brake light circuit, not your running lights.
The reason this is important is because there 4 components on this truck that monitors the brake lights. ABS, Cruise control, ECU and your trailer brake controller. I know the cruise control trickles a small amount of current in the brake light circuit. I imagine the ABS and brake controller does too. LEDs are so efficient that they will start to illuminate with this trickle current.
Once youve confirmed that its your LED brake lights that are dimly illuminating, the problem becomes a lot simpler, and its probably in the wiring near the trailer brake controller.
Heres a schematic for a 1994 truck, the colors are probably the same.
Last edited by Prototypemech; Mar 11, 2026 at 07:50 PM.
LED tail light bulbs on these trucks are the worst. Get rid of them and swap back to incandescents. That'll solve 1/2 of your problems.
This is gunna blow your mind, but stick with me here. How do you know your running lights are very dim, and not a trickle current barely activating your brake lights?
If you find the 8v terminals on your tail light bulbs socket, have someone step on the brakes; does the 8v jump up to 12v? If so, then thats your brake light circuit, not your running lights.
The reason this is important is because there 4 components on this truck that monitors the brake lights. ABS, Cruise control, ECU and your trailer brake controller. I know the cruise control trickles a small amount of current in the brake light circuit. I imagine the ABS and brake controller does too. LEDs are so efficient that they will start to illuminate with this trickle current.
Once youve confirmed that its your LED brake lights that are dimly illuminating, the problem becomes a lot simpler, and its probably in the wiring near the trailer brake controller.
Heres a schematic for a 1994 truck, the colors are probably the same.
I will pick up some halogen bulbs and verify this. I think I should start by cutting out all the extra junk the previous owner added to make trailer lights, and just rewiring the taillights to the harness, and probably pick up both the taillight harness and trailer light harness from a scrapyard truck and rewire those in as well to be safe. I did notice that the factory plug that would plug into a harness for trailer light plugs (the cylindrical grey one with two prongs) has a pin that is completely corroded off.
I will pick up some halogen bulbs and verify this. I think I should start by cutting out all the extra junk the previous owner added to make trailer lights, and just rewiring the taillights to the harness, and probably pick up both the taillight harness and trailer light harness from a scrapyard truck and rewire those in as well to be safe. I did notice that the factory plug that would plug into a harness for trailer light plugs (the cylindrical grey one with two prongs) has a pin that is completely corroded off.
there should only be 7 pins max in those 2 plugs, so one missing is normal
