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How to test for a parasitic leak on 6-volt positive ground (30 Plymouth)


hursst
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Hello,

   I have a severe parasitic leak on the battery of my 1930 Plymouth.  I have to disconnect the battery almost immediately, before the battery starts discharging.  For example, a few months back, I got some gas, checked the oil, brake fluid, etc, messed around for about 10-15 minutes, and the battery did not have enough power to start the car, just in that short amount of time.  Luckily, I was able to push start the car.

 

   I tried testing it myself with a multimeter, but I have no idea what I am doing with electric.  I know to start with the battery to see how much draw is occurring, but I get no results, because I don't know what I'm doing.  

 

   Can I test at the battery to see if a parasitic leak is occurring, using a multimeter with the Plymouth's 6-Volt Positive Ground system?  If so, how do I do it and what are the settlings for the multi-meter?  Thanks for any advice!

 

-Chris

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You can, but you should start with a test light to see how bad it is. Here is what i would do. When you shut the car off with a charged battery (that you know is going to get discharged), disconnect a battery cable, "ground" (positive) would be a good choice, and then hook the light between the battery post and the cable you disconnected. A bad draw will make the light pretty bright, and would be too much for the multimeter, so always start with the light.

 

Now that you have a glowing light, indicating a draw, disconnect accessories until the light goes out. I would start by disconnecting the wire at the generator cutout, and I mean the one going from the cutout to the wiring harness. If it makes the light go out, your cutout is stuck. If that is the problem, maybe you can take the cover off and unstick it.

 

An electric clock, if it has one, is another likely area for trouble.

 

 

Edited by Bloo (see edit history)
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8 minutes ago, Grimy said:

My money is on a stuck cutout, given how quickly the battery is discharging.

I agree, it's really the only thing that would be wrong, but still allowing the car to be driven without other noticeable issues.  If it was a heavy short somewhere, then the symptoms would be very different.

 

Only one wire to disconnect on the generator cutout, to find out for sure.

 

 

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Thanks, I will investigate this in the next few days, when I have some time to get out to the garage, and report back.  Thanks for the replies!

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A clamp style multimeter that measures up to 50 or so DC amps is a very handy tool to own for this type of work.  I’ve been able to successfully nail down a couple of ignition off battery drain problems on my vehicles.  Very quickly.  

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Someone mentioned a stuck cut-out!

If it is a stuck closed cutout,when the car motor is no running and 6 volts is going back to the generator, It wants to run like a motor ,(a very weak motor that will run with no load of belts etc.on a bench) it will soon get heated up not turning. 

All this is easy to check just by disconnecting the lead to the to the cutout and look for an arch draw.

If the cutout is stuck or shorted? for fun loosen the fan belt..the genertor should want to weakly turn .If an adjustable brush type gennie this can vary the speed..

Backyard method test work sometimes.

Edited by Flivverking
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Finally got back out to the garage.  Yes, it is a stuck cut out switch, as most folks predicted.  Thanks for all the comments and suggestions!

 

I guess the next question would be, why would it stick?  I freed it up pretty easily.  Should I replace it, or just monitor it to see if it keeps doing it?  I filed the points a little to maybe make them a little cleaner to resist sticking...

 

-Chris

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Don't want to seem like the prophet of doom here, but may I respectfully  suggest that you check that your generator armature has not been damaged because of the cut out sticking closed.... even for a short period of time.

As Flivverking said, the generator tries to act as a motor if the cut out sticks closed with the engine off. But it cannot rotate because of the friction of the fan belt ( if correctly tensioned} . While this is happening lots of current flows from the battery through the generator. Not only discharging the battery quickly but also overheating the generator causing  the armature to get very hot very quickly, stressing the windings and insulation of the armature and solder joints on the commutator . Many generators were burnt out because of cut outs sticking closed when the  engine was switched off. As to why it's sticking one common cause is- as the points open and close material is removed from one contact and deposited on the other( just like the points in your distributor) causing a hole on one and a mountain to build up on the other (figuratively speaking], if enough material builds up it causes a mechanical latching of the contacts, thus causing the problem. Have a good close look at the cut out contact  points and at a minimum remove the cover band on the generator and have a look at the armature for signs of overheating , hopefully you will find it looks good.

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On the point of generator damage agree---while  it's too late now, that kind if draw should've had the generator warmer than usual from internal heating, but that would/ve taken experience to be detectable...

Also, , IIRC from my generator days, relay and voltage regulator points often have a subtle wiping action, and polishing, rather than just filing, was recimmended when cleaning the points...details should be online...

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