Hello. My 1977 CJ5 headlights only work on low beam. When I hit high beam, they go out. The dimmer switch has
been replaced. I tested one of the headlights on another car and the high beam was good.

I checked the 3 prongs and got the following:
headlights off: all negative.
low beam on: middle prong positive, other two negative.
high beam on: all negative.

Does anyone know which of the two remaining prongs should be hot on high beams?

Any idea how to fix this?

thanx

Your post wasn't clear on which "prongs" you're testing. Are you talking about the pins on the connector that connects to a headlight, or are you talking about the pins on the connector that connects to the dimmer switch? The only thing that makes sense to me is that you're talking about the connector that plugs onto the back of the headlamp, so I'm going to assume that's what your talking about.

With that in mind, let's talk about some basic circuitry and what you should be getting.

Let's assume for the moment that the dimmer switch is good, since you're just replaced it; you could have gotten a new bad part, but that's not likely, especially since you have the same exact symptoms after replacing it.

When you turn the headlights on, 12 volts is sent to the dimmer switch on the center wire. Depending on the position of the dimmer switch, that 12 volts is routed either to the low beam output wire or the high beam output wire of the dimmer switch connector.

Let's also assume that at least one of the headlamps is good, because you tested at least one of them in another vehicle.

The connector that connects to the headlight should have three critical wires:

(a) One wire connects to ground. That same ground is used for both high beam and low beam, and since low beam works, we'll assume that the ground is good. That pin on the headlight connector should read 0 volts at all times with a voltmeter, not negative. It should also read 0 ohms to ground with an ohmmeter.

(b) One wire comes from the low beam output wire of the dimmer switch connector. That pin on the headlight connector should read 12 volts when the headlight switch is on and the dimmer switch is in the Low Beam position. We can assume that that wire is good, because your low beams work OK, and you're getting 12 volts on the pin when you're in Low Beam.

(c) One wire comes from the high beam output wire of the dimmer switch connector. That pin on the headlight connector should read 12 volts when the headlight switch is on and the dimmer switch is in the High Beam position. We can assume that the 12 volts is missing from that wire, since your high beams do not work, and you do not read 12 volts on any of the headlight connector pins when the dimmer switch is in the High Beam position.

So, that leads me to believe that the wire going from the high beam output pin of the dimmer switch to the high beam input pin on the headlight connector as been broken somewhere along its path. Now, it could also be that the high beam pin-to-wire connection in the headlight connectors are bad (unlikely, since they'd both have to be bad) or that the high beam pin-to-wire connection in the dimmer switch connector is bad (a distinct possibility).

A quick way to eliminate the dimmer switch connector as the problem: Look at the high beam indicator lamp on the instrument panel. If that lamp is working when you go to high beam, then the dimmer switch connector must be good, and the problem lies somewhere in the wiring between the junction for the instrument lamp and the headlights.

But, if the high beam indicator lamp on the instrument panel doesn't light up during high beams (which I suspect is the case), then the problem is either: (1) A bad dimmer switch connector; or (2) A break in the wire going from the high beam output pin of the dimmer switch connector to the junction where the high beam indicator lamp ties into the wires going to the high beam connectors on the headlights.

So here's what I would do (assuming that the high beam indicator lamp doesn't come on). Take the connector off of the dimmer switch and clean the connector pins as best as you can with a wire bruch. Then spray some WD-40 on the connector pins, and then install and remove the connector from the dimmer switch several times. Then check to see if the high beams work. If they still don't work, suspect an open wire. Since the dimmer switch in probably on the floor of your CJ5, the wire may have worn through from the last 20-25 years of foot traffic. Visually inspect the entire run of that wire.

Good, luck, and let us know what you find after trying ALL of the above.

RobRowe: Thank you very much for your comprehensive answer. You are correct, I referred to the prongs on the headlight. I will try what you say tomorrow.

One other question: could it be the headlight switch? I ask because when I got the jeep, the tail lights did not work. I traced the problem to a burnt wire going to them (the brake lights did work). It was burnt just past the switch. I replaced the wire and they work fine now. Maybe I should have put this in the first post, but I figured the headlight switch only sends power to the dimmer switch and therefore, would not be the problem.
thanx again
dave

If the dimmer is on the floor the chances are it is there or between there and the lights.. I'd forget about the switch.
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />

Dave, you are correct. As far as the headlights are concerned, the headlight switch just sends power to the dimmer switch, and the position of the dimmer switch determines whether the high beams or the low beams receive power. Since your low beams work, it's not your headlight switch. It's also not the wire between the headlight switch and the dimmer switch.

Check out the stuff I suggested and then get back to us.

Finally got around to doing this and your were right, RobRowe, it was the connection to the dimmer switch. The Jeep was out of service for some repairs, so I decided to wait. Anyway, I feel stupid now, but at least it works! Thanx

Glad to hear it's working now, Dave. Good wheelin'!