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Alternator Gauge- What's "Normal"

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So_Calif_Guy

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Alternator Gauge- What's "Normal"

Morning. Last night I replaced my alternator with a Moss Motors rebuilt. Red dash light is now off and overall, the car seems to be running well, except... With the original alternator, the dash voltage almost always read about 14 - 14.5 volts. With the new alternator in place, the guage reads a solid 12 volts. I don't know if this is fine, better than the old alternator or if it's a problem. Folks, can you share with me what you gauge reads under normal condition? Drove it about 20 miles last night with lights on, high beam, low beam, nothing seems to get the voltage above 12 volts. Might be nothing, but it's worth the check.... Thanks to all.
 
Re: Alternator Gauge- What's "Normal"

IF the gauge is accurate, 12 volts is too low.

When the battery is discharging, it puts out about 12.6 volts (but a few tenths are lost in the wiring) so the gauge would read about 12 volts.

But it takes more than that to charge it, so 14-14.5 is normal once the voltage regulator starts regulating.

The joker of course is that the numbers above assume the battery is nearly fully charged. If yours has been deeply discharged (as might happen if you drove for some time with a duff alternator), then it can take some time for the voltage to come back up. Or the battery may have failed with a shorted cell, in which case the voltage will never come up above 12v or so.

Another one of those places where an ammeter is more useful than a voltmeter
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I'd try another 20-30 minute drive at speed with as little load as possible (headlights on doesn't leave much margin for charging the battery) and see if the voltage starts to come up. Maybe run over to Harbor Freight and pick up a cheap DMM
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Re: Alternator Gauge- What's "Normal"

My 73 TR6 reads about 14.5V normally with a fully charged good battery and that's where your car should read IF your gauge is reading right, stick a multimeter on it and check.
 
Re: Alternator Gauge- What's "Normal"

So the car is a '75 TR6. Drove it yesterday and the gauge read as high as 13 volts. Good info from the forum. Today, I'll put a trickle charger on the battery to assure that it is fully charged, and try it after that. Thanks for the help and suggestions...
 
Re: Alternator Gauge- What's "Normal"

No matter what the condition of your battery, it should read between 14-14.5 constantly at the battery terminals while the car is running with a multimeter attached. Charging your battery with a battery charger before hand will have no affect on the reading.

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Re: Alternator Gauge- What's "Normal"

Check out any set of curves for lead acid battery voltage under constant current charge; you'll see that they start down around 2.0 volts/cell (ie 12.0 volts for a "12 volt" pack) for a battery that is 90% discharged (even lower for lower charge rates).

And with the relatively small stock alternator on a TR6, a constant current charge is what happens.
 
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