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When 12v is not what it seems?
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ABloch
Posted 2019-04-01 1:11 AM (#580221)
Subject: When 12v is not what it seems?



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Newly acquired 1956 Fury that has the brake light switch relocated to the brake pedal. Wire number one goes to the brake lights. Grounds are good and if I run power direct from the battery they both light up.

Here is the mystery... The second wire shows 12v. to the switch. Once the pedal is depressed and the switch activates the voltage disappears. If I disconnect the wire and touch it to ground one would expect a spark. Nothing. When testing for continuity I am unable to trace where the phantom 12v is coming from.

I am fairly adept at wiring. I am using a Fluke multimeter (set on DC) Battery shows 12.7v at the terminals and 12.2 at the switch. Being a '56 most of the wires I see under the dash are ALL beige cloth so trying to find the red from the circuit breaker atop the headlight switch is not working out for me.

I have come to the collective to seek guidance.....Anyone have any suggestions?



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wizard
Posted 2019-04-01 1:39 AM (#580223 - in reply to #580221)
Subject: Re: When 12v is not what it seems?



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Yes, since the wires are new I suspect bad contact at the splice to the original wire. The original wires have verdigris and needs to be bathed in citric acid for to remove the verdigris. A bad contact might give 12 volt, but as soon as you give some load like the brake lights, the bad contact cannot supply the amps needed.
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57chizler
Posted 2019-04-01 12:07 PM (#580248 - in reply to #580223)
Subject: Re: When 12v is not what it seems?



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Yeah, the multimeter draws so little current that it will read voltage from a faulty connection but the faulty connection won't handle the current load from a device or a direct short to ground.
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ABloch
Posted 2019-04-01 6:52 PM (#580267 - in reply to #580221)
Subject: Re: When 12v is not what it seems?



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I have never experienced this so I thank you for your insight. I was completely flabbergasted.

I went ahead and by passed the dead wire and all is good now.
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1959Dodge
Posted 2019-04-03 1:22 PM (#580347 - in reply to #580221)
Subject: Re: When 12v is not what it seems?



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Location: Yorba Linda, Ca
"Most Likely"

The way that brake light switch is wired, is it returns one of the brake light bulb's terminals to ground.
(we'll call it terminal "G")

The other terminal on the bulb is directly connected to +12 volts.(we'll call that terminal "H").
The way the brake switch is wired is one terminal on the switch goes to ground (Or negative),
the other terminal is wired to terminal Terminal "G", on the bulb.

So if the brake pedal is not depressed, the switch is in the "Open" position, (IE not connected to gnd),
so you will see 12 volts at the terminal on the switch that does not go to ground,
the 12 volts goes through the lamp's filament and back up to the ungrounded terminal.
(if you short that terminal to ground, then the brake lights will come on.
If you hold the brake pedal down you will measure close to "0" volts at both switch terminals.

We are leaving the turn signal switch operation out of the above info, to keep things simple!

Gary


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Old Ray
Posted 2019-04-03 5:32 PM (#580358 - in reply to #580347)
Subject: Re: When 12v is not what it seems?



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1959Dodge - 2019-04-03 11:22 AM "Most Likely" The way that brake light switch is wired, is it returns one of the brake light bulb's terminals to ground.


Gary, that is the way many interior / courtesy lamps are wired and also the horn circuit via a relay, they are ground side switched.

New cars with LED lights might use a similar design but the Forward Look cars (with a turn signal switch) I think, use a conventional system of having the bulb base grounded and the hot circuit switched to the bulb terminal, including the brake lights?
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1959Dodge
Posted 2019-04-05 1:31 AM (#580402 - in reply to #580358)
Subject: Re: When 12v is not what it seems?



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You are right about the way the 56 plymouth was wired "from the factory",
In fact I looked at a wiring schematic of a 56 Plymouth and it is as you described.

But who knows what kind of "jerry Rigging" was done to the car from 1956-present?

It is easy enough to check, the base, (the big part of the bulb) should go to chassis ground.
If you measure 12 volts there, then it has been "Jerry Rigged".

My parking brake light on my 59 Dodge is wired the way I described, the switch
connects the light to ground when the switch is closed, (IE brake off).

Gary
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