The Stovebolt Site Forums The Shop Area The Electrical Bay How do I test my 6 volt generator?

I have a 1949 3600 with the original motor and electrical system. I am trying to figure out the best way to verify if my system is charging the battery? I am sure it is something simple, but I am not sure how to do that.

Can someone kindly reply with the steps to rule out a bad generator?

The proper way is to check it with a volt meter at the battery with the vehicle running. If you don't have one you can disconnect the negative battery terminal while the truck is running, if it dies you are not charging, if it stays running you are. Do not disconnect the positive, more potential for unwanted sparks.

First, momentarily attach a jumper wire from the BAT terminal of the regulator to the ARM terminal, for no more than a second or two. This "polarizes" the generator, establishing "residual magnetism" in it. With all the wires removed from the generator, there should be at least 1/2 volt present at the ARM terminal of the generator with the engine running. If this voltage is not present after polarizing, don't bother with any other tests- - - - -you've got a bad generator. If the residual voltage is there, reconnect the wiring, and run the engine with a jumper wire connected between the FLD terminal of the generator and ground. The voltage at the ARM terminal should increase rapidly as you rev up the engine. Do not allow it to go above 8 volts or so. If it will do that, you've got a generator that's capable of charging the battery. If it's not charging, the trouble is in the regulator or the wiring. Do the same series of tests- - - -ground the Field wire and measure voltage at the ARM wire of the regulator to check the condition of the wiring.
Jerry

PLEASE don't disconnect battery terminals with the engine running- - - - -you can do major damage to anything that's voltage-sensitive when the generator voltage spikes to 50 volts or more momentarily. I've seen that on an oscilloscope dozens of times. Definitely don't do that on a vehicle with an alternator, and NEVER do it on one with a computer!
Jerry

Hotrod is correct according to the books. However I have done this to just about every vehicle I have owned without mishap except for the once I pulled the positive cable instead of the negative. Turns out my alternator was over charging and the battery was letting out gas, nobody was hurt but it was scary. Probably might have happened with either cable. Definitely a shade tree maneuver, but it gives you an answer pretty quick.

I have a volt meter and can certainly check it that way, but what am I looking for on the meter? More voltage than the battery has by itself?

When the motor is running well above idle, you should see about 7.2 v or so.

If the battery is very low on charge, the charging voltage at fast idle should be about 6.5 to 6.8 volts. As the battery charges up, the current flow should decrease (ammeter closer to center) and the voltage should go up. With a fully-charged battery, the charge current should be 5 to 8 amps, and the voltage should be around 7.2 to 7.5 volts volts.
Jerry

the challenge I have with this is that I will not get that reading. I use an 8 Volt battery, so the voltage is more than that when the battery is charged. Is there another location where I can check the output of the generator to see if I am getting that voltage?

Dump the 8 volt, or find a pro who can adjust the regulator to charge it. Then buy a BUNCH of light bulbs, particularly headlights. With the regulator set properly, you'll fry lights at a record rate. An 8 volt battery is a solution to a nonexistent problem. Your voltage regulator setting needs to be 9 volts to keep an 8V battery charged.
Jerry

Senor Lincoln, realizing this forum is very old BUT I have a question. I followed your advice (thank you) on my '54 3600 (all original) and everything checked out as far as the generator and wiring. over 1/2 volt at the armature terminal on the generator, re-hook wiring up and there is a voltage increases as you rev the motor, check at the regulator as you suggested and it's the same, so the wiring must be fine... NOW what? Am I to assume it's the regulator at this point? Just for info, 3 years ago (and maybe 1,000 miles ago) I had the generator professionally rebuilt and put on a new regulator. Been working perfectly until the other nite. I flipped the headlights on and the gauge on the dash needled down and wouldn't climb back up even at high RPM's and my headlights were dim and didn't seem to 'brighten' as I revved the motor. Regulator?

Ugggh! Before throwing away your 8 volt battery, try this..

1) loosen the negative cable at the battery. Suggestion... put one of those on/off battery pole switches on it so you never have to do it by hand again.

2) start the truck, then remove the negative cable at the battery. (trying to keep sparks down by doing it this way)

3) LEAVE the cable off no matter what happens until you turn off the ignition switch. Then put the cable back on.

NOTE: This isn't something you want to make a habit out of without that switch since any sparks near the battery can cause an explosion.

4 and most importantly) Leave the 8 volt battery in and enjoy the benefits of a little peppier start, a little brighter lights, a little stronger everything without changing anything.

My 2 year battery is an 8 volt, been in my truck for 3 years and shows 8.37 volts. Do I care if the storage battery is only 85-90% charged? No. No I don't. That's the small price you pay for not having a properly engineered system (8v in a 6v system)

I can hear it now especially as a Kansas farm boy back in the day... Hey Earl, Trucks not starting again.. go out to the shed and get the tractor battery. AGAIN. Because anyone in north America in the wintertime knows a 6 volt truck is very iffy when it gets below freezing. I agree its not a perfect solution. As a electrical engineer I get all of that. I do most of my trucks with 12 volts to stop the madness completely. Its called common sense and fulfilling a need with what you already have.

Besides this is a universally known trick that has been used for a million years on all makes of the old 6 volt vehicles. Show cars/trucks have 8v to give the presentation just a little better flavor. Open mindedness and helping people with what they already got.. no need to rain on anyones parade with the top down.

The mindset that a 6 volt system is inadequate is ridiculous. If 8 volt was the way to go the OEM's would have spec'd a 8 volt battery. I wouldn't call the 8 volt route a trick. I would call it a band-aide that usually ends up bringing on more problems....

Well, it is Sunday, so a sermon is a good thing (and my low opinion of an 8v conversion will be rested - as the 8v aspect of this post has been resting since 2009).


ol green,

Do you have a 6v/8v/12v battery?

Regardless of your battery voltage, if you want to determine if your alternator/generator combination is good, take it to a reputable auto-electrical repair shop for a bench test (take your battery with you, too). First, ask the repair shop (over the phone) what it would cost to do the bench-tests and adjustments.

Good luck,

Yeah, everything is as it was in '54, 6 volt, rebuilt generator, NOS regulator. No 'reputable' shop available, I'm a DIY'er on this stuff because of $$$. Like I said, everything checks out fine (+ 1/2 at the armature on the gen when grounded and running, voltage increases to 8 volts and above as you rev the engine, same reading up TO the regulator (so wiring seems fine) BUT I have 6.2 volts at the battery at rest and 6.1 volts at the battery when it's running and if you rev the engine there is no INCREASE of volts at the battery. It's gotta be the regulator, right?