I have a 2004 sti. The alternator was changed before I bought the car and since then the MIL light comes on with the code P0558 (alternator circuit low). My plug has 3 wires so appears to have the smart charge feature. All the alternators look identical, with 3 pins. One alternator has 3 pins with one of the pins a dummy and does nothing (internals wired differently)
The other has 3 pins and are all wired up internally to support the smart charge feature.
If you put on an alternator with a dummy pin the computer can't handle it and throws up the MIL.
Some motor factors haven't a clue what I'm on about, so my question is this. Do I pay to hardwire a fix which will bypass the smart charge feature or try and get a alternator and hope it's the right one? If I go the bypass route does that mean I can use either of the alternators with no ill effects?

 

I think Kev at scoobyclinic could maybe help you with this.

 

.....