The high beams on my 1966 Mustang don't turn on. I tried replacing the headlights but that did not work. The low beams and all other lights work fine. Any suggestions???

 

Welcome.

This is a common enough problem. First check the dimmer switch on the floor. The wiring diagram is available HERE, in the FAQs sticky of this forum. If you have a multimeter things would be easier, you could just do a resistance check of the switch.

There's only three wires to the switch, power(15) low beam feed(12) and high beam feed(13.) The color code is on the diagram.

The switch sends the power to low OR high. You know you have power to the switch because the low beams work. So chances are good that either the switch, the high beam feed wire connector or the high beam feed wire itself is open. If you don't have a meter and want to check the switch, WITH THE LIGHTS OFF, jumper wires 13 and 15 together (refer to the color code for the wire colors.) and then turn the lights on. (use a fairly heavy solid wire jumper or you'll have a free light show with arcs and sparks.) If the high beams come on the dimmer switch is bad. If they don't the wiring is bad.

 

Thanks for teh advice.

I pulled the dimmer switch and used a jumper wire to test the high beam and low beams independently. The low beams operate but not the high beams. I suspect this means it is a wiring issue and not the dimmer switch. Any suggestions where I should go from here?

 

The next place to check is the plug at the firewall, look for power/continuity. A little corrosion in that plug would kill the lights.

 

I believe I have isolated the wiring problem between the firewall harness and somewhere under the dash. I checked the continuity from the harness connection inside the engine compartment to the left and right headlight and it is good. I also tested the continuity from the harness to the dimmer switch and it is dead. As you know it is difficult to track the wiring under the dash. Any further suggestions would be greatly appreciated.

 

I believe I may have found my problem, but for some reason the lights (high beams) still don't work. I followed the electrical diagram you provided and everything from the main disconnect on the fire wall to the head lights is wired and operating correctly. I used a meter and tested the continuity in all wires. But... from the main disconnect to the dimmer switch it appears wires 12 and 13 are crossed. When I use my meter to test the continuity from wire 13 at the dimmer to wire 13 at the fire wall disconnect nothing happens, appears to be broken wire, but if I move the test lead to position 12 at the fire wall and leave at position 13 on the dimmer the meter buzzes. The same things happens when I connect to 12 at the dimmer and 12 at the disconnect (nothing), but if I leave the meter at 12 at the dimmer and move the lead to 13 at the disconnect the meter buzzes.

If the wires are simply crossed why wouldn't both the low beams and high beams work? Isn't the dimmer simply a toggle between the two settings? I'm obviously missing something.

The other thing I noticed. If I apply a jumper wire on the dimmer connection from 12 to 15 and turn on the lights the low beams work. But when I use a jumper wire from 13 to 15 the high beams don't turn on and the courtesy lights on the doors and under the dash flicker and you can hear some sort or noise/click like a short.

Any guidance of where to go from here would be appreciated.

 

First, the apparent crossing is baffling, especially because the low beam circuit works. There are other indicators to work with, so that goes on the back burner for now.

The power lead (15) passes through the light switch. The light switch has a built-in thermal protector. The clicking you hear may be the thermal protector cycling, the dimming could indicate this cycling or a high current drain (short?) somewhere in the lighting circuits. These light switches are know for oxidizing over time and failing, so it may be bad. It may be able to handle the current required for the low beams but not that required for the high ones. But you want to be sure that's the problem before you replace it. (I should have mentioned this last night but I was trying to keep it simple, and I was hoping it was the dimmer switch not the light switch. They are not the easiest part to change.)

Disconnect EVERYTHING and check all wires for a short to ground. Disconnect the wires at the back of the headlights, the firewall connector, the dimmer switch and the light switch itself. The plug is on top of the switch. If there are no paths to ground, reconnect the headlight wires and the firewall connector. Run a jumper from the positive side of the battery to the 13 slot on the dimmer wiring harness. This will eliminate the light switch from the circuit. The high beams should light. I've got my fingers crossed.


Pulling the God Damn light switch.