Alternator weirdness
 

Any advice would be greatly appreciated because my 2003 wrx started getting harder to start until I realised the battery wasn't fully charging .. Went to get it checked and everything seemed fine with the alternator as it was reading 14v at the battery when running and 12.8 when the engine was off .. So I bought a new battery and that lasted two weeks before it too started to die .. So I bought a multimeter and this is what I've discovered .. I'm getting 14v at the battery when the lights are on and the engine is running and it drops down to 12 ish when I turn the lights off .. What's going on and do I need a new alternator???

How often are you using the car ?

Every day'

Oh right certainly a problem then,
They have a intelligent alternator so only charge when they sense load.

ok thanks .. Sounds like it's on the way out then .. I'll drive it with the lights on until I can get a replacement!

Yeah, the thing is it shows the correct Volts with load on,
have you got a multimeter that does Amps?
If so can you do a load test to see if it`s drawing anything when turned off ?

I know lol that's what's freaking me out .. If I have the lights or the AC on the alternator works perfectly .. I did a parisitic draw test and it didn't even register at 200mA so I've pretty much ruled out everything other than a funky alternator .. I've ordered one anyway as this one will probably totally crap itself in the middle of nowhere

Ok now I'm even more confused .. I've just replaced the alternator with a brand new one and now I'm only getting 12.8 ish with the lights on and engine running .. Is this because the battery is full and it knows not to charge or am I going to have the same problems .. Doing my head in now .. Any help would be really appreciated!

So pissed off with this now .. Went out after testing the voltage in the battery and when I switched on the lights the charge warning light AND my handbrake light came on and my windscreen wipers are slow .. On a better note the battery did have more charge in it

I would have a good check over all the wiring and follow the 3 pin alternator plug back into the loom and check for any signs of damage.

When fitting a new alternator that has brand new brushes it can take a little while for the brushes to bed in properly amd make full contact so voltage output will rise a little after a few miles. You should however still be seeing at least 13v and you should not be seeing the warning light on the dashboard.

As youve now had 2 batteries and 2 alternators with the same faults showing i would think you likely have a wiring issue. Also not all 3 pin alternators work via the smart charge and will put out the same voltage regardless of load.

Dubzy