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I assume it works the same as any other voltmeter - it's monitoring overall system voltage. If the car is off and the key is in the on position, it will be giving you battery voltage. If the car is running it's giving you overall voltage. The only way to get a direct alternator output reading is an ammeter. Normal voltage is usually around 14-15v, and a lower\higher voltage can signal a poorly functioning battery or charging system. The battery and voltage regulator usually buffers the charging system, and a wildly fluctuating readout can signal an issue as well. Its pretty normal for the voltage reading to fluctuate slightly as electrical draws on the system fluctuate - ie, the battery is recharged from starting, or the A/C is cycling on and off, while running the defroster, etc..
 

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Yeah that last sentence doesn't make much sense.

I'm pretty sure the electrical light comes on if the system voltage drops below 10v, or if the alternator is no longer producing power (thrown belt, or some other failure). If you suspect your charging system has a fault somewhere, your dealer's service department can hook up the SD3 computer, and get live monitoring of a lot more sensors that would indicate different systems turning on and off, exact live voltage readings, as well as any stored codes that could indicate a momentary problem or inconsistent sensor \ module.


Bear in mind there is a twin thread http://www.maseratilife.com/forums/showthread.php?t=2975 to this regarding voltmeter fluctuation and a possible link to an impending F1 pump \ relay issue. It will be interesting to see if anything comes of that, but in the mean time it's one of those things to be aware of, but probably not freak out about too much unless things are really varying from the norm.
 
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