sometimes my voltage guage will be around 14 or more but suddenly it will drop to like 10 and in the red, car looses rpms and allmost stalls.only happens when im stopped and idling.do i need new alternator,please help.can't listen to my stereo at full power unless this is fixed

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1987 Trans AM GTA
305 5SPD
Hypertech airfoil
gutted maf
kn filter
High Flow Cat,
3"Walker pipe
Dynomax Super turbo Muffler
Custom Air Intake.

"Like a wounded duck i will continue to hobble alone"

 

Make sure the belt is tight. If it's slipping you will only notice it at idle.

 

Because the voltage drops so much (to ~10 volts) suddenly, I would not suspect a belt or a loose electrical connection (the drop would be gradual). Even if a plate inside the battery were to come loose from the other plates, that should not cause the voltage drop because the alternator is providing the voltage/current.

My first guess would be something is suddenly drawing a lot of amperes (such as >300) to cause such a dramatic, and sudden, drop of voltage. Perhaps a positive wire (e.g. primary battery cable to the starter from the battery) shorting to ground, either externally (shorting to the chassis) or internally (shorting to a ground inside the solenoid).

Another possibility would be something which is connected with heavy gauge wire to the battery, such as a power amplifier(s) for your stereo (if you have one). Again, when the problem occurs, the wire that gets very warm or hot after 10-20 seconds of drawing that much amperage, will more than likely lead you to the problem. Since the amplifier is protected by a fuse (it is, isn't it?), the problem would probably be between the amplifier and the battery. ---Does the amplifier(s) work at all when the problem occurs? Try removing the positive power wire to the amplifier(s) at the battery, not at the amplifier. If the problem does not return, then this wire would be suspect. If you do not remove the wire at the battery, it could still short to ground somewhere.

If it's a thinner wire that cannot handle that much current, it'll lead you to the problem by signaling you with smoke (and then possibly flames).

Just a guess. Let us know what you find.

[This message has been edited by Stuart Moss (edited May 29, 2001).]

 

i have new belts and amp still works will amps are going down.

 

In order to troubleshoot, I would need to know how many amperes the alternator is putting out when the voltage is normal and when the voltage drops to 10 volts.

If you don't have an ammeter to measure at least 100 amperes (to put in series with the output of the alternator output, such as a clamp-on type which would be the easiest way), you can get an idea by just measuring the voltage drop that is created by the resistance of the output wire to the battery positive terminal.

To do this, you'll need a digital multimeter (DMM) set to DC millivolts. Put one probe on the output of the alternator and the other output on the positive battery terminal. You'll just be reading the voltage drop that is created by the resistance of this wire from the positive output to the positive battery terminal.

An example would be 200mV (0.2 volts) when normal [dependent upon the resistance of this wire and amperes it is carrying]. If, when the volts drop to 10, this number increases, then the alternator will be putting out more. And conversely, if it decreases, then the alternator will be putting out less amperes. If you know the resistance of this length of wire, you could determine the number of amperes passing through. But the actual number is not as important (to me) as knowing if there are more or less during the 10 volt condition.

The answer to that will give me an idea of where the problem is.

Barring that method of finding the problem, I would touch as many wires, especially the thick ones, as I could after at least 15-30 seconds into the 10 volt condition to see if any are warm/hot. If so, that would indicate an overcurrent condition.