Left Headlight Problem

[86 911]

Okay, this is really strage. What happens is that the left headlight bulb will be okay when I switch to high beam light, but the low beam light is not coming on. The right headlight is fine though. The left headlight bulb is good. I took both headlights out and put the right headlight into the left headlight socket and it does the same as with the left light. I tested for volts, and both headlight contacts read exactly the same: 14 V when brights are on. 5 V when brights are off. 12 V on the low beams. The low beam on the right works fine, but the left low beam does not come on, although it registers 6 V with the digital amp meter. Why isn't my left headlight low beam coming on? I know for a fact that both bulbs are good and the amp meter is reading the same on both headlights.

[ianc]

Do they use separate grounds? Is the low beam ground bad?

ianc

[86 911]

No, they are on one ground. The grounds are not bad since I got a good reading from them.

[bell]

the voltage shouldn't change between the high/low beams........it should show the same voltage on both (with no bulb in the socket)

could be a corroded wire in the connector, probably the ground.....if it is corroded the high beam could be pulling just enough current for it to ground and illuminate.
check where it grounds under the hood also, should be a 10mm bolt....mine worked loose once.

[86 911]

Thanks Bell. I checked the ground, and it had a good signal to it. When the high beams off, should there still be 5 V going to them? Could the yellow headlight wire that operates the low beams have corroded somewhere and not be able to handle the load, but able to get a 12 V reading?

[bell]

if it's showing 5 volts it's possible that the wires are in the wrong locations on the bulb, with the car not running the voltage reading at the light should be real close to what it is at the battery.
check it like this......(per chance you're overlooking something)
unplug the bulbs....
take multi-meter ground lead and stick on negative post of the battery.
turn on low beams, check the 3 wires at the headlamp socket....only one should read 12v+.....mark it.
then hit your high beams and check again, the low beam wire should show nothing.
the 3rd wire will be your ground.

looking at the back of the bulb these wires should correspond like this.....each line is a terminal

_
[ ]

left=ground
top=low beam
right=high beam

check it like that and post your results.....

[bell]

i re-read your last post....

the wire could be corroded where it enters the terminal in the plug, with both bulbs unplugged there should be 12v+ on one wire, the other should read nothing.......this is providing the headlight switch is working properly.
if your readings stay the same i would probe the wire before it goes into the fender (under the hood).....if the readings are the same the problem is upline somewhere, if it checks ok there then the problem will be between the point of where you probed to the headlamp socket.

[86 911]

Thanks Bell. I'll go ahead and do as you suggested and post some results tomorrow.