Not an electrician

[13fx]

To say I have a kindergarten level of knowledge for electrical work might be generous. Might just be my lack of knowledge here, but I think I have an issue on a tractor with a single wire alternator. With the tractor running if I put the negative lead on the negative post, or the body of the alternator my multimeter goes nuts giving me readings all over the place. When I put the positive lead on the alternator nothing changes. With my basic knowledge of electricity I wouldn't think I would get any reading with only the negative lead hooked up. Can someone please explain what is going on?

 

[JMOR]

Digital meters are very sensitive devices and they don't like noisy (RF interference, etc.) and pick up enough to yield unreliable readings.

 

[wore out]

To say I have a kindergarten level of knowledge for electrical work might be generous. Might just be my lack of knowledge here, but I think I have an issue on a tractor with a single wire alternator. With the tractor running if I put the negative lead on the negative post, or the body of the alternator my multimeter goes nuts giving me readings all over the place. When I put the positive lead on the alternator nothing changes. With my basic knowledge of electricity I wouldn't think I would get any reading with only the negative lead hooked up. Can someone please explain what is going on?

Digital multimeters are confused by the "electrical noise" made by charging and ignition systems (IF you have a ''gasser").

A high $$$ digital meter will usually work better, or an analog meter won't be affected by the "noisy" environment.

 

[Destroked 450]

To say I have a kindergarten level of knowledge for electrical work might be generous. Might just be my lack of knowledge here, but I think I have an issue on a tractor with a single wire alternator. With the tractor running if I put the negative lead on the negative post, or the body of the alternator my multimeter goes nuts giving me readings all over the place. When I put the positive lead on the alternator nothing changes. With my basic knowledge of electricity I wouldn't think I would get any reading with only the negative lead hooked up. Can someone please explain what is going on?

What alternator do you have, the Delco’s I use don’t have a negative post, they have a bolt hole in the case to attach a ground wire with a bolt

 

[13fx]

Ok, so there's no guarantee that things are ok, but the screwy readings are most likely my cheap multimeter picking up interface from the ignition system.

 

[13fx]

What alternator do you have, the Delco’s I use don’t have a negative post, they have a bolt hole in the case to attach a ground wire with a bolt

It says Delco Remy on the case but below there it appears to have some Chinese marks on it so my question that and suspect that it's a Chinese knockoff

 

[Destroked 450]

Is this a recent conversion
Was the tractor originally 6 volt positive ground
Delco 12 volt alternators only work with negative ground

 

[Geo-TH,In]

Digital meters are very sensitive devices and they don't like noisy (RF interference, etc.) and pick up enough to yield unreliable readings.

Jessie,
Not all digital meters will go nuts when measuring 12v when the alternator is charging.
I keep these meters in a to-go bag. Each meter has a special feature.
The meter in the middle I use when I test 12v batteries, 12v charging and cranking voltages.
It doesn't go nuts.

 

[13fx]

Is this a recent conversion
Was the tractor originally 6 volt positive ground
Delco 12 volt alternators only work with negative ground

It was a prior owner converted it from 6 volt positive ground to now 12 volt negative ground.

 

[Brown David]

Jessie,
Not all digital meters will go nuts when measuring 12v when the alternator is charging.
I keep these meters in a to-go bag. Each meter has a special feature.
The meter in the middle I use when I test 12v batteries, 12v charging and cranking voltages.
It doesn't go nuts.

I've got two cheap meters currently, (pun intended), and have had a few previously.

I've never encountered this phenomenon of insane meters.

 

[used red MN]

The trouble with cheapo analog meters is the majority of them have a scale that is in the range to check a 12 volt DC system is 0 to 50. So you do get an idea of what it is charging. Here is an example meter. Example meter

 

[RobCons]

I am a little confused. When you touch only one lead, either one, from the meter to the tractor and the other lead is not touching anything the meter goes crazy?

 

[BarnyardEngineering]

I've got two cheap meters currently, (pun intended), and have had a few previously.

I've never encountered this phenomenon of insane meters.

Have you ever touched JUST the negative lead to an alternator for no apparent reason?

 

[BarnyardEngineering]

I am also confused. What was the purpose of touching just the negative lead to the alternator? It's really not a valid test for anything.

If you want to test alternator output voltage, you touch the negative lead to the chassis of the tractor, and the positive lead to the large terminal on the back of the alternator.

 

[Janicholson]

I am a little confused. When you touch only one lead, either one, from the meter to the tractor and the other lead is not touching anything the meter goes crazy?

When only one lead is connected, the other lead (and the original) become antennas. Antennas are devices that convert magnetism into electricity. Things with electrical systems produce magnetism (so does electrical wiring in our homes). this magnetism is what makes radio stations work. If there is variability in the magnetism there is variability in the electricity produced by the "antenna". Modest to cheap meters sample voltage several times per second and attempt to display what ever they encounter in that very tiny sample. If the voltage is all over the spectrum, the display will be crazy. A professional level meter, usually more costly, filters out the variability by averaging the voltage. An analog meter with a needle that moves against a scale samples continuously and the needle cannot respond to the tiny changes in the voltage fast enough to show them by moving.
When you touch the second lead to the tractor, the effect is to shut off the antenna. This causes a far smoother signal to the meter, and gives it a chance to read pretty well. alternators and generators are not producing perfect direct current. If we hook up a imaging tool, resembling a heart monitor in a hospital, there will be visible ripples in the pattern shown. The generator's pattern will be more ragged and spikey, the alternator more smooth and regular, but still not the same as a flash light battery, pure DC. Because I teach these things to university students (and neighbors) I can explain them. But the learner's responsibility is to continue to pursue more understanding. This was 4 minutes worth. Jim

 

[JMOR]

When only one lead is connected, the other lead (and the original) become antennas. Antennas are devices that convert magnetism into electricity. Things with electrical systems produce magnetism (so does electrical wiring in our homes). this magnetism is what makes radio stations work. If there is variability in the magnetism there is variability in the electricity produced by the "antenna". Modest to cheap meters sample voltage several times per second and attempt to display what ever they encounter in that very tiny sample. If the voltage is all over the spectrum, the display will be crazy. A professional level meter, usually more costly, filters out the variability by averaging the voltage. An analog meter with a needle that moves against a scale samples continuously and the needle cannot respond to the tiny changes in the voltage fast enough to show them by moving.
When you touch the second lead to the tractor, the effect is to shut off the antenna. This causes a far smoother signal to the meter, and gives it a chance to read pretty well. alternators and generators are not producing perfect direct current. If we hook up a imaging tool, resembling a heart monitor in a hospital, there will be visible ripples in the pattern shown. The generator's pattern will be more ragged and spikey, the alternator more smooth and regular, but still not the same as a flash light battery, pure DC. Because I teach these things to university students (and neighbors) I can explain them. But the learner's responsibility is to continue to pursue more understanding. This was 4 minutes worth. Jim

https://imgur.com/k9ojuue

 

[Brown David]

Have you ever touched JUST the negative lead to an alternator for no apparent reason?

No.

I have seen wierd/random/changing readings from a meter with only one lead connected to whatever was being checked though. Never with both connected.

If one lead oddities is what everyone was talking about, I misunderstood. I assumed it was with both leads connected.