how do i tell if my alternator is shot?
My gauge stays above 13 then it will start to jump around, then it will sit at above 13.

 

If it's at 13 when you start then it's working. The reason it jumps is probably because the idle is low. My car has the normal 14.5 volts if engine is above 1000rpm. When I'm sitting at idle for a couple of seconds the needle will drop to 13 and wait, then a little engine surge and it's back to 14.5. I wouldn't worry about it as long as it stays above 13 and you don't notice a drop in voltage. I've had 2 alternators go bad in the past couple of years and one almost killed me when it quit on the highway.


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, Jon (91 RS too many mods to list)

 

Generally, as long as it is between 13.0-15.0, you're okay. Slight fluctuations are normal between that range. Less than 13 volts at idle is also common, especially with more load (lights/fan/wipers/etc.)

FYI, the regulator in the alternator will compensate for temperature so the battery receives a higher voltage when its cold (due to its higher resistance). For example, if it's very cold (20°F), mine will output about 14.9 volts (I've never seen it more than 14.9), and when it's hot (90° +), it'll put out about 14.0