What does "GEN" measure, and from where?

[ArkShark]

What does the "GEN" light measure, exactly? Voltage? Current? From the battery or alternator?

My alternator passed on last week, and the GEN light was on the entire time.
So, now I have a fully charged battery, and a brand new alternator.

The GEN light comes on while at idle, but goes off when the RPM's are up.

Is it telling me I have low voltage somewhere? If so, where? Or low current?

Thanks

 

[DDLS3]

Generally the light is connected to a voltage sensor. If your light is on at idle it could be that your idle is too low, or your alternator that you just replaced is not putting out enough volts at idle. Connect a voltmeter to your alternator lead and read the output that will tell the story. It could also be a faulty sensor.

 

[ArkShark]

12.43 volts from the alternator @ idle. (which is 500rpm) And the light still comes on. Although it kinda fades in and out.

Is it possible that my chrome 135 amp replacement alternator has a slight larger pulley, causing it not to kick in at 500rpm? Would a different size pulley help?

Think I should contact the manufacturer of the alternator and ask them?

EDIT: Alternator man'f said it won't kick in charging until 750-800 RPM's.

So what do ya'll think? :
Live with the light coming on when at idle?
Try a smaller pulley?
Have the alt. changed to turn on at lower RPM? (if this is possible, and cost-effective)

 

[brusso]

The gen light gets it's signal from the smaller wire in the two wire plug on the side of the alt. Also I think that 500 rpm is way to low. It should be something like 600 or so in drive, if its an auto, or 600 or so in neutral, if its a manual trans. Without looking at the decal, I think that it is 650 in drive. Reguardless, idle set while in drive(under load) is significantly less than in park. It may be 700-750 in park. Don't mind my ramblings, you get the point.

Oh yah, 12.43vdc is way to low even with the engine off. The batt should be between 12.9 and 13.1 at rest.(technically 2.2vdc x 6 cells = 13.2vdc at rest)
In theory, to charge a battery, you need to apply a voltage 1 volt above battery static voltage. ie. 14.2

 

[JohnZ]

A fully-charged battery will typically show about 12.5-12.6 volts; any time the alternator is putting out less than that, the light will come on, as the electrical system is drawing current from the battery instead of the alternator.

 

[DDLS3]

ArkShark

Yes if your pulley is larger then you will turn the alternator slower. Changing to the correct size pulley will likely cure your gen light on. 500 rpm for a standard trans and 600 for an auto sounds about right. Kick it up 50 to see if it keeps your light off, won't hurt a thing.