volt meter goes crazy with engine running

[Greg Buechner]

When the engine is running on my 1950 side mount 6 volt 8N the digital volt meter starts displaying numbers all over the place when I bring it near the engine. Not even hooked up to anything. Just come within a 8 inches of the engine or battery and the numbers jump all over the place.

Is there suppose to be that much interference coming from the motor?

 

[Bruce (VA)]

Yes.

See tip # 47.
50 Tips

 

[oldtanker]

Yep Bruce is right. The old systems are not shielded as well as the new ones. They didn't need to be. Who worried about radio interference on a tractor that old?


Rick

 

[Greg Buechner]

Thanks for the info. I really need to memorize the 50 tips before I go asking questions.

 

[Bruce (VA)]

Yes, we have monthly quizzes; failures get sent to the JD board for 30 days!

Where are you in ME? I've got living relatives in Lincoln & Millinocket & a lot of dead ones in Lamoine.
50 Tips

 

[Dean]

Greg:

You will find that a DVM is of very little usefulness when working on your tractor. A good analog meter is MUCH preferred.

Dean

 

[36 Coupe]

I saw some nice analog meters for 10 bucks.Read the instructions and learn to read the ranges.I have many meters in my shop but still use one I bought in 1952.I have 2 dead digital meters on the shelf.Your winky blinky digital meter is picking up ignition noise.

 

[kev8n]

It pick up noise almost anywhere you go with the leads open. The input impedance is so high that charges on all kinds of common objects will be sensed. If you short the leads together it should read very close to zero.

Non-electronic analog meters (VOM's) are not subject to this behavior because the input impedance is relatively very low so these stray voltages do not develop a great enough signal.

fwiw
kevin

 

[teddy52food]

Just goes to show the power of a magnetic field.

 

[LarryT]

Go analog. about the only thing the digital is good for is the functions when nothing is running. and you cant even time a mag with a digital.

 

[Anonymous-0]

What you have is known as a "cheap" digital meter. If you want to work on your N with a "cheap" meter, you need to use an analog meter. It will not be effected by the RF. By the way my Fluke digital meter works fine on the tractor, but it cost over $300.00.

 

[kev8n]

Note that he indicates that it does this "...not even hooked up to anything". *ANY* DVM will behave this way when the leads are floating -no matter what price.

An FET-input analog meter will do the same when the leads are hanging out in space.

Most meters are just fine when connected in the circuit.

fwiw

kevin

 

[Anonymous-0]

"*ANY* DVM will behave this way when the leads are floating -no matter what price. "


Not true. Mine works fine, even when the leads are not connected to the circuit.

 

[kev8n]

It is not a matter of not working correctly.

When the leads are disconnected on the VOLTAGE function, (not current, not resistance), the extremely high input Z will allow the meter to sense whatever electric field is present and this is present everywhere except in some shielded enclosure or closed loop. The elecric field strength of the space near the earth alone is 100-300 volts/meter without any electical devices at all.

Yes, it is possible to read near zero at any given time with a digital voltmeter but it is normal for the thing to read random fluctuations and sometimes even over-range blanking the display with the prods open-circuited- no matter where you are. If shorted together, they will read essentially zero. If you don't experience this I would suggest that your meter is the one which is malfunctioning.

I am only pointing out to the originator of the thread that this is not atypical behavior and to be expected. When the voltmeter is not in the circuit the voltage is undefined. When the leads are shorted the conditions are defined as zero and that is what you then get.

kevin

 

[Anonymous-0]

I appreciate your schooling for a retired Electical Engineer. I am trying to point out to the OP that a cheap digital meter will not work well when troubleshooting his tractor, whereas a good digital meter or a cheap analog meter will perform as needed.

 

[kev8n]

I was responding to the *original* observation that when connected to (nothing) there were random voltage indications and *your* statement that this is because of the low-quality meter used, though the instrument was not even specified by the poster.

My claim is that this is normal behavior. Perhaps re-reading in the context of the original posting would make this clear.

I use all manner of instruments including Fluke, Keithley and Hewlett-Packard most of which are NIST traceable and calibrated, as well as Chinese "throw-aways".

I had no intent of casting aspersions on your credentials, John Fluke company, nor the forces of nature. My apologies for umbrage you make take but I stand by my claim for the reasons stated.

It is agreed that a cheap analog would probably be fine and is what I would normally use as well, but this was not the question posed originally.

g'day
kevin