alternator about to go?
 

this is for my kid's subaru. the alternator output is in the vicinity of 12.8v on average. It sometimes dips to 12v even when revved up. On idle, it jumps around between 12.4-12.8v and is not constant. (i have a cigarette lighter voltmeter)

should i go ahead a get a new alternator?

benhogan, have you checked the back of the alternator to see if it has a voltage regulator, similar to those on MB alternators? If so, it may be that the voltage regulator brushes are worn and the voltage regulator needs to be replaced. Replacement is a simple DIY fix.

Low voltage can also be the sign of a bad battery, bad cables/connection or poor grounds. If you have an ammeter check to see what amps are being produced.

Inspect, clean, tighten the cables and grounding points before deciding the alternator is bad.

If you don't have an ammeter then after doing the cable check and the voltage is still low, swap out a known good battery and re-check the voltage.

You can't have high voltage and high amperage at the same time. It's a trade-off between the two values.

High amperage is usually a sign of either a weak battery or a current drain (aftermarket stereo amplifiers are a common cause).

I wouldn't put too much faith in the plug-in voltmeter.

i have a clamp type ammeter. does it matter if i put it on the neg or pos line?

Not really. I'd put it on the positive line but the negative would work. Be sure it is on the battery feed and not a secondary lead.

I'm sorry but I don't know much about subaru but It could be a loose belt. Easy to check.

PaulM

12.8 is fine if your only reading it from the 12v lighter socket. There's a decent voltage drop to the lighter.

Test at the battery and check the amp output from the lead on the alternator (very easy on a Subaru)

In general alternators work or don't work, very rarely will they "be going bad".

You could check for A/C current as well if you want to get fancy.