The stock gauge on my S4 shows 10V+/- when stopped (with the brakes on) and 12V while moving. I’m running a duel alternator pulley, but no crazy stereo setups. The lights and dash dim a bit when stopped. Advance auto tested my alt and indicated it was leaking AC current. (They didn't say anything about low volts/amps/etc.)

After reading a few threads, it seems this is normal. If I'm not upgrading, should I just keep my current alternator?

 

it sounds like it's going bad to me...
I would get a new one if I was you, regardless if you were upgrading or not.
What about the alternator belt?

 

your alt is toast.. why would you think this to be normal??

leaking AC means there are blown out diodes/retifiers

anytime you are under 12.6 volts you are running on battery, not on the alternator.

At idle, you should be at 14.4 volts, and never under 13.5 volts.

 

Which means I need a new one too....crap.
Thanx Icemark

 

The S4 alternator is WEAK (60amp) at best why don`t you see if you can round up one from an S5 (80amp) or better yet find an FD (100amp) alternator. There are diagrams, if you search, that show how to alter the wiring so the S5 and S6 (FD) alt.will work in the S4. You can get the plug that fits the upgrade at the bone yard. I found several on other makes of japanese cars besides Mazdas. If you have trouble finding the plug let me know.

 

You are wrong. The stock S4 alt is 70 amps peak not 60 amps.

 

So I would be safe with just going and getting another stock one?
And how much would one from an s5 cost?, and is it hard to fab the wiring to make it work?

 

Thanks. That was quick.

I must have misread a few threads. The new alt will be on my car tomorrow.

 

How right you are. I stand corrected

 

Yes.
Not much.
No.

All found through searching.

 

CHECK YOUR BATTERY. just take it to autozone.

holy crap my alternator shitted on me, ran the battery down and killed it (unbeknownst to me), then I had it rebuilt at a shop, only to fry it immediately by hooking it up to a bad battery.