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Old 04-04-2021, 10:11 AM   #1
RalphG
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Default Stray Voltage?

I was going to test the voltage on the 52 Merc while it was idling. Just a cheap little voltage/multi tester that seems to work fine on everything else but it went a little wild when I got close to the car. Random readings jumping all over the screen. Obviously there was no way to get any kind of accurate test on the voltage. Is that normal for these systems? Where is it coming from?
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Old 04-04-2021, 10:31 AM   #2
5851a
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Default Re: Stray Voltage?

Do you have non resistor plug wires? Seems to be something about generator charging fields also.
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Old 04-04-2021, 10:41 AM   #3
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Default Re: Stray Voltage?

My guess is that you are picking up interference from the high voltage ignition circuit. My other guess is that you left the flux capacitor energized and you will end up back in 1952 if you exceed 88 mph.

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Old 04-04-2021, 10:50 AM   #4
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Default Re: Stray Voltage?

Digital or analog??
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Old 04-04-2021, 11:49 AM   #5
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Default Re: Stray Voltage?

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Originally Posted by 1931 flamingo View Post
Digital or analog??
Paul in CT
Most digital volt ohm meters will get a signal from
the plug wires and jump all over the place. G.M.
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Old 04-04-2021, 12:18 PM   #6
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Originally Posted by G.M. View Post
Most digital volt ohm meters will get a signal from
the plug wires and jump all over the place. G.M.



George, my doctor hooked me up like that for my last stress test.
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Old 04-04-2021, 12:29 PM   #7
RalphG
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Default Re: Stray Voltage?

This tester is digital readout. I notice when I have the Merc idling in the shop it just overcomes the shop radio with static. Ignition wires maybe? They might be almost as old as the car. I don't know but they seem to work well for their intended purpose otherwise.
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Old 04-04-2021, 12:39 PM   #8
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Default Re: Stray Voltage?

Normal with cheep meters. I've even seen it with carbon core wires.
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Old 04-04-2021, 03:32 PM   #9
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Default Re: Stray Voltage?

Use an analog meter - digital is a no go.
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Old 04-04-2021, 03:35 PM   #10
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Default Re: Stray Voltage?

Yep, get an analog meter.
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Old 04-04-2021, 05:28 PM   #11
1931 flamingo
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Default Re: Stray Voltage?

Harbor Freight should have a cheap one good enough for your needs.
Paul in CT
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Old 04-05-2021, 08:46 PM   #12
RalphG
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Default Re: Stray Voltage?

I was discussing the subject with my brother on the weekend. He remembers a neighbour of my grandmothers commenting that he always knew when my dad drove up (across the street) with the 52 Merc as it would cause interference on his tv. This was in the early 1960s. Spark plug wires I guess. Not a big problem. I get ahold of an analog tester one of these days.
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Old 04-06-2021, 06:26 AM   #13
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Default Re: Stray Voltage?

Ralph: I bought 10 of these at a dollar store in FT. Worth about 15 yrs ago, gave them all away thru the years, I keep one in the trunk of my’41. Cheapest I could find it on eBay was 6 bucks.
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Old 04-06-2021, 08:58 AM   #14
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Ralph: I bought 10 of these at a dollar store in FT. Worth about 15 yrs ago, gave them all away thru the years, I keep one in the trunk of my’41. Cheapest I could find it on eBay was 6 bucks.
That looks exactly like one I bought many years ago. I still have it but it quit working years ago which is why I bought the new digital meter (Princess Auto).
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Old 04-06-2021, 12:52 PM   #15
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Default Re: Stray Voltage?

You just about need both types. Digital is best for some things, but analog is far better for other types of measurements.
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Old 04-07-2021, 01:41 AM   #16
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Default Re: Stray Voltage?

Ralph you are a mobile emp. It's pretty weird that you can effect anything outside of the car system to interfere with outside radio waves. I'm sure it's its just a coincidence of circumstance. or the winter was really long!. Digital meters can be problematic. RF interference with sensitive digital equipment can be problematic. But a flathead doesn't have the ability to do that outside of a ft of the car or something being plugged into to direct grounded internal power system. Even if you have a magneto. It's just not that powerful.


Analog works most of the time and it can be used to monitor airborne electric static with the right setup. Best to not overthink it. Never had an issue on any meter.


If you think your plug wires are bad and arcing. Go out at night and crank the motor up with the lights off. You'll see if things are sparking under the hood.


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Old 04-07-2021, 09:05 AM   #17
RalphG
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Default Re: Stray Voltage?

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Ralph you are a mobile emp.


If you think your plug wires are bad and arcing. Go out at night and crank the motor up with the lights off. You'll see if things are sparking under the hood.
.
There is really nothing to indicate the spark plug wires are bad other than the interference they cause on other devices. They appear solid and intact and I don't ever recall seeing any unusual lights in the engine compartment in low light situations. I found my old analog meter but it is still dead. :-) Once the Merc is legal again I'll get it checked at my brother's place where he might have an analog meter.
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