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A head light question:

B

Brent Zappe

Member
Yesterday I got a call from a friend who has a 1968 Buick wild cat. He has a head light problem Which I never thought about and till yesterday. The question was his head lights go dime when he has them on. Then he asked me what should the voltage be at the head light? Then I felt like a dummy because I have forgot about this one. Well I had stop working on cars for the past ten years. I told him it could be the light switch might be going bad on the dash or it could be a bad ground to the lights but about the voltage to the light I think should had be 12 volts or around that amount. Now I not to Shure if I told him right so I am posting this just to see what every body has to say.
 
The voltage at the headlight should be very close to the battery voltage, so 12.6 not running and 13.5-15 running.
 
Under the cercumstances what you told him is probably good as any. If he doesn't know to compare voltage at light with that at battery,your telling him isn't going to help when it come's to testing remainder of circuit. "Going dim"could mean the lens are dirty or maybe the silver mirror surface has peeled off. Then like many of us,it could be his eyes and have nothing to do with the lights. Pardon the pun but we are shooting into darkness.
 
Newer cars would be to check some relays. On my 1996 ford explorer the headlight switch AND the socket took a dump. Had to junk yard the wire harness from a low miles vehicle and carfully solder the harness with lineman splices and heat shrink tubing. Made a very neat job. Worked perfect after that.
 
Electrical problems are always fun to find. There must be a bad connection somewhere, either in the switches or wiring. He might start with hotwiring the headlight straight off the battery and make sure they are working right to begin with. From there trace the wires back to the switches and fuse box and make sure the wires are undamaged and connections are good. A nearly 50 year old car perhaps the fuse box is corroded or a mouse has chewed on the wiring.
 
Should be pretty close to battery voltage.

First thing I would check is the ground strap from the engine to the firewall.
 
Should be pretty close to battery voltage.

First thing I would check is the ground strap from the engine to the firewall.
 
The fellows have covered the voltage pretty well. I can tell you that you can have several bad connections make the lights be very dim. One of the farm trucks would barely light the head lights. It is an 1988 Ford 9000. It has the floor dimmer switch and the junction blocks on the firewall. When we started we only had around 8 volts at the head lights with 12.5 at the battery. The power supply from the power side of the junction block was corroded. The spades on the back of the light switch and plug where corroded a little too. The junction block was badly corroded. Spend an afternoon cleaning and replacing connections. When finished we had around 11.5-12 volts at the each lights when not running. A half a volt drop after going through the entire light circuit is not bad.
 
Ford had that junction block problem forever, made worse by adding huge quantities of dielectric grease to it. But I was under the impression they'd fixed it by '85 or so?
 
This reminds me of when I was with Ford around 1990.

We got a Tempo in the shop with no low beam headlights. My tech in that area tore his hair out for a couple of hours and then found out--by coincidence both low beams had burnt out at the same time.
 
Thank you for every body's help and I mean that. All of you confirm more less what I thought I knew but I want just to see if my thinking was okay. I quiet working on cars many years ago and went to farm tractors. Since than I still have friends who try to get me on things and yes do to the fact of my getting older means some times I forget. again I like to say thank you for making feel like I did not forget this one. You made my day.
 
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