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Old 10-10-2019, 09:08 AM   #1
1930deluxe
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Default Positive Ground

When the car is a positive ground the frame and body is positive charged. Where does the taillights and headlights ground so they work? How do I use a voltage tester to see if I have current to the headlights, taillights ect?
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Old 10-10-2019, 09:19 AM   #2
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Default Re: Positive Ground

Ground is positive and the hot wires are all negative . Any hot to ground will light the lights. You don’t have to treat it differently.
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Old 10-10-2019, 09:41 AM   #3
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Default Re: Positive Ground

There is no difference between negative and positive ground circuits except for the direction of current flow; the circuit is the same.

The hot wire is switched to the bulb socket contact. The socket shell is connected to ground through the various body and frame hardware connections, back to the battery ground post to complete the circuit. I suspect more lighting problems are traced to rusty ground paths than bad wire connections.
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Old 10-10-2019, 09:41 AM   #4
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Default Re: Positive Ground

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When the car is a positive ground the frame and body is positive charged. Where does the taillights and headlights ground so they work? How do I use a voltage tester to see if I have current to the headlights, taillights ect?
The headlight bulb sockets ground to the headlight bucket which grounds to the headlight bar which grounds to the frame all through the bolts that hold everything together. Similar for the taillights. The current runs from the light switch through the wire that enters the light socket at the bottom and then through the bulb (causing it to light)and then to the light socket that is grounded. Take the bulb out and test between the pin in the socket and the ground with the lights on, you should have about 6 volts. If you really want to measure current flowing to the bulb they have test equipment you can place over the wire to do that but it is not commonly used.

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Old 10-10-2019, 09:51 AM   #5
katy
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Default Re: Positive Ground

Look at the wiring diagram:
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Old 10-10-2019, 09:54 AM   #6
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Default Re: Positive Ground

With the voltage meter turned to voltage testing, you would turn the light switch on, then ground the red probe to ground, headlamp bucket or bolt, then hold the black probe to the end of the wire and it show show voltage on the meter. Remember there are two or three wires to check depending on your harness on the headlights and the brake and tailamp. The brake light will only show voltage at the taillight when the brake switch is activated.
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Old 10-10-2019, 10:59 AM   #7
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Default Re: Positive Ground

Kinda as said, nothing really changes. Everything will eventually ground thru the frame to the positive battery cable and the positive battery terminal.

Checking for power is the same whether using a meter or simple test light. If the test light has a 12v bulb, it will just show dim while checking 6v.
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Old 10-10-2019, 11:49 AM   #8
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Default Re: Positive Ground

With the modern digital multi-meter set to voltage, you can connect either probe to power and the other to ground and it will still get an indication but it will also show if polarity is off by adding a minus symbol to the digital reading. A test lamp will still light up but it has to match the voltage it is designed for.

Tail lamp brackets should be well bonded to the fender or what ever it's attached to. Fenders with tail lamps should be bonded to the body and so on.
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Old 10-10-2019, 09:48 PM   #9
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Default Re: Positive Ground

Monday I will see why the lights are not working. The motor runs fine, the horn works fine but the lights are not working.
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Old 10-10-2019, 10:43 PM   #10
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I have never attempted to understand the positive ground system on early fords...thanks to all who have posted their knowledge on this...I'm 59 yrs old and still learning..��
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Old 10-11-2019, 12:16 AM   #11
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Default Re: Positive Ground

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I have never attempted to understand the positive ground system on early fords...thanks to all who have posted their knowledge on this...I'm 59 yrs old and still learning..��
I think a common mistake with not understanding positive ground is just that folks over-think it.

Other than polarity, wiring, testing and trouble-shooting positive ground is no different than negative ground.

We are so used to think that the "hot" lead has to be positive. Well if positive is ground then the negative side is the "hot" side. Just remember that when using a volt meter to test, put the red (+) lead to ground and use the black (-) lead to measure voltage.
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Old 10-12-2019, 08:43 AM   #12
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Default Re: Positive Ground

Ford changed to the positive ground system in the early days of the Model A so as to get the best efficiency from the low voltage system. The direction of electron flow was what the engineers were looking at. They were using the power house generator with a model T cut out so the whole system was a running engineering change in the first 2 years of production. Model A systems used a tiny fraction more energy than the model T and it had no magneto so they wanted it to be as efficient as possible.

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Old 10-12-2019, 08:50 AM   #13
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Default Re: Positive Ground

My first 2 cars were early 60's MG midgets when I was 15 years old. I was told then, the reason positive ground was used was because of poor cloth wire insulation. Positive ground system was better at preventing corrosion? The positive ground was hard to get my one semester of auto shop brain wrapped around!
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Old 10-12-2019, 11:14 AM   #14
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Default Re: Positive Ground

im half dumb and havent broken a multi meter yet. its all the same with the polarity reversed. get some sand paper and get to scratching and shining. use your meter set to continuity or ohms to test that you are grounding. if its a junker i just scratch at the body. a nice car I alligator clip a length of wire to the ground on battery and use that as one leg of the test. On my car I have added a wire from battery ground to the engine. I was getting a glowing radiator support bolt on the first few starts of the car.
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Old 10-12-2019, 01:14 PM   #15
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Default Re: Positive Ground

Adding bonding is way to get a good ground path without taking the car apart and actually cleaning the pathways that can get corrosion or crud that adds resistance to the ground path. Most wire was poorly insulated with resin and cloth until vinyl coverings improved things in the 1950s. I doubt if this was a consideration when the wiring was new and clean. The manufacturers have never figured a car was going to last as long as some of them have. Life expectancy was around 5 years in the early Model T days and 10-years beginning with the Model A. That life limit has only changed relatively recently due to requirements set in the lemon laws that were passed by the government. 20-years is closer to the standard now days.
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Old 10-12-2019, 08:25 PM   #16
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Default Re: Positive Ground

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Originally Posted by 1930deluxe View Post
When the car is a positive ground the frame and body is positive charged. Where does the taillights and headlights ground so they work? How do I use a voltage tester to see if I have current to the headlights, taillights ect?
There is what is called "conventional theory" of electron flow through the system, and what is called "electron theory" of electron flow. Conventional theory (now disproven) said that current flow is from the positive terminal of the battery, through the system and back to the negative terminal.
The electron theory is the accepted theory now, and says that current flows from the negative terminal, through the system and back to the positive terminal of the battery. Therefore, a positive ground system makes more sense!
Think about this: In a modern negative ground system, electrical current flows out of the negative battery terminal, through the ground cable to the frame, engine, and body, then through the starter, coil, lights, etc. ending up at the positive battery cable and flows back the positive battery post.
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Old 10-12-2019, 09:19 PM   #17
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Default Re: Positive Ground

When i got my Tudor the rear lights did not work, the previous owner did not add ground wires to cure the problem caused by having fiberglass rear fenders.
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Old 10-13-2019, 06:49 PM   #18
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Talking Re: Positive Ground

If you painted your fiberglass fenders with a metallic paint, then you wouldn’t need a ground wire!
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Old 10-14-2019, 11:55 AM   #19
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Default Re: Positive Ground

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If you painted your fiberglass fenders with a metallic paint, then you wouldn’t need a ground wire!


The more metallic in the paint the better.
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