Saab 92 Forum : Saab92x.com / Erratic voltage at idle

-B- · 2012-04-01 21:19:31

Did a search here and NASIOC and came up empty. At idle the voltmeter is erratic in rhythm with the erratic idle, which at times can be smoother than other times. It always stays in a healthy range but I'm of the thinking an 05' should be just a touch smoother, and that the two may be somewhat related. The alternator looks to be fairly new.

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jon_s · 2012-04-01 22:58:32

Is the battery original? Mine has been doing this lately (lights dimming), but the battery is 7 years old, so I suspect that first.

-B- · 2012-04-02 09:04:53

new battery when the short block was replaced 4k ago.

tonka92x · 2012-04-02 10:24:50

Yup, they are related. Alternator output varies with RPM, so if your engine RPM is varying so will your alternator output.

Scargo · 2012-04-02 12:06:05

according to my Passport (haven't cross checked it with a real meter), my voltage regulator has the car running at 12.5 - 12.7 V while driving or at idle. Engine off, it's like 11.7V. And it's a new battery. facepalm

tonka92x · 2012-04-03 13:03:59

With the engine off you should see 12.0-12.7 volts with a fully charged battery. With the engine running you should see 13.0-14.2V. Below 13.2V the battery will not charge.

With the engine running check the voltage right at the alternator and at the battery - use the mounting bracket as the ground when checking the alternator, the battery negative when checking the battery. They should be approximately the same, within a few tenths of volts of each other. If they are and the voltage is still below 13.0V you have a bad alternator or a broken/loose connection.

If the difference is more than a few tenths of a volts you likely have a poor connection. Check the voltage between the alternator output and the battery positive, and between the alternator ground and the battery negative. The one with the greater voltage will be the side the poor connection is on.

Another thing to try is to charge the battery with an external charger, then see if the alternator can keep the voltage above 13.0V. I've had this happen on several older vehicles, as the alternator ages it can keep a charged battery at full charge, but it can't charge up a flat battery.