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Headlight ground

M

mcconneje

Member
On my 67 Mustang, I am experiencing a situation where the headlight on the on the right side of my car is very dim, but the headlight on the left side is as bright as normal.

I have cleaned all of the contacts on the wiring connections on the wire which runs from the light to the main wiring harness. I am thinking that the problem may be a ground, but search my wiring diagrahms as I may, I am not finding where the headlights are grounded, which obviously makes the simple step of checking the ground a problem.

So, my question is: where is the headlight ground on my 67? Other thoughts and comments on the problem are appreciated, too. Thanks. Jeff
 
Probably like the 66's...there's a grounding wire that attaches to the RHS of the radiator support, top back side.

Have you tried replacing the lamp itself? Usually they go dim before they die.
 
Check the connectors also. I had to put in new connectors & larger ground wires on my '68 to get acceptable voltage drop.

The factory wire is ~18 ga, and with thath an the connector issue, the drop was 5 volts before I reworked it.
 
I thought of that. I tried to clean the connections, but I am seriously considering just replacing the headlight to harness wire. My ohm meter says it is carrying juice, but it really looks old and corroded.

I will keep your advice in mind. Thanks.
 
"mcconneje" said:
I thought of that. I tried to clean the connections, but I am seriously considering just replacing the headlight to harness wire. My ohm meter says it is carrying juice, but it really looks old and corroded.

I will keep your advice in mind. Thanks.
Click to expand...

Use the DVM to measure the resistance from the headlight plug to the firewall plug (you'll have to pull that, of course). Any resistance above 1 ohms is cause for rejection when I refurbish harnesses. Checking for voltage doesn't work, because you're not pulling a load while you're measuring voltage. The true way is to have the headlight on and then measure voltage at the headlamp. It can be done, but with some difficulty. It's only 12VDC, so the worst that can happen is you bite your tongue, your hair falls out, and you begin to look like Frank. J/K.
 
I pulled the headlight, and measured the voltage difference from ground on the headlight itself to a known good ground while the headlight was on (5 volts!). However, when 'cold' the connections measured less than ohm or so, IIRC. Based on the measured amps, that works out about right. One ohm in the ground of a head light will eat up nearly half your voltage (1 ohm x 7 amps = 7 volts lost).

As Mid pointed out in another thread, measuring 1 ohm reliably is tough. My meter reads .4 ohms with leads shorted, and it's a good meter. That's why I measured voltage loss under load - but Mid's correct, it isn't ususally as easy as the headlight was.
 
I was right? Seriously? You have to tell my SWMBO, because she doesn't believe anything I say anymore!
 
"Midlife" said:
I was right? Seriously? You have to tell my SWMBO, because she doesn't believe anything I say anymore!
Click to expand...

Must be a genetic flaw on the X chromosome, my SWMBO has the same issue :craz
 
Ditch the old wiring method and do the Headlight relay Mod. I did it, and it works great.

Do a search here. The schematic is floating around.
 
"70_Fastback" said:
Ditch the old wiring method and do the Headlight relay Mod. I did it, and it works great.

Do a search here. The schematic is floating around.
Click to expand...
+1. The wiring job Ford did was kinda hokey by 1960s standards, not that some modern cars are much better, and there was quite a bit of v drop when everything in there was NEW. :rofl

Most 1990s Chevys benefit from the relay mod too...way to go, GM engineers, LOL.
 
I'd agree on the relays - you'll loose some serious voltage through the switch, plus it won't handle the amps for halogens.

But, most of the schematics I've seen for the relays don't address upgrading the ground. Without doing both, you're not getting the full benefit.
 
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