Headlights Flickering On And Off .......why?

Ford TSB 89-17-11 says this is caused by overheating in the headlamp circuit. You'll need to replace the headlamp and dimmer switches, several connectors, and then bypass the wiring in the circuit with new 12ga wire run alongside the original harness.

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

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
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.

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

BKM48198 said:

Thanks for all that info. From what I am getting I should just rewire the fog lights with better wire and add in the relay. Can I use the wiring from the switch to the relay or will that need to be replaced also? Looking at the picture I am assuming the Red wire is all new and the orange is the stock wire. I have been using my Fog Lights every morning for years and never had a problem until now, if the multi function switch ( turn signal- high beam-wiper control ) seems to be working OK will it need to be replaced also or should I just do the wiring and leave the other switches alone?

Click to expand...

You are correct...
The headlight and multifunction turn signal switches and the wiring that directly connects to them may be damaged due to the heat generated by the poor design by Ford. Try using them, but be prepared to replace them if it doesn't work satisfactorily.

Multifunction turn signal switch:
Before you think about replacing the multifunction switch with one from the junkyard, here are 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:


The following diagram is for 90-93model cars.

Turn signal switch wiring:

BKM48198 said:

If the flickering is being caused by the fog lights overheating the circuit would it be OK to drive with the fog lights off for now until the wiring is changed or will the headlights still be likely to flicker on and off?

Click to expand...

It may be OK and it may not. The overheating has already damaged the switches and wiring connectors., so it is a roll of the dice. Try it and see what happens.

You have to remove the plastic that covers the ignition switch.
Saleen0679 was nice enough to dig this up for us awhile back:
http://www.mustangandfords.com/how-to/engine/5657/

I highly recommend that you closely inspect the ignition switch and wiring since the are prone to problems as well. Do a search using my user name and the word recall for search parameters and you'll get the full story on how to do it.

To make it simple as possible, I set it up so that you just connect the old foglight wiring up to the relay. The relay doesn't pull much current, less than 1 amp, so it doesn't make the stock wiring heat up.

Making a new wiring harness for the foglights does take some time and effort, but it is worth it.