Question for electrical gurus....
 

I recently had my alternator rebuilt, and it has been working great for a couple of months. Recently, my tach has been bouncing around occasionally. Also, over the last couple of days, my battery seems to be draining (not a draw though) seems like my battery is weak, but If I let the car run for about 5 min and watch the voltage at the battery, it will all of a sudden jump from really low to an immediate 14.5 volts. Then even though the car is running, sometimes it will also immediately dump back down to low again. Does this sound like a bad voltage regulator?? or is there some kind of starting 'regulator' or something that might be causing the voltage to drop?? It's kinda spooky driving it, because it is so intermittent. Don't want to get stuck somewhere!! The only way I know that the voltage has dropped is when I open my door my door buzzer sounds weak (like a dying bird!!) :) ...any ideas??

Bad VR. First check all connections-- if you post the year and model of your car it will assist in diagnosis-

sorry, it's a 1976 911S Targa.

No problem, many folks here include that information in their autosignature line, accessed through the user cp button.

So you have a '76 with an external voltage regualtor. Since the alternator is putting out 14.5V, you know that it's working OK. But the fact that it's 14.5v makes me suspect that the external voltage regulator is going bad: the set-point for charging should be closer to 14.0v. The VR is a metal can located on the electrical console on the port rear of the engine compartment, behind the plastic watershield. If you change it, make sure you get the same manufacturer as your alternator, e.g. bosch for bosch, motorola/marchal for motorola/marchal. (If you use a different brand, the setpoint is usually different)

The only other thing to check is to verify that all the connections in the "blue wire" circuit are corrosion and crud-free. This is the blue wire that begins on the back of the alt, runs to pin #11 on the 14-pin connectors on the electrical console, runs forward through the wiring harness to the back of the oil pressure gage, then through the bulb holder and into a red wire with black stripe to the ignition switch. If you pry out the gauge, don't pull off the wires, and for sure, don't inadvertently short any of them together. What might be happening is a loose or intermittent connection in that circuit resulting in no-charge, then you hit a bump or something and reestablish continuity, which turns the juice on, but since the battery is so low, the alt goes to maximum output, pretty close to 14.5V.

Good luck!

My alternator must have been updated at some point, because I actually have the internal VR. Is there any way to test prior to buying a new one?