[signalflare66]

My 1966 Mustang high beams are not working. I tried replacing both headlights but that did not work. The low beams and all other lights work fine except for the high beams. Any suggestions???

[RF Mustang]

Did you check the condition of your dimmer switch ?? Those things get pretty raunchy being on the floor for many years and get pretty corroded. You can find issues with its connector or the switch itself.

Pull the connector off the dimmer switch on the floor and clean the contacts with come contact cleaner as best you can. connecting and then disconnecting will help get some of the crude and oxidation down.

A new dimmer switch is less than $10.

If its not the dimmer swithc or connection, it is most likely the high beam feed to the head lights.

[signalflare66]

I pulled the dimmer switch and used a jumper wire to test the high beam and low beams. 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?

[Ivy66GT]

Hi and Lo headlights both get their power from the floorboard dimmer switch. If one works and not the other the problem is either in the switch of the wires between there and the headlights. There are two wires, green/black and red/black, from the switch to the bulbs with one plug on the firewall that they go through. I would say your problem is somewhere in those two wires. The schematic doesn't really say which one is Hi and which is Lo but that should be easy to tell.

[signalflare66]

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.

[signalflare66]

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.

[Ivy66GT]

That is somewhat strange. You may have more than a single problem.

Yes, the dimmer switch should just connect +12V (the 15 color wire) to either the 12 or 13 color wire. I doubt the wire color should change from 12 to 13 between the dimmer switch and the bulbs. Also, the shop manual schematic says the 12 wire is the HI beams, not the LO. I have been thinking of some possibility that could do what you describe. I can come up with a wild guess but you are going to have to investigate further.

You are using the meter to buzz. That may not be the best for this purpose. The meter likely buzzes if the resistance is below a few Ohms which may not truly be a direct connection. You may be getting a buzz because you are measuring across the headlamp bulbs which will be down in the 1 Ohm range and look like a direct connection.

Here is one scenario which may, or may not be what is going on. Let says the harness has been whacked hard somewhere before the firewall so that the #13 wire was cut and is shorted to ground. At the same time the #12 got cut and shorted to the #13 wire but on the headlight side of the cut in #13. Measuring either wire from the dimmer switch to the firewall would say open circuit. But by going from 12 to 13 you are measuring across a pair of LO beam bulbs so you get a buzz. Putting +12V on wire 12 lights the Lo beams which it should not because its really shorted to #13. Putting +12V on wire #13 is putting power directly in a short to ground so everything flickers. To make this story complete the LO beam contact in the dimmer switch would also be fried from the short circuit which is why nothing happens when you flip it to LO. You would be getting the the LO beams when you flip the switch to HI because #12 (HI) is shorted to #13 (LO) somewhere along the way. I know that sounds kinda far fetched but it I think it would explain what you describe.

Take some more measurements with that kind of a situation in mind. Hard telling what you will find.

[MizzouMike76]

Hope this helps:

Vintage Mustang Wiring Diagrams

And while you're there, here's an upgrade to your original wiring using 30A relays. Took me about 2 hours and cost about $10; made a huge difference in headlamp brightness:

Brighter Headlights

Michael

[signalflare66]

I finally located the issue. It ended up being a screw someone installed that clipped one of the wires in the harness and was grounding out.

Thanks for all the help.

[Ivy66GT]

Good work. I was pretty sure there was a short involved there somewhere.