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Locating a short circuit
#1
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R Howe
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When my headlights are on, they flash in accordance with the documentation stating that there is a short circuit in the headlight wiring somewhere. I have been trying to locate this short but am probably working under a misconception. I have disconnected the ground cable from the ground pole of the battery. I then take my voltmeter and check the voltage between this ground cable and ground terminal of the battery. Is it not true that when everything is shut down there should not be any voltage shown within this portion of the circuit or am I, as I said, suffering under a misconception?
Thanks for your expertise and help with all of my questions.

Posted on: 3/30 13:51
Roger Howe
Whitewright, TX
1937 120C Touring Sedan
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Re: Locating a short circuit
#2
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HH56
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There are many tie ins and places for feedback between circuits so an ohm meter may not narrow things down too much.

If your car has the inline fuse between the headlight switch and instrument/reading light switch then if it is not blowing that would rule out a lot of possibilities in the dash and rear lights. If it does not then you can include all the wiring to the tail lights and dash illumination.

Can you narrow down the places to look by changing the dimmer or headlight switch combinations and have the problem go away. If you can then concentrate on just the hi or low beam or bulbs that are on when the short happens but if it is constant problem on all positions then it will probably be a matter of disconnecting one wire at a time until the problem goes away. The dimmer switch itself has been a source of problem before.

Some other common places that have been found on other year cars are anywhere the wire passes a hole in the body or chassis and at the sockets themselves. On sockets, sometimes the old wire insulation shrinks back and leaves a space where the bare conductor can touch the side of the hole where it enters into the grounded socket shell.

Posted on: 3/30 14:20
Howard
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Re: Locating a short circuit
#3
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BigKev
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Not sure with that year of Packard, but if there is a self resetting breaker or self healing fuse, the could be weak and causing a cycleling when under the load of the headlights.

A multimeter with a amp settings might we useful to determine the load the headlights are putting on the system. The removing disconnecting the bulbs at their sockets and measueing the load again to see if there is anything else pulling a load.

And ohmeter can be useful with the battery disconnected and then it used with the to see where their is a path between ground and feed.

My battery was being drained and I couldn't figure out why as everything was off. By using a ohm meter I tracked it down to a sticky trunk light switch.

Posted on: 3/31 19:40
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Re: Locating a short circuit
#4
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DavidPackard
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This thread has a discussion about thermal relays (aka circuit breaker).

https://packardinfo.com/xoops/html/mod ... &order=ASC&status=&mode=0

The total discussion goes on for at least a few posts.

Posted on: 3/31 20:46
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Re: Locating a short circuit
#5
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Gar
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What year is your Packard?

You could have a weak ground (normally the positive wire on a 6 volt system most often negative on a 12 volt system).

As mentioned in the earlier threads; 1-a dirty relay or a through hole fitting could also cause your problem.
1-Relay will get warm/hot with usage and can cause the contacts to expand slightly. If the contacts are dirty or pitted, this will generate more heat and cause great expansion resulting in possibly dropping the circuit temporarily.
2-wire insulation could be frayed or missing a rubber grommet where it passes thru a piece of metal (firewall/fender). You could also have a corroded connection.

Assuming you are not continuously blowing fuses... (with the motor turned off, using an accurate DC volt meter: check battery terminals (+ to -) and record voltage; check battery to relay/ammeter (input side) and record voltage; check output side, record voltage; check input side of switch, record voltage; output side (dimming off), record voltage...and continue all the way to the light socket. Time consuming and tedious. If any voltage varies at any device, you can by pass that device and continue testing.


I however, am a believer in the 50% rule. Split the wiring circuit in half and trouble shoot at the source (ie:battery) to the midpoint of the circuit. So battery to light switch then... light switch to something between and light socket (junction box/wiring connectors).

Voltage should drop very little from end to end. Also grounding issues can cause problems as well.

Best of luck

Posted on: 4/7 14:37
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Re: Locating a short circuit
#6
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R Howe
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Hi, Gar. Thanks for the tips. I shall definitely perform those also. BTW, my Packard is a 1937 120C Touring Sedan.

Posted on: 4/7 18:15
Roger Howe
Whitewright, TX
1937 120C Touring Sedan
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Re: Locating a short circuit
#7
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Greenfield
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I'd have to agree that if you disconnect the ground cable from the battery and if you hook a voltmeter on the battery ground and the ground cable, and you have voltage then you have a short somewhere or what is referred to as a parasitic draw/leak. But the question is where, and how do you isolate it? Most importantly though is heat generation and whether the short could cause a fire.

Ordinarily, if you have a fuse box and turn everything off, you can pull fuses until the voltage goes to zero, then at least you can figure out what circuit is involved. You might not have a fuse box though to work with.

How many volts are present when everything is off?

Posted on: 4/20 15:43
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