Symptom of a Bad Alternator?

[engineer2009]

Hi All,

I have a 1996 Sentra GXE with 114K miles. During moderate acceleration, the battery light goes on. During steady speeds below 55mph, the battery light stays off - but at speeds above 55mph, the battery light comes on.

With the car in park and engine running, I attached a voltmeter and got the following readings:
1. At idle, the voltage was 13.5 V
2. At moderate rpm's, voltage was about 13.5 V
3. When rpms are increased and just before the battery light comes on, the voltage drops to 12.2 V. It is also 12.2 V while the battery light is on.

I checked the fuses, electrical connections, and belt tension and they all seem good. The battery hasn't drained overnight and the engine starts right away in the morning. Though I did lose power once while on the highway when I had my lights and AC on - speedometer suddenly registered zero and I lost acceleration; I released the gas pedal for a few seconds and the power kicked back in.

Can an alternator fail with the above symptoms - good at low rpm's and bad at higher rpm's?

Thanks for any input.

[Angry01]

I would check your alternator to be sure. but to me it sounds like it may be your alternator.

[fugiot]

Hmm, I dunno, I always thought your car would die at idle with a bad alternator. I'd still check it, but I've never heard of anything like that. But the battery light is a good indicator to check it anyway.

[engineer2009]

I replaced the alternator over the weekend and everything runs good. Advanced Auto guy tested it and found that it was only outputting 9A - so it makes sense that at low rpm (small electrical loads), it runs good, but at high rpm (higher electrical demand for engine), the battery light goes on.

Thanks all for your advice.