The Stovebolt Site Forums The Shop Area The Electrical Bay peculiarity with voltage reading at battery

So, I have a 216 on the engine run stand. I got the ole gal running right nice. I gave the generator on just for the purpose of the fan belt to work. No wires hooked up to it and no voltage readings from the posts or from either post to ground. I believe that the generator is toast which is fine. It has good bearings to serve as an effective idler pulley for the fan belt.
While the engine is running, voltage on the multimeter fluctuates from around 14 volts and 7 volts back and forth in rapid succession. With the engine off, the voltage at the battery is 12.3 volts.
For further reference, it is unknown to me whether the coil which came with this engine is for 12 volts or 6 volts. I have installed a ballast resistor since I am using a 12 volt battery. I attached a jumper wire across the ballast resistor and check the voltage again, but the erratic readings continued unchanged.
What the hey?

It sounds like whatever means you are using to charge the battery while it's running has some issue. 14 volts is pretty good for a charging system at work, 12.3 is just slightly low for a battery that is assumed to be fully charged at rest.

Regular?

If you mean regulator, I don't have a regulator on my engine run stand and there are no wires on the generator, nor any voltage coming from the generator posts when test to ground or from when testing post to post.
Just to make sure, I am going to remove the brushes from the generator and start it up and see what my multimeter reads.

I have been able to start the engine which has a 6 volt starter and it runs just fine with with 12.3 volt battery (admittedly low) and it runs very well.
Currently, I have the battery charger on the battery until I am ready to test again after I remove the brushes from the seemingly already dead generator.

Try checking the generator output with an analog (needle type) meter- - - -not a digital. With no wires connected you should be getting between 1/2 and 1 volt. Most digital meters just produce unreliable gibberish when used on a DC generator.
Jerry

Where is the meter connected? I think it may be expected for the voltage to jump around while the points open and close if you are not connected directly across the battery terminals.

The generator is not connected to anything. There are no wires attached to the generator.

I now have the engine purring like a kitten. I readjusted the valve clearance while the engine was fully warmed up. No clacking nor misses. Temperature needle is holding steady between the 8 and the 0 of the 180 on the original gauge just like every 216 and 235 that I have ever owned and driven. The oil pressure is at 14 pounds.
I believe that I need to stop buying multimeters at the Dime Store.
I am now moving on to the Engine Forum for specific engine questions.
Come on over, it'll be fun.

That.

Carl: You told us where your voltmeter WASN'T connected, but you never told us where it WAS connected "While the engine is running, voltage on the multimeter fluctuates from around 14 volts and 7 volts back and forth in rapid succession."

(Almost) any digital voltmeter will bounce around a lot when reading varying DC voltage. My cheapy ( less than $20 ) digital meter actually has a "MAX" button, but no "MIN" button.

It also has "NCV" (non-contact voltage) setting that beeps when you get close (6 inches or so) to a hot wall socket. Doesn't tell you what the voltage is, just that it is there. The lower the voltage, the closer you have to be. It works great to tell if your electric fence is working a few miles from the fence charger. Doesn't work on DC though.

I've got a couple of Simpson brand analog multimeters with the old school sweep needle gauge. They're a lot more stable and reliable for checking DC generator/regulator systems than any digital meter I have.
Jerry

Mike and Bill. My bad. I misread Mikes question about where my "meter" was hooked up for where my "generator" was hooked up.
For the record, while the engine was running, I used my multimeter to test voltage at the battery posts and at both sides of the ballast resistor to see what voltage drop I would have down stream of the ballast resistor.
The meter jumped back and forth between 14 and 7 volts regardless of where I measured it.
Hope that clears it up.

While the engine was running .....

The battery + to ground should stay fairly constant. Maybe vary by a few tenths of a volt as the points open and close if the battery didn't have much of a charge. I'd expect to see around 13 volts here. You should see the same on the battery side of the ballast resistor to ground.

If the meter was hooked to "both sides of the ballast resistor" I'd expect to see the voltage to jump between zero volts when the points are open and 4 to 6 volts when the points are closed. Because your meter probably updates 5 or so times a second "your mileage may vary".

If the + meter lead was hooked to the coil side of the ballast resistor (or the + side of the coil) and the - meter lead was on ground I'd expect to see the voltage to jump between zero volts when the points are open and 7 to 9 volts when the points are closed. "Your mileage may vary".

Error in the previous statement. It should read ...

If the + meter lead was hooked to the coil side of the ballast resistor (or the + side of the coil) and the - meter lead was on ground I'd expect to see the voltage to jump between 13 volts when the points are open and 7 to 9 volts when the points are closed. "Your mileage may vary".

If the + meter lead was hooked to the coil - side and the - meter lead was on ground I'd expect to see the voltage to jump between 13 volts when the points are open and 0 volts when the points are closed. "Your mileage may vary".

An analog voltmeter can also be used as a dwell meter. It will read the average of the "points open" and "points closed" voltage. With the engine running and clean points with little or no resistance, points closed voltage at the downstream (distributor) side of the coil will be virtually zero since the points are grounded. Points open voltage will be the same as battery terminal voltage, since there's no current flow. A 66% dwell reading (30 degrees on a V8 with a 45 degree cam lobe separation) will be 1/3 of battery voltage, since the points are closed 2/3 of the time. A 40 degree dwell setting on a 6 lobe distributor cam will show the same 1/3 of battery voltage reading. Simple math!
Jerry

Nope it won’t
A good meter reads RMS not linear average
1/3 duty cycle is
12 squared
Then average that
Then take the square root
That’s the number you will get for a old fashion iron vane meter
And the same value you will get for a modern decent meter that is true RMS

Junk meter assume a sine wave and give you all sorts of silly number
But no meter will give you the linear average


Edit: 1/3 duty square wave at 12V peak will give you 6.93v on a true RMS meter
It’s because RMS is about the equivalent DC required to preform the same work (or rate of energy transfer)
Voltage X Current is Watts (joules / second) aka the rate of work
But current is also proportional to voltage for a fixed load
So averaging only the voltage is insufficient to extract the effective DC equivalent voltage value of an arbitrary waveform, it requires the Root Mean Square value to compare it to DC
-s

RMS pertains to AC only. DC is straight linear.

Your method would work but I would suggest verifying the meter behavior first on a known waveform before trusting it

DC mode on some might average linearly but you would need to experiment on known waveforms beforehand as it’s not guaranteed what a meter will do with a pulse train when set to DC mode.

AC with true RMS is a guaranteed way for this method to work everytime but ya just got to calculate the expected rms first.
-s