The Stovebolt Site Forums The Shop Area The Electrical Bay How to test 12V generator

I finally got my truck into 'reasonable' running condition. Put a voltmeter across my battery and started the engine. I read about 12.6 V regardless of RPM from 900 to ~2000. This doesn't seem right to me. The generator was recently rebuilt and has maybe 2-3 hours run time on it. I went through the process of polarizing it when I installed it - I assume that it took. This is a GM generator with terminals marked A and F. I am confident (but not positive) that it and the regulator are correctly wired - I did the wiring.

Bob

Your reading might just indicate the battery is at full charge. Maybe

When charging the voltage should reach 14.4 or thereabouts, if the battery is discharged. How easy does it start? What is your battery voltage when running the starter-need a helper.

Ed

I have outlined the test sequence for diagnosing a generator/regulator type charging system several times, so there should be plenty of information available in the archives. Here goes, one more time!

Step 1:
Disconnect the Armature and Field wires at the generator. Start the engine and run it at a speed somewhat above idle. Connect a voltmeter between the ARM. terminal and the generator housing. You should find somewhere between 1/2 and 3/4 volts DC present at the ARM terminal. This is called "residual voltage". It comes from a slight amount of magnetism retained in the armature core. If not, polarize the generator again by connecting a jumper lead from battery voltage to the ARM post momentarily. Engine running or stopped doesn't matter for this procedure.

Step 2:
Reconnect the wires at the generator. Monitor voltage at the ARM wire either at the generator or the regulator, and connect a jumper wire from the F terminal of the regulator to ground. The voltage at the ARM terminal should rise without limit as the engine speed is increased. Do not allow it to go above 15-16 volts. If the above tests are good, your generator is OK.

Check the voltage at the voltage at the BAT terminal of the regulator with the engine at fast idle. If it is not within a couple of tenths of a volt of the ARM terminal, you have a bad regulator, dirty regulator points, or one that's out of adjustment. Battery terminal voltage with a good condition charging system should be in the range of 13-14.5 volts, depending on the state of charge of the battery. Voltage below 13 indicates problems. Voltage below 12.6 or so indicates a "no charge" condition.

Now, if things go as usual, people will look at the above procedure, decide it's too difficult, and install an alternator! Pity!
Jerry

Bob, I know you did not ask this but: Seems like if you have updated with 12V, you should go with alternator. Get rid of all the clap trap wiring/charging components under the hood. Much more reliable.
Just compare the number of components, total wire lengths, number of wires and number of termination points sometime. It's time to get into the 60's at least.

"Alternators have several advantages over direct-current generators (dynamos). They are lighter, cheaper, more rugged, and can provide useful charge at idle speed. They use slip rings having greatly extended brush life over a commutator. The brushes in an alternator carry only DC excitation current, a small fraction of the current carried by the brushes of a DC generator, which carry the generator's entire output. A set of rectifiers (diode bridge) is required to convert AC to DC. To provide direct current with low ripple, a polyphase winding is used and the pole-pieces of the rotor are shaped (claw-pole). Automotive alternators are usually belt-driven at 5-10 times crankshaft speed. The alternator runs at various RPM (which varies the frequency) since it is driven by the engine. This is not a problem because the alternating current is rectified to direct current." Source, Wiki of course.

Get a transistor, sister.

Sorry Jerry but as you suspected, here I am. The yang for the yin.

Heres a couple shade tree troubleshooting tips to help isolate should a no charge condition occur out on the road far from your tools/shop/garage. That said HRM's above technique should be used for proper troubleshooting and repair.

With the engine running at fast idle or higher, remove the brush access cover from the generator (generally a strap), you should see atleast one of the brush assembly's on the top side. With a stout wooden stick, gently push down on the brush to force it to have better contact with the commentator. If the brushes are worn down or some grit of grime gets between the the brush and commentator this may get it charging. It may work good enough to get you home where you should repair the generator.

Sometimes the BAT, field or armature contacts in the regulator can stick. With the engine running at a higher than idle RPM, gently tap on the regulator to see if it starts charging. If so like above it may get you home. You'll need to clean & check the adjustment on all three regulator contacts.

Course make sure all your electrical connections are clean and tight, 2 at the generator and 3 at the regulator.
Good luck
Dave

Jerry, thanks for the info and my apologies for your having to repost. It should be posted in tech tips. It is concise and easy to follow. I had 2.7 v on the armature, no wires connected, 1800-200 RPM. Wires connected, field connected, voltage rose to 17V after a few seconds. So I guess that means I move on to the regulator.

The tips from others were also worth remembering - thanks.

Bob

Good news! Your polarizing worked, apparently. Now, remove the cover from the regulator, after disconnecting and taping the "BAT" wire to prevent getting it shorted against anything. It's very easy to touch the wrong place inside the regulator with that cover and get a "positive smoke check". Use a piece cut from a clean business card or an index card, and burnish the points thoroughly with it. DO NOT use a file, sandpaper, or emery cloth on the regulator points. They open and close with a slight wiping action, and even very fine scratches will make them stick. Two of the point sets are held closed by springs, the current regulator behind the ARM terminal and the voltage regulator behind the FLD terminal. If either one of them gets dirty the generator field current gets interrupted and the voltage can't rise automatically.

