Headlights Will Not Turn Off

The multifunction/turn signal switch or it associated wiring is the problem. The fog light circuit causes excessive current draw and causes the multifunction/turn signal switch wiring to overheat. The high beam uses multifunction/turn signal switch to control the high beam lights. Therefore the fog lights cause problems for the high beam headlights. When you remove the cover from the ignition switch and multifunction/turn signal switch you will see evidence of the wiring overheating and sometimes the contacts fall out of the plastic connector shell. I suspect that this is what has happed to your car.


Multifunction turn signal switch:
Before you think about replacing the multifunction switch with one from the junkyard, here's some things to be aware of...

The problem is more common in GT models because they had fog lights on the same power wiring as the headlights. Ford undersized the wire and that caused problems.

A word of caution about multifunction switches is in order here. The multifunction switch (high/low beam, wiper, turn signals) are different for different years. 87-98 will work in any 87-89 car. The 90-93 switches only work in 90-93 cars. You can't put an early model switch in a late model car, nor can you put a late model switch in an early car.

Supposedly you can move the pins around to make the switches work in model years that are different from the car the switch came out of. I cannot verify that and haven’t tried it.

Other possible problem sources for the turn signal & headlight malfunction are the ignition switch, multifunction switch and the plastic shell that holds the turn signal wiring connector pins.

The following diagram is for 87-89 model cars.

Turn signal switch wiring:


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
I=W/E
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
26.77
- 20.00
6.77 amps excess current

Add a resistor in series to reduce the current? It won't work and here's why...
Added resistance required to reduce fog light current to permissible 20 amp limit
R=E/I
14 volts/6.77 amps = 2.06 ohms
Resistor wattage
W=EI
14 volts x 6.77 amps = 94.78 watts
You would need a 2 ohm, 100 watt resistor.

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.


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

The multifunction switch controls the high beams and the low beam power. The wiring overheats from too much current draw. This causes the connector pins to loosen up from their shell and fall out They can short together to prevent the high beams from turning off, or the wires can overheat and short together and cause the same problem. You won't know until you remove the cover and take a look at the wiring.

There should be a tech article in the archives here on how to rewire the fog lights so that they do not share the same wiring as the headlights. I did this years ago on several of my GT's. If not, google for it as I've seen it on many mustang sites. Another better idea from Ford in the 80's.

I bet if you place your hand on top of the dash when you put the fog lights and healights on, it's warm.