headlight problem
Thread Starter
Registered User
Joined: Aug 2013
Posts: 66
From: N. CA.
headlight problem
On a 67 cutlass which has two low beam and two high beam lights the lights won't work but when I hit the high beam switch all the lights come on. What the heck is that about.
Registered User
Joined: Dec 2011
Posts: 7,286
Wrong assumption.
The low beams are low/high
they each have two filaments
thus the 3 terminals on a lo [/hi] beam bulb- one wire connects to one end of both filaments, the 2nd wire feeds the one filament, and the 3rd wire feeds the other filament.
High Beam lamps have but two wires - one to each end of the single filament
It is entirely possible, and in fact very likely, that both low beam filaments are burned out, but all 4 hi beam filaments are serviceable. Thus, in "Low," only the defunct low beam filaments in the low/hi lamps are fed electricity, to no avail. In "high," the high beam lamps are fed, and illuminate, and the high beam filaments of the lo [/hi] lamps are fed, and also burn, because they are still intact.
Start with the simple
then, if the bulbs test OK, you can delve into the wiring and switches and whatnot.
The low beams are low/high
they each have two filaments
thus the 3 terminals on a lo [/hi] beam bulb- one wire connects to one end of both filaments, the 2nd wire feeds the one filament, and the 3rd wire feeds the other filament.
High Beam lamps have but two wires - one to each end of the single filament
It is entirely possible, and in fact very likely, that both low beam filaments are burned out, but all 4 hi beam filaments are serviceable. Thus, in "Low," only the defunct low beam filaments in the low/hi lamps are fed electricity, to no avail. In "high," the high beam lamps are fed, and illuminate, and the high beam filaments of the lo [/hi] lamps are fed, and also burn, because they are still intact.
Start with the simple
then, if the bulbs test OK, you can delve into the wiring and switches and whatnot.
Last edited by Octania; Nov 15, 2013 at 03:18 PM.
Thread Starter
Registered User
Joined: Aug 2013
Posts: 66
From: N. CA.
Wrong assumption.
The low beams are low/high
they each have two filaments
thus the 3 terminals on a lo [/hi] beam bulb- one wire connects to one end of both filaments, the 2nd wire feeds the one filament, and the 3rd wire feeds the other filament.
High Beam lamps have but two wires - one to each end of the single filament
It is entirely possible, and in fact very likely, that both low beam filaments are burned out, but all 4 hi beam filaments are serviceable. Thus, in "Low," only the defunct low beam filaments in the low/hi lamps are fed electricity, to no avail. In "high," the high beam lamps are fed, and illuminate, and the high beam filaments of the lo [/hi] lamps are fed, and also burn, because they are still intact.
Start with the simple
then, if the bulbs test OK, you can delve into the wiring and switches and whatnot.
The low beams are low/high
they each have two filaments
thus the 3 terminals on a lo [/hi] beam bulb- one wire connects to one end of both filaments, the 2nd wire feeds the one filament, and the 3rd wire feeds the other filament.
High Beam lamps have but two wires - one to each end of the single filament
It is entirely possible, and in fact very likely, that both low beam filaments are burned out, but all 4 hi beam filaments are serviceable. Thus, in "Low," only the defunct low beam filaments in the low/hi lamps are fed electricity, to no avail. In "high," the high beam lamps are fed, and illuminate, and the high beam filaments of the lo [/hi] lamps are fed, and also burn, because they are still intact.
Start with the simple
then, if the bulbs test OK, you can delve into the wiring and switches and whatnot.
I will check the lights, thanks for the info.
Old(s) Fart
Joined: Mar 2007
Posts: 50,803
From: Northern VA
It is unlikely that both low beam filaments burned out at exactly the same time, but it is possible that you were driving with one burned out for a while and didn't realize it, then the other burned out. In any case, there are only three possible causes of both low beam filaments not working.
1. Both are burned out
2. The dimmer switch is bad
3. The wire between the dimmer switch and the LH low beam is bad (possibly at the firewall connector).
The ground wires are common between the low and high beam circuits, so this cannot be caused by a bad ground. The high and low beams get power from the same wire going into the dimmer switch, so that cannot be the problem either.
1. Both are burned out
2. The dimmer switch is bad
3. The wire between the dimmer switch and the LH low beam is bad (possibly at the firewall connector).
The ground wires are common between the low and high beam circuits, so this cannot be caused by a bad ground. The high and low beams get power from the same wire going into the dimmer switch, so that cannot be the problem either.
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