The third set of points, behind the BAT terminal is the cutout relay, which is spring-loaded in the open position when the engine is not running. It prevents the battery from discharging itself through the generator anytime generator voltage is lower than battery voltage. With the cover removed, reattach the BAT wire and start the engine. as engine speed increases above idle, the cutout relay should snap closed and the voltage at the battery posts should rise to 13-something or more. If it does, carefully reinstall the cover and go about your business- - - -you fixed it! If not, start shopping for a regulator, and remember- - - - -there are two kinds of regulators- - - -original equipment "Delco" and "Wrong!" There are usually a bunch of reasonably-priced Delco regulators on Ebay. Good luck!
Jerry

I believe my 12 volt generator and my spare ones do not have a band covering the brushes. My six volt generators do.

If the generator has a stepped-down housing, there won't be a spring band. The short-housing 6 volt units with the same diameter housing all the way to the end are the ones with a band.
Jerry

Jerry - thanks for the help. My system is NOT charging - the cutout relay stays open at "idle" speeds of 1500-2000RPM. I am thinking that I should go back out and manually close the relay manually and check the voltage at the battery to see if it rises. Problem might be wiring - I did the whole truck myself, but everything else works. I just hate to replace a regulator with only a couple of hours use on it. But it was purchased too long ago to return.

Bartamos - assuming that you are still following this thread. I essentially agree with all you have to say about alternators. However, I am trying to restore this truck to look like it did in 1946. Yes, a 12V battery does not look like a 6V, but it is hidden under the floor boards.

Bob

"DANGER WILL ROBINSON!!!!!!" DO NOT manually close the cutout relay! It will probably stay closed after the engine stops, and try to discharge the battery through the generator. Instead, do the field wire grounding procedure again, with the wiring connected, and see if the cutout closes then. If it does, chances are you've got an open circuit in either the current or voltage regulator point circuits. They're in series, so a dirty set of points or a broken wire inside the regulator will interrupt the field circuit and prevent the generator from making enough voltage to close the cutout.
Jerry

Thanks for the warning, "ROBOT" (there seems to be some issue over the exact name used in the series). But that is exactly why I post my thoughts. Give that a shot later this afternoon.

Just checked Ebay- - - - -there are at least a dozen Delco original-equipment 12 volt regulators listed as "aircraft" equipment for prices from $30.00 to $50.00 each. They will also work in automotive applications, just as well as the $250.00 "refurbished" ones that are also there. I'm probably going to buy a few of them just to have them on hand. With a quick bead blast and paint job of the cover with gloss black enamel, I could flip them for a $100.00+ profit, assuming I wanted to do that (which I don't!)
Jerry

Well, it looks like everything is mostly OK. I did as you suggested, watched the BAT relay close and noted that the BAT voltage went from battery voltage (12.7) up to 14-something. What RPM range would you expect the generator to start charging the battery? I always thought that charging started just a little above idle, but did not amount to much current until the RPM got "above" idle.

It looks like you might have an open circuit, or at least high resistance somewhere in the Field control circuit. Here's how that works:

The "residual voltage" that I had you test at first leaves the armature and travels through the generator field coils to the "F" terminal on the generator. From there, it travels to the "F" terminal of the regulator. It must pass through two set of points, one at the "voltage regulator" coil behind the F terminal, and then through the "current regulator" points behind the "ARM" terminal, before it finds its way to ground. If the residual voltage gets back to ground through both sets of points, it increases the magnetism in the generator field housing. The stronger the magnetism, the higher the voltage output at the ARM terminal goes. When the armature voltage rises to about 1/2 volt higher than the battery voltage, the cutout relay closes and current goes to the battery. Anytime ARM voltage drops below the battery terminal voltage, the cutout relay opens to keep the battery from discharging itself through the generator.

If you have to make the cutout relay close by grounding the field, something is wrong with the control circuit- - - -dirty points, and/or a broken wire somewhere in the regulator housing. Have you cleaned the points as I suggested? At least half the regulator "failures" I've seen are simply caused by dirty or oxidized points at either the voltage or current regulators.

Regulator adjustment consists of three operations, setting the closing voltage of the cutout relay, setting the maximum current, and maximum voltage by adjusting the spring tension on the two spring-closed point sets. I'm probably one of the last generation of auto mechanics who studied DC generators and mechanical-point regulators, back in the early 1960's. If I don't pass some of that knowledge on, it's going to go away when I do!
Jerry

It is fine to restore to original stock. I do like original. My idea of original is the body and most of the interior. I don't see fighting a bunch of outdated unreliable systems and wiring and safety issues. As you said the battery is hidden under the floor boards. The hood closes also. I salute your sticktoitiveness to originality. You have chosen your level of "original" with conversion to 12V. I go a little further.