I think either my battery or my alternator is bad, but I can't determine which, and would appreciate some input. Here are the tings I've done/measured:

- No voltage drop between alternator and battery, or along wires leading to and from alternator and battery
- Voltage at rest across battery terminals, after having left the car alone for a while: 12.00 V
- Voltage across battery terminals while idling : 9.54 V
- Voltage across battery terminals while at 2-3K rpms : 9.68 V
- at higher rpms, there is a scratchy whirring sound coming from the alternator

Now all this would make me think that the alternator is bad...BUT:

- the car never stalls out - keeps on running
- both the alternator and battery are relatively recent (relatively)
- Weirdest thing: the voltage at one point shot up from 9 V to close to 12...but it only did this once

Anyways, I would really appreciate any help!

Thanks

Before you replace anything.
Check the D+ wire to your alternator-
Whats the voltage with:
1 car off ignition on?
2 car running?

Pull the regulator off the alternator and see if the brushes look
bad.

another little hint is to measure battery volts only at the battery posts
and NOT the clamps at the battery, so you don't get snookered by dirty
connections.

rfbbr,
A fully charged battery will read 12.5 with hte engine off. A good alternator will bring it up to 13.5, maybe even higher, at idle. With headlights on and blower on the voltage should drop some what but come back up with higher revs. If yo do not have the above carefully attach your volt meter pos lead to the B+ terminal on the alternator (carefully, if you ground it sparks will fly). Connect the neg lead of your voltm,eter to a good ground. Run engine. If you dont get at least 13.5 when yo rev engine the alternator is bad. If yo get good readings from alternator but not at the battery, check all wires and connections.

Len

Set up your voltmeter at the battery and hang it on the hood. I use alligator clip wires from Radio Shack to make this easier. Make sure voltmeter is hooked up and grounded right (ie your constant will be your battery which you know is 12v before car is on). Set everything up so there's no interference from other sources.

Turn on your car, headlights, defrost and let it run for while keeping an eye on the voltmeter. If it stays steady and doesn't bleed voltage I'd start wiggling and flexing ground wires, battery wires, wires at alternator ect. Keep an eye on voltmeter and look for a pronounced spike, drop in voltage and that will tell you if you have a corroded/failing connection. If everything checks out fine and voltage never wavers I'd suspect your alternator again. I had a bad alternator diode that exhibited similar symptoms.

What do think guys?