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Discussion Starter #1
hi Guys

I wondered if anyone could or has tested their rear brake light voltage at the plug? I am surprised at the reading I got and wondered if the voltage is limited by the fuse in the fuse panel?

thx!
 

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Fuses don't limit voltage. Well at least they shouldn't unless things are not sized right but it should start blowing at that point. I would expect near 12 volts at the lights, maybe more depending on the charging system. Too small of wire size would add resistance to reduce voltage as well.
 

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Too much resistance in the headlight switch and a bad ground could be problematic.
 

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Discussion Starter #7
Thanks Guys

I have one battery between two cars . I jumped to the battery empty mustang with the other car using Jumper cables. Got strong head lights and my reading was 5 volts at parking light position tail lights. I will inspect properly again! Good to know its not a fuse limiting the system.
 

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Thanks Guys

I have one battery between two cars . I jumped to the battery empty mustang with the other car using Jumper cables. Got strong head lights and my reading was 5 volts at parking light position tail lights. I will inspect properly again! Good to know its not a fuse limiting the system.
All that shows is that you have resistance somewhere in the circuit.

Using a multimeter you can narrow it down.

But first I’d go through and look for loose wires at any connection point.
 

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If you don't have any bulbs in you'll read full battery voltage no matter what. The correct way to measure voltage is with all the bulbs in and working properly. You measure across the bulb with the bulb lit. Subtract this voltage from measured voltage at the battery, this will give you the amount of voltage drop or amount of voltage lost of the wiring and switch due to resistance. Ohms law 101
 

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Discussion Starter #10
If you don't have any bulbs in you'll read full battery voltage no matter what. The correct way to measure voltage is with all the bulbs in and working properly. You measure across the bulb with the bulb lit. Subtract this voltage from measured voltage at the battery, this will give you the amount of voltage drop or amount of voltage lost of the wiring and switch due to resistance. Ohms law 101
Hey Huskinhano

How am I too measure across the bulb? If the bulb is in the socket I cant get to the back of the bulb or inside of the rubber plug when in use? Am I going to craft something?

Thx
 

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If you're testing the voltage by back probing the light unit, the pre-bulb voltage should be battery voltage and the post-bulb voltage would be about 6 (due to the resistance in the bulb filament).
 

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Hey Huskinhano

How am I too measure across the bulb? If the bulb is in the socket I cant get to the back of the bulb or inside of the rubber plug when in use? Am I going to craft something?

Thx
Unfortunately you would have to scrape a little of the insulation off the wire and close as possible to to the negative or shell on the bulb. This way you'll see resistance from the shell of the bulb to the socket and to the contact points
 

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Discussion Starter #13
If you're testing the voltage by back probing the light unit, the pre-bulb voltage should be battery voltage and the post-bulb voltage would be about 6 (due to the resistance in the bulb filament).
I probe the wired rubber tail light plug itself with the lead s and get 5.1 volts. By your description I am guessing thats the "Pre Bulb " test?/
 

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Remember that it’s called a circuit. Juice needs to start at the battery but also return to the battery. So grounds are as important as power.
 

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Discussion Starter #16
Thanks Dave.
Yes I Kinda know that one. I have been wondering where I am going to try to find/trace a ground point in the "circuit" . Both wires run into the rear fender and I dont really want to open the rear interior or the door sill. I think it runs up to the dash area but the schematics dont seem to help but tell me they go to the turn signal.

I have proven to myself that the lower post in the rear socket grounds to the socket itself with continuity. But I'm thinking there should be more. Should be a better ground going on. Maybe even the brake light switch on the pedal might be corroded. or somewhere else ?
 

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One way of locating voltage drops requires a constant for accurate answers. Your constant is at the battery connections it self. Using continuity checks is a losing proposition as your meter usually produces only 1.5 volts from itself for measuring. Leave those test procedures for componet testing or wringinng out wiring. By testing in a "live" condition the circuit failures will be exploited by the current the loads draw. So one can take advantage of that fact. By making an extenstion for your meter that can reach far enough to the rear of your car.you can accurately see the drop instantly power or ground. Then work your way back till you find the offender or offenders.
 

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Temporarily attach a jumper wire between a good ground and the taillight housing. I've found the '65-66 taillight housings to be notoriously poorly grounded.

Hint: Years ago I made myself a jumper wire that I've kept in my toolbox for probably 30 years, now. It's an old telephone handset cable with all the conductors wired together and soldered and terminated on each end with alligator clips. Easily expandable to 3 feet. Always a bunch of 'em to be found floating around.

 

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I made one alteration to my electrical system that fixed a multitude of electrical issues. In addition to the "factory-like" engine ground from the head to the firewall, I added a second cable from the post on the block where the negative battery cable attaches directly to the front frame rail using the anti-sway bar mounting bolt. It instantly increased the brightness of every bulb in my car and I was able to read full battery voltage at nearly every point in the car.

738939
 

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Discussion Starter #20
Yes
I made one alteration to my electrical system that fixed a multitude of electrical issues. In addition to the "factory-like" engine ground from the head to the firewall, I added a second cable from the post on the block where the negative battery cable attaches directly to the front frame rail using the anti-sway bar mounting bolt. It instantly increased the brightness of every bulb in my car and I was able to read full battery voltage at nearly every point in the car.

View attachment 738939
Thanks Yes ,I did something similar.
 
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