The Stovebolt Site Forums The Shop Area The Electrical Bay Can I check charging system with voltmeter?

I need to troubleshoot the charging system on my 52 3100. The manual says to hook up an ammeter. My multimeter can measure amps, but it maxes out at 10 amps, and evidently the generator should be putting out 30-40 amps. Can I use the voltmeter setting to do the same checks? I'm trying to decide if my no-charging problem is the generator, regulator, or battery. The wiring harness was replaced last summer.

You can check the charging system with a voltmeter, but it may or may not tell you the true state of the charging system. In general, alternators should put out about 14+ volts when they are charging correctly, but this will not tell you what the alternator will do under load. To do that, you would need to have the voltmeter connected while driving. If the voltage drops under load, the alternator is probably bad. (I'm assuming you have an alternator that has a built-in regulator.)

That's why technicians use ammeters to check charging systems. Some local auto parts stores have this capability. You might check with NAPA or another local vendor. They'll be happy to sell you a new alternator if the old one is defective.

The generator will only put out 30-40 amps if it has to, depending on the charge of the battery or the load on the system. Baldeagle is right...hopefully your local autoparts store can test. If your generator/alternator is putting out the proper voltage, then chances are good that it is OK.

why not just pick up one of those dash guages and stick it in between the battery and the generator and see what you have..

Are you still running the original 6-volt system? If so, the generator can be put into full output by removing the "F" wire at the voltage regulator, and attaching it to ground with a jumper lead. At a fast idle, the voltage should jump to 7 volts or more when charging against a fully-charged battery. If you don't get a noticeable voltage rise with the field grounded, try polarizing the generator by shorting the armature wire at the regulator to battery voltage momentarily. Some oldtimers just arc a screwdriver blade from the Bat. to Arm. terminal, but I prefer to remove the wire and touch it to the battery wire for half a second or so, just long enough to get a pretty good spark. This sets up the "residual magnetism" in the generator housing that triggers the field current. Grounding the field wire puts the generator into full, unregulated output so don't rev the engine up much. If this test works, the generator is good, and you need to start looking into regulator or wiring problems.
Jerry

I bought an ammeter from Sears.. After putting the battery on a charger for two days, I followed the directions in the manual for testing the system. With the all the lights on, the generator was putting out about 20 amps, which didn't seem to bad to me considering the battery was fully charged. So I took my battery in to have it checked, and found it had a bad cell. The NAPA guy wanted to know if I wanted another 8 volt battery. That's how I found out I had an 8 volt battery. He said people used them to get faster cranking from the starter. I chose to buy a heavy duty 6 volt to replace it. But I got to wondering how my 6 volt charging system could ever have managed to charge an 8 volt battery. Doesn't the voltage of the charging system need to be greater than the battery voltage before current will flow through the regulator and on into the battery?

I grew up around a top-quality independent shop in the 1950's, where we did a lot of charging system work, and 8-volt batteries were pretty common back then. They were the cure to a nonexistent problem. A 6-volt charging/starting system will work just fine, if it's properly maintained. When an 8 volt battery is installed, the voltage regulator must be adjusted to 9.5 volts, which overloads the generator, and burns out lights in a hurry. You'll need to check the charging voltage against a fully-charged battery like I described before, and the max voltage should be 7.5 volts at a charge rate of 10 amps or less. If it's higher, you'll boil the water out of the battery, and possibly damage it from overcharging. Be sure all the ground wires are attached properly, and all the charging system connections are clean and tight. A ground wire from the regulator base to the generator housing is a good idea, ditto on one from the engine block to the body.

There are 3 adjustments on a mechanical-point voltage regulator, closing voltage on the cutout relay, current regulator, and voltage regulator. Most 6-volt generators produce about 35 amps max, and the current regulator should be adjusted to prevent the generator from overloading itself when charging against a low battery. The voltage regulator sets the max charge voltage against a fully-charged battery to prevent overcharging. The cutout relay assures that the generator and the battery are disconnected when the engine stops. If the cutout is adjusted too low, or the points stick, the battery will discharge through the generator when the engine stops, ruining both the generator and the regulator. As a general rule of thumb, adjust the cutout to close at about 6.5 volts, the current regulator to 35 amps, and the voltage regulator to 7.5 volts. It takes a lot of experience to get these adjustments right, so find an oldtime DC system pro if you can, and pay him to do his thing!
Jerry

As luck would have it. last summer my voltage regulator had started making funny noises, so I took it and the generator in to the old pro in town. He made some adjustments ( I don't remember what they were), and said it was working fine now. I know he looked up my truck in his manual to make sure I had a 6 volt negative ground system. So I figure he adjusted the voltage regulator down from whatever it was set for that 8 volt battery to what would be correct for a 6 volt battery.
Today I checked the voltage between the regulator's field terminal and the ground and got 8.5 volts. Is that high enough that I need to take it back to him for adjustment?

You're checking at the wrong place. Check the voltage at the battery terminals with the engine running at a fast idle. It should be 7.5 volts, (MAX) when charging against a fully-charged battery. With a low battery, the terminal voltage will be lower, as it doesn't take as much effort to force a charge into a low battery. Actually, the state of charge of the battery determines the charging voltage. That's why you really need an ammeter in the circuit to do a good check. Charge CURRENT,(Amps) should be less than 10 when you make a voltage regualtor check/adjustment, with the voltage regulator cover installed. With the cover off, the voltage will be slightly higher, because the cover conducts magnetism. Make an adjustment, replace the cover, and check voltage at the battery. Repeat if necessary. If it's showing more than 7.5 volts, you've got a problem!
Jerry

Hotrod,
Following your excellent instructions, I adjusted the regulator to decrease the voltage at the battery (fully charged and receiving less than 10 amps according to the ammeter) from 8.5 down to 7.4 volts.
Thanks to everyone who responded to my questions.
I feel empowered - and lucky to have this bulletin board as a resource.

Hotrod Lincoln, it sure looks to me like you should be writing a techtips article on six volt charging systems. Would you be willing to consider that?

I'd be happy to share what I know! I was among the last few electrical techs to grow up around 6 volt systems, I guess, and they're definitely a different breed of cat!

The main thing to remember is that a 6-volt system is terribly intolerant of any resistance in the charging circuit. It needs big wires, with clean, tight connections, especially at the battery cable terminals. There's just not enough voltage to overcome any resistance, and still have enough juice to do much charging. There's a "series resistance" check that we used to do with a low-reading voltmeter, to test for high-resistance connections, all the way from the generator armature to the battery hot post. How about a tech tip article on that one for a beginning procedure? Email me, and let's get started!
Thanks!
Jerry

You guys sure answered my questions about the charging system! I have an 8 volt battery and it won't restart the truck after running a long while. Now I know why! I am rewiring the truck and am using a 6 volt battery. I also bought 1 gauge battery cables and new grounding strap, but haven't cranked it yet because I'm still chasing short circuits (probably the headlight switch). I'm lucky too in that there is a guy near me who has an alternator shop and still rebuilds generators. He's rebuilt mine and has agreed to set the regulator when I get the truck over to him.