Alternator interchangeability- 2003 Forester using an earlier alternator?

[Stevo F]

I replaced the alternator on my 2003 Forester a few months ago with an OEM new alternator. Now I'm finding, with a voltmeter, that either it takes a few seconds to art charging or not at (it takes several starts for battery voltage to go from 12 to 14 volts). I installed a crappy Duralast alternator and it pushes 13.5- 14 volts all the time.

I'd rather use OEM than Duralast (it was originally on my Outback when I bought it and I replaced it with OEM on that car). I already have an unused Subaru remanufactured alternator that I had purchased for my '98 Legacy, but sold the car before I could install it. My question is whether I can use the older alternator on my Forester. I'm not that concerned that it's only an 85 amp vs. my current 90 Amps, but the electrical plug is a different shape, although it has two wires like on my 2003 Forester. I figure I could just wire in the 90's style plug, but are there are design differences between 1990's and 2000;'s Subaru alternators that would prohibit me from using the older alternator?

 

[nvu]

I went reverse, 2006 alternator in 98 impreza.  Swapped from black rectangle to green oval plugs and works fine. 

Though 13.5-14v at the battery is kinda of standard nowadays, it's okay for newer batteries.

Edited Friday at 01:44