Electrical New Fog Lights Won't Turn On

Discussion in 'Fox 5.0 Mustang Tech' started by dnbonds, May 2, 2014.

  1. So I ordered brand new Fog Light Assemblies from LMR. The old fog lights were missing the lenses and wouldn't turn on so I assumed they were fried from all the water during rainstorms. So I install my new fog lights and they don't turn on. I checked all the fuses and none of them were blown or missing.

    Any diagrams or troubleshooting that I could go through?

  2. SEE Mustang GT Fog Light Fix to fix the foglight problem. The stock wiring isn't up to the job and is overheating. The headlight switch & turn/multifunction switch are affected by Ford's wiring problem. Sometimes it overheats so badly that the plastic shells of the wiring connectors start to melt. This will show you how to add a relay to the fog lights to relieve the overload on the headlight wiring.

    Be careful not to use bulbs rated at more than 55 watt each with the stock fog light wiring. Using oversize bulbs can result in overheating the wiring harness and electrical fires. Definitely do the fog light fix first.

    I did mine differently, but I had to build my own wiring harness for the fog lights. This is more trouble than it is worth for most folks. I left all the wiring on the stock light switches in place and used the fog light wiring to power the relay coil. The other side of the relay coil is connected to ground. I have an inline fuse that picks up power from the battery side of the starter solenoid. It is connected to the relay contact. The other relay contact is connected to the new wiring harness I made for the fog lights.


    The advantage of making your own foglight wiring harness is that you can run 100 watt fog light bulbs. The stock wiring harness will not use 100 watt bulbs without overloading and causing a fire.

    Unless you are good at electrical wiring, have the skills and tools (crimp tool, soldering gun, heat gun for the heat shrink tubing, etc.) I recommend that you stick with the Corral method.

    Technical explanation of why the wiring and switches overheat.
    You asked for it...

    I= Current
    E= Voltage
    R= Resistance
    W= Watts

    Two 55 watt fog lamps =110 watts. Find the current in the circuit
    I= W/E
    110 watts/14 volts = 7.85 amps for fog lights alone.
    Since the lighting circuit supplies headlights, taillights, and parking lights, etc.

    56 watts 2 each GE Part # L3156 corner light 28 watts each
    90 watts 2 each GE Part # 9004 headlight 45/65 watts each (low beam)
    63 watts 2 each GE Part # L194 parking light 31.5 watts each
    56 watts 2 each GE Part # L3157 tail light 28 watts each
    265 watts Total

    Total other exterior lighting current
    265 watts/14 volts = 18.92 amps
    18.92 amps other exterior lighting current
    + 7.85 amps fog light current
    26.77 amps with all exterior lights and fog lights on.

    The 12 gauge power feed wire to the exterior lighting switch is rated at 20 amps
    - 20.00
    6.77 amps excess current

    7.85 amps used by fog lights
    -6.77 amps excess current
    1.08 amps to run the fog lights left if you stay within the 20 amp limit of the wire.
    With 1.08 amp of current, the fog lights probably won’t produce any useable light.

    Added resistance required to reduce fog light current to permissible 20 amp limit
    14 volts/6.77 amps = 2.06 ohms
    Resistor wattage
    14 volts x 6.77 amps = 94.78 watts
    You would need a 2 ohm, 100 watt resistor.

    Light bulb ratings from http://www.roadparts.com/catalog/section30.pdf
    Radio shack resistor catalog - no matches , and no combinations that could be used to make a 100 watt, 2 Ohm resistor.
    See RadioShack.com

    Ohm’s law – in case you have any questions about my formulas - Ohm's Law Calculators
    dnbonds likes this.
  3. So I have a problem neither of those will fix. The Light Blue with Black stripe wire that goes from the fog light switch to the 15 amp fuse doesn't have any continuity. I can't seem to get behind the fuse box to check the condition of the wire. Is it OK to just attach the fog lights to the headlights? I understand the headlight switch will work both headlights and fog lights now
  4. is there power to the fuse box fog light fuse?
  5. No. That will burn the headlight switch up. It can barely handle the load of the headlights as is.

    Have you pulled the headlight switch out and looked at the contacts at the back? Very common to overload the switch and basically melt it
  6. i would put my money on the switch,assuming theres power going to it...??...and ya,dont start hacking it up and adding to the headlight switch,you'll be replacing that also.
  7. The fog light's have been an issue since day one on these cars. They would heat up and cut off, cool off and come back on. Very embarrassing!LOL. Sooner or later it would finally burn out a switch and then none of the lights worked.
    I bought my vert new, and must have taken it back to the dealer a half a dozen times back in the day while still under warranty trying to fix the issue.
    They never did really fix the problem.

    I'm not much of an electrician but later I fixed it myself. I went to the auto parts store and bought a cheap aftermarket fog light kit. I shelved the aftermarket fog lights but used all the wiring, relays, ect, ect, from the kit. I cut the factory plug from the back of my stock driving lights and wired them up to the new kit. I put the little provided toggle switch low on my dash near my knees so noone see's it.
    Been working great for 20 years like that. I like it because I can run 100 watt bulbs, and run my driving lights with or without my headlights on.
    If all else fails try it!
    Good Luck!
  8. I took a few electronics courses so I am very comfortable reading circuits. Jrichker sent me all the wiring on my 89 and I wrote down just the circuitry for the fog lights to determine my problem. I am at work and can't post a picture of the circuitry but I know for sure that the wire going from the fog light switch to the #15 fuse is bad. There is no continuity for those two connections.

    I tried to get behind the fuse box and see if the wire was cut or bad but couldn't see it. I tried to unscrew the fuse box and tried to push it down under the dash for better access to the wiring. I almost got it but I didn't want to exert too much force and break it.

    The switches are all good, no burn marks whatsoever. The continuity on the switches are good as well. The connectors are not burned or melted either. I haven't checked to see if it is getting power from the other side of the fuse yet (it actually pulls power from the same line that the head lights do).

    Since I cannot reach the factory wiring I am thinking about running new wire and creating my own wiring harness like Jrichker's method. I'll probably just put an inline fuse right behind the switch and if anything happens its easy enough to pop the switches out and check it.

    Thanks for everyone's inputs. Very appreciated :nice:
  9. my LX aint got no foglights :rolleyes:

    i do have a pretty dummy switch in that slot though...about as effective as the factory GT one...

    anyway looks like everyone got u covered...
  10. Apparently nothing electrical in a fox is effective lol... The stock alternator starves the system, the wiring scheme is a rainbow that differs from year to year, wiring a stereo is a nightmare, and the headlight system overheats itself lol.

    Someday I hope to rip everything out of this car and redo the wiring.
  11. Amen! 80's car mfg at its best... but it works across multiple models !!!

  12. typically when the fox foglights go out, it's the fuseable link dr side out of the main harness, or the relay itself under the dash. If you want them to come on with the headlights, wire a relay into the headlights, and to the the foxlights
  13. I didn't see any type of relay or fuseable link anywhere in the circuitry. Although, being ford, they probably didn't include it in the diagram :shrug:
  14. they're blue where the harness exits the firewall under the hood dr side
  15. Ok thanks :nice: I'll check on those tomorrow after work