Is Alternator With Max Voltage 13.5 Defective?


Old 10-17-06, 06:36 PM
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Is Alternator With Max Voltage 13.5 Defective?

My wife's 2001 Pontiac Montana van has been displaying the red battery symbol on the dash display sporatically for months. It comes and it goes. Once though the battery drained so much that all the electrical devices in the van started shutting down.

I took it to a Pontiac dealer to check this problem, but they couldn't find it. At that time they couldn't get the problem to occur. I've also had Advance Auto test both the battery and the alternator. The serpentine belt is practically new.

I tested the battery this evening after my wife got the red battery symbol again. However, when I was testing it never displayed.

These are the voltages I got:
- Engine running, warmed up, at idle: 12.9 volts
- Same, except increased engine RPM: 13.5 volts maximum
- Right after turning engine off: 12.5 volts

In my experience alternators should put out at least 14 volts. So, I suspect that the alternator is going bad.

What do you think?
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Old 10-17-06, 08:45 PM
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It's been awhile but I believe most alternators are supposed to put out about 14.5 volts. If the battery is not fully charged or is bad, it will cause the voltage read to be lower than that though.

a fully charged battery is 13.2 volts (2.2 volts/cell- 6 cells)

If the alt pulley is glazed it can allow the belt to slip. If it is not tight enough, same thing. I have seen a lot of tensioners go bad and not keep the belt tight enough although it appeared the belt was tight.
Old 10-17-06, 09:35 PM
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13-13.5vDC is good for a running engine with some accessories on.

With all the accessories on, i.e. lights, radio, heater/ac, the voltage should not fall below 12vDC.

Keep the car running with these accessories on for about 15 mins, then take a reading to see voltage.

Check your cables going from the alternator > batt > starter.

If anyone of these cables are not connecting properly, you will lose charge.

That charge indicator light is controlled by the diode pack on that alternator.
Old 10-17-06, 10:10 PM
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I've seen a voltage meter at walmart that plugs into the cigarette lighter. This would be handy to monitor the voltage when the red light comes on but I think your alternator is on the way out.
12.9 volts at idle sounds low.
From my experience, the voltage will drop for only about 1 second when an accessory is turned on then go back to what is was before.
Old 10-18-06, 07:32 AM
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Testing voltage drop between positive terminal at alternator and positive terminal at battery showed over 2 volts drop.

After cleaning the positive battery terminal and the end of the red battery cable there, the voltage drop from there to the positive terminal of the battery dropped to 0.17 volts, and the idle voltage across the battery terminals climbed to 14.3 volts.

Interestingly, both the nut holding the battery wire at the alternator and the connector post (bolt) were pitted, an obvious sign of electrical arcing due to the corroded connection. I feel confident that this was also the cause of the intermittent red battery light problem we've had for months.

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