Matt Harwood Posted June 15, 2020 Share Posted June 15, 2020 (edited) Over the weekend I was driving a 1940 Buick around and noticed that its ammeter doesn't move much. It would twitch a bit now and then, particularly when I turned on the headlights, but it never showed much discharge or charge. It would flicker a bit with the turn signals on, too. Enough to tell me it's probably working. The idle on the car is a bit high, so the generator--if it's working properly--is likely spinning fast enough to make plenty of juice even at idle. I plan to adjust the idle a bit. The battery has been on a tender full time and before I fired it the first time I primed it, so we didn't drain the battery much, if at all, by cranking and cranking to start. Still, my gut says that the generator isn't putting out enough juice. But since it didn't show much discharge with the headlights on and the engine running, maybe it was working just fine. I took some readings at the battery and it shows 6.75 amps with the engine running, headlights on or headlights off. With the engine off, it shows about 6.2 volts. That suggest the generator is pushing juice into the battery, no? Not as much as I'd expect, but some. Am I fretting over nothing? The car doesn't exhibit any signs of a low battery or weak electrical system, it always starts instantly and runs great, it's just not showing the usual wide swings on the ammeter that I'm accustomed to on my own car. Any thoughts? Edited June 15, 2020 by Matt Harwood (see edit history) Link to comment Share on other sites More sharing options...
JACK M Posted June 15, 2020 Share Posted June 15, 2020 I think it is working. Leave the headlights on for a half hour or so without the car running then see if it shows a better reading. Probably will have to take it for a drive to bring it back up. 1 Link to comment Share on other sites More sharing options...
FLYER15015 Posted June 15, 2020 Share Posted June 15, 2020 Matt, Would this be of help ? Not calling you Buick a tractor, BUT..................... Guess that would be like throwing mud at myself.......... Mike in Colorado 1 Link to comment Share on other sites More sharing options...
Rusty_OToole Posted June 15, 2020 Share Posted June 15, 2020 Sounds like your electrical system is working perfectly. Battery is good, generator is working. It may show more charge right after you start the car if you rev the engine, then it is charging up the battery but once the battery is charged you don't want the generator putting out too much juice. 6.2 volts seems a little on the low side, a fully charged six volt should be closer to 7.2. You could check with a volt meter with and without the engine running to see how many volts the generator is putting out. Link to comment Share on other sites More sharing options...
maok Posted June 15, 2020 Share Posted June 15, 2020 Polarize the generator. Link to comment Share on other sites More sharing options...
TerryB Posted June 15, 2020 Share Posted June 15, 2020 1 hour ago, Rusty_OToole said: Sounds like your electrical system is working perfectly. Battery is good, generator is working. It may show more charge right after you start the car if you rev the engine, then it is charging up the battery but once the battery is charged you don't want the generator putting out too much juice. 6.2 volts seems a little on the low side, a fully charged six volt should be closer to 7.2. You could check with a volt meter with and without the engine running to see how many volts the generator is putting out. I think you have the charge rate listed for a 6V battery. Each battery cell is about 2.1v for a total of 6.3V typically. A decent charge rate would be about 7.5V. 2 Link to comment Share on other sites More sharing options...
Bloo Posted June 16, 2020 Share Posted June 16, 2020 How many volts at the battery when running at fast idle or higher? I would also expect about 7.5 volts. The shop manual will have a better answer but it will be real close to that. If the battery is fully charged it won't draw much current. 1 Link to comment Share on other sites More sharing options...
Matt Harwood Posted June 16, 2020 Author Share Posted June 16, 2020 It shows 6.75 volts at a reasonably fast idle speed (1940 Buicks still have hand throttles so I wicked it up to maybe 2000 RPM). It doesn't fluctuate even a little bit. Turn the headlights on, battery voltage stays at 6.75. Sitting with the engine off it's about 6.2 volts. The fact that it stays rock solid at 6.75 even with the headlights on says to me that the generator is generating. I suspect that the regulator is a bit out of adjustment and could be made to push more voltage, but since the battery appears to stay topped off, it starts easily every time, and it keeps up with the headlights, I think it's working OK. It just seems weird not to see a heavy charge after starting or a big swing at a red light with the headlights on. Link to comment Share on other sites More sharing options...
edinmass Posted June 16, 2020 Share Posted June 16, 2020 (edited) It’s working fine. 6.2-6.3 indicates a good battery. Some cars have cutouts, others have cutout regulators, and many have thermostatic shunts. A good running car will not draw down the battery at all when starting. If it’s a three brush generator you could turn it up a bit, but the system will cut the charging back automatically. I set my early cars up that have cutouts and a shunt to charge at 12 amps upon start up, and down to 6-7 when warm and running on the shunt. Overcharge can caus gassing of a regular style battery. Optimas seem to be immune to overcharging below ten amps. Cadillacs seem to cut back on the amperage with the shunt faster than the others, around four minutes.. Pierce Arrow take about ten minutes. Duesenbergs about split at 7 or 8 minutes. I would leave the car the way it is. If the system is only lightly charging, and your drawing more amps, the battery will easily supply them and no damage will be done. When going on tour, and night driving is in my future, I turn up the third brush till the car charges at two amps at normal road speed with the lights on. When I get to a light it will show froM 7-15 amps draw. All which are normal. Edited June 16, 2020 by edinmass (see edit history) 1 Link to comment Share on other sites More sharing options...
Bloo Posted June 16, 2020 Share Posted June 16, 2020 The battery is just not drawing current because it is charged. 6.75v sounds like the regulator is set a bit low to me, but I would check the manual. It's possible it is normal. There will be voltage vs temperature listed, because (most) voltage regulators are temperature compensated. No third brush on a 40 Buick, It's a 2-brush generator and a 3 unit regulator, so charging voltage is the main thing to set to deal with any overcharge/undercharge. 1 Link to comment Share on other sites More sharing options...
Rusty_OToole Posted June 16, 2020 Share Posted June 16, 2020 6 hours ago, TerryB said: I think you have the charge rate listed for a 6V battery. Each battery cell is about 2.1v for a total of 6.3V typically. A decent charge rate would be about 7.5V. I guess I phrased it badly. I meant, when the engine is running and generator charging. I have run a couple of wires into the passenger compartment and connected a volt meter temporarily, to check the charging system while driving. 2 Link to comment Share on other sites More sharing options...
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