[leeburch1]

The headlights on my wife's red 1973 convertible go off and on while driving and the frequency of the event increases and the 'off'' duration increases until finally they go off and stay off. I can turn the lights off for a bit and they will come back on but will soon repeat the problem. I have looked in all the specified places for a headlight relay but havent found one. Even changed the headlight switch. Nothing has helped. (This problem does not appear to be related to the weak dash lighted reported in an earlier post). All suggestions welcome.

[Asheron2]

Try wiring up a relay to power the headlights. Use the current power to power the relay which will then power the lights directly from the battery. Put a fuse in line too.

[79stangghia]

Check you grounds you might have a short in the system some place. Have fun finding it. Also, good job replacing the light switch, that was my first guess.

[mustangducati]

Hi, I am not sure if the 73 is similar to my 69 but I had the same problem. Long story short, it was the high/low beam switch. On the 69, it is a switch you press with your left foot. Hope that helps! Good luck.

[silver69]

my 72 did the same thing, new switch did not help, all grounds good. went to relays and lights are brighter and no blinky blinky - scary as crap by the way!

[leeburch1]

Thanks to all who responded.
I wouldn't have the foggiest idea how to wire the headlights to a relay without messing up the dimmer and all the other lights. You are sure right about that last part though.

[silver69]

It is quite easy to use the relays. Here is a crash course in the wiring - The headlight switch sends 12V+ to the dimmer switch on the floor when you turn on the headlights. The dimmer switch sends this 12vdc+ to either the high or low beam curcit.
Buy a 30 amp relay from Advance auto (5-6 $) and it is an easy wire in.
First UNPLUG your leads to the floor mounted dimmer switch and set the test meter to DC volts. now turn on the switch, put one side of the test meter to a good solid vehicle ground
and the other into the wiring harness plug, one of the three will show 12 VDC. That lead is your supply voltage off your switch. giving yourself a few inches to work with splicing wire, cut that lead about 2 inches from the plug and strip back both cut ends and crimp a butt splice or solder one on each of the cut supply wires
Bring a NEW 12VDC+ lead from the battery to this relay(the relay is only a high amp capacity switch which uses low voltage for triggering - just like your starter solenoid)
The relay will have 4 -5 terminals or leads on it, 1 is ground for the trigger, one is the actuating 12+ for the trigger(pull in coil) usually small gauge wire like 16-18 Ga.
Then 2 will be heavy #10 Gage wire
with no continuity between the wires until a 12- and 12+ are applied to the small leads.
Take one of the small leads and attach it to the chassis ground, free of rust and paint. take the cut wire coming from the switch and attach it to the other small gage lead. you are 1/2 way done!!
Connect the 12+ coming from the battery to one of the larger gage wire sticking out of the relay. Now connect the wire that you cut from the plug to the last spot on the relay- boom youre done.
This is what you have done,
You have taken the weak old switch and overload away from the switch and wiring.
you cut on the lights and the relay gets 12+ signal to the pull in coil on the new relay completes the curcuit
giving you all kinds of power to the headlight switch to send through to the dimmer which simply chooses high or low beam.
You are done man!!

[silver69]

It was late last night when I responded and forgot to mention that there are several ways to use a relay for lighting and this being the simplest(for me to explain anyway)
Also after you find your supply voltage lead to the dimmer switch and cut it into 2 pieces, one coming from the headlight switch and one going to the dimmer switch - disconnect the battery cable before doing anything else, also put in a 30 amp fuseholder and fuse in the new line you run to the battery. Dont cheat, bring it from the + side of the battery to your relay. Read this a few times and soak it in, its easy once you digest it and follow the logic. Imagine current as water in a hose and the relay as the spray nozzle, when you pull the trigger(12V+ supplied by the headlight switch) the water(current) flows. The relay uses less than .2 amp to function versus all the combined lighting load(20+Amps) and the reason the lights are blinking is because that load is too much for the old wiring and lights now. Replacing the switch might fix it but all that wiring is 35 years old and that is a long time for any electrical circuit. The wires and relay are overheating, tripping off to prevent a fire, cooling off and coming back on - over and over and over.

[snoman1973]

I have owned over 11 mustangs and was haunted buy this very same demon years ago i found the problem thru trial and many errors it turns out to be a Bad body ground in my case the ground w=strap that went from the back of the engine to the fire wall i t was loose and refastened it and no more problem

[frehley77369]

I had the same problem, but my headers cooked the wiring harness between the firewall to the radiator support around the area of the shock tower. It was fun replacing it, because it goes to all lights and horns in the front of the car.

[drummingrocks]

Another thought: Does the car have halogen headlights? I'm not sure about a '73 model, but the '65-66 cars are notorious for overheating the headlight circuit if halogen headlights are used. The wiring on early Mustangs wasn't designed to carry the heavier current load from brighter halogen headlights.

What about the voltage regulator on your '73? Is it the original?

[fastbck65]

If you have the original headlight harness it may be delapidated and the metal tabs in the harness have loosened from the plastic. Bend your headlight tabs slightly and re-connect the harness to the headlight. If the problem quits, then you need a new harness soon. My '73 had the same problem.

Just a thought. With both headlights going out at the same time, I would venture to guess at something else. I just like trying the simple things first.

[Pawk72]

Upon diagnosing the problem I found that I was loosing 2 Volts of power from the headlight switch to the headlights. The problem was the wiring was so old that it was causing high resistance to the headlights making the wires over heat.

As many people said replacing the wiring and installing relays was the fix. While you there you should do a H4 sealed beam upgrade with a set of Hella, Bosch or Ciebe lights. They give a much better light output.

Also wire the light to the ignition power source so they turn off with the ignition off, so you don't kill you battery as fast(you know this will happen).American cars are wired this way and I still don't know why.

[leeburch1]

Thanks guys, I am still looking. The problem has gone goofy. I notice a blinking of the interior lights (door lights) when I turn on the light switch. Also the first switch detent is out so I am changing the light switch again. All (?) grounds have been cleaned and terminals checked. Also the pc is old so I plan to change it out also. Thanks again for all the advice and encouragement.