Yes,that is crazy high AC voltage.Usually the dash lights go nuts w/AC.

Anything else to check before I declare the alt as the problem?

Anything else to check before I declare the alt as the problem?

30 what? 30 milliamps is just fine (30 ma), but 3.0 amps is not; 30 amps should have blown the fuse in your meter.

 

More editing (2nd): 30V AC might be normal since the battery wasn't fully charged at this point. I'm going out to my car and see what is "normal" for a working alternator and fully charged battery and report back.

 

As to why the battery went dead again: Was it fully charged to start with? Running it at idle for an hour or driving to the market and back will not fully charge it.

 

Also, it is winter time and the possibility exists that your battery isn't holding a charge very well right now. Batteries do not like cold weather.

 

Only way to tell to have it load tested at your local auto parts store. However, be aware that they want to sell you a new battery. If the battery is less than three years old it should still hold a decent charge. Five years is about all you can usually expect from a battery unless you're very careful to keep it fully charged all the time.

 

Edit: After re-reading your post, have the autoparts place check your alternator output also. 14.5V DC is good but unusually high. I have a perfectly good alternator and the most it puts out at cruising speed is 13.7V. It's also possible that the battery is bad enough that it won't filter out the AC portion (called ripple) of the alternator output. There's some electronics here that are a little past what you can get here on the USMB. But checking out the battery AND the alternator at a shop you trust is the only sure thing.

Edited January 19, 201115 yr by edrach

Okay time to get some comparable readings.

 

Went outside and measured the volts and ripple on my '97 Impreza. Took readings at idle and about 2000 rpm and the readings were essentially the same. 14.02 VDC and 25mv AC (that's .025VAC). Pretty cold for here since moisture is freezing on the car this evening.

 

So 30VAC is certainly out of the "normal" range.

 

However, a well discharged battery (and yours was certainly that considering you had to jump start it) will not have the capability to filter out the AC component of the alternator output. Conclusion: Have both battery and alternator checked out.

Without any doubt, the alternator diodes are bad. You should never have that much AC voltage if the alternator was ok, even if the battery was nearly drained. I would be concerned if the level was even 2 volts. A new alternator is needed and it would be good to check the condition of the battery also. Get it fixed ASAP so damage to the battery is limited.

 

As a side note about measuring AC voltage across the battery. With older analog meters the DC voltage isn't blocked out when you measure AC voltage and will be added to the reading. So you have to know your meter operation so you don't get fooled. As far as I know none of the digital meters have this issue so it makes seperation of the two voltage components easy. If there is any doubt about how the meter is set up just place the meter in the AC voltage mode and measure a DC voltage. If the DC is blocked the meter will read zero after an initial surge is made by placing the probes across the voltage source.

Edited January 20, 201115 yr by Cougar