Fog Light Relay???

Discussion in 'Fox 5.0 Mustang Tech' started by TrophyHead, Jul 17, 2013.

  1. Having a issue with my fog lights not working. I have voltage in and out of fog light switch. No voltage to fog light connector. Everything else is working on my headlight harness.(high beams, blinkers, marker,parking, ect) After looking at the veryuseful dot com wire diagrams it appears there is a fog light relay. Where is this located?

    On a second note it appears I have a 90+ headlight harness in my 88 because the parking lights have the newer style connector. Fog lights never worked.

    Any help where that relay is or any suggestions on a solution would be great. Thanks.

  2. burnt or broken wire i bet

    i believe the relay on very useful is part of the foglight fix and not an oem part of the lights
  3. Yeah it says coming out of fog light switch tan/orange is goes right to fog light. I must have a broken wire or burnt wire. I just can't believe this is my only issue. Gonna have to pull the tire tomorrow and do my first troubleshooting with wires tucked in fender. I'll do more investigating and report back
  4. Double post
  5. No fog light relay from the factory, but they really do need one. Re-wire them per the Corral write-up that mikestang posted. You'll be happy you did. And you can turn on your fogs with your parking lots. Really looks badass when driving around at dusk.
  6. yeah it is crazy how Ford did the wiring on these cars. All of the gauges have a common power wire,the headlights and foglights go to that one switch, the stock alternator wiring are known to catch fire, the starter/solenoid issues

    Put your hand on the dash over the headlight switch when you have the lights and foglights on for a while. I bet it will become very warm. On almost every fox with foglights the switch eventually overheats and melts after repeated cycling on and off. I'd do the foglight mod while you are in there as a precaution.
  7. Good idea Scott. Think I will go ahead and rewire them like that.
  8. Be careful not to use bulbs rated at more than 55 watt each. Using oversize bulbs can result in overheating the wiring harness and electrical fires. Definitely do the fog light fix.

    SEE Mustang GT Fog Light Fix to fix the 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.

    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 fog light 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
    Radio shack resistor catalog - no matches , and no combinations that could be used to make a 100 watt, 2 Ohm resistor.

    Ohm’s law – in case you have any questions about my formulas - Ohm's Law Calculators
  9. I really like that fog light wiring method jrichker. I have to wire my lights from scratch as well, so this seems like a great way to do it. Thanks for the info.