Lo Beams not working, I'm stumped!

[hshift]

I have a 91 Olds Ninety Eight. Neither low beam light comes on. The hi's work fine, as well as all signals/parking lights etc. In trying to figure this out, I replaced the dimmer switch which is mounted on the steering column (no easy task) which made no difference.

Then I removed the bulbs and connected them directly to the battery, and they light up, so I think they are OK. Then with the low beam bulbs out, and a voltmeter connected to either side plug, I get 11.6 volts at each plug with the dash headlight switch on, and it shows zero when I turn the switch off, so I think the switch is good as I'm getting juice to the plugs, but when I put the bulbs back in, nothing! I'm assuming the two wires connected to the lamp sockets are pos and neg, is this true? I also assume that 11.5 volts would be enough to light the bulbs, as the battery shows 12.5 volts and the bulbs light up when jumpered to the battery directly, right? Any ideas appreciated.

[Tech II]

OK, here is what I want you to do....disconnect the 3 wire harness to one of the headlights....the low and hi beams use the same ground, so since the hi beams work, the ground is ok.....

When testing POWER CIRCUITS(by this, I mean circuits that carry high current), always use a test light....voltmeters only show potential voltage, not working voltage....you are only testing OPEN CIRCUIT voltage with a voltmeter, this means that, current is not flowing in the ciruit, and will not show a voltage drop due to a bad connection.....you can have a simple series circuit with a battery, a switch, and a bulb....you close the switch, and the bulb doesn't light.....you remove the bulb, and insert a voltmeter....it reads battery voltage, so why doesn't the bulb light?

Because that switch has a bad contact, which creates resistance....when the switch closes, current flows......but since the resistance is higher in the switch than in the bulb, most of the voltage is dropped across the switch, rather than the bulb, so the bulb is either dim or not lit at all....

So, in power circuits, always use a test light.....

OK, enough of the lesson.......so with the harness to one of the headlights disconnected, turn the high beams on......attach the test light to the blk wire and the lt green wire....that test light will be bright......now change the dimmer switch to lo beams, the test light goes out....now with one lead still connected to the blk wire, attach the other lead to the tan wire.....the light probably doesn't light.......now remove the leads, and attach your voltmeter to the tan and blk leads.....bet it reads near bat voltage.......so that means in the tan lead circuit, there is a voltage drop(we know the ground is good because the hi beams are ok)......that tan lead goes back through the dimmmer switch and from there to the headlight switch.......

You say you replaced the dimmer switch with a new one?

I don't have a wiring diagram, but I don't think the headlight switch is a double throw switch......by that I mean, there is only one power input to the dimmer(instead of two seperate circuits, which are used on newer cars).....so that means, since the hi beams work, the tan circuit between the lo beam headlights and the dimmer switch has a voltage drop......this could be in the form of a broken wire making an intermittant contact, a bad electrical connection in a harness in the circuit, or there is a soldered joint in the circuit that feeds both low beams, and it could be bad.....

Hope this explains why you have over 11 volts on your meter, but the bulb does not light....

[hshift]

Thanks for the response. I messed around with the car today. Just for the hell of it I threw another dash switch I got from the junkyard since I was out there for another car of mine, and it still didn't work. After reading your reply, I started checking the wiring. I found that if I jumper a wire from the tan line coming out of the dimmer under the dash directly to the tan line at either lamp socket, both bulbs light, so I'm thinking I got a wire break somewhere between the dash and the headlights. I will get on it again with a test light this weekend, and see if I can trace down the break.

[Tech II]

P.S. Your avatar would be really "kewl" if you could find a .GIF, that moved the laser side to side.....

I wouldn't even bother looking....I would just cut both wires and install the jumper permanently, instead of trying to find that wire in a harness....

[hshift]

Yeah, took a look at it tonight, I'm just gonna run a new line. Thanks for helping me figure it out.

[hshift]

OK, I ran a new line from the tan wire at the dimmer switch to the driver side bulb socket and Voila!.... LOW BEAMS! My only issue now is the digital display on the dash still says "Low Beam Light Out", but I can live with that. Thanks for the help and check out my avatar now!

[Tech II]

kewl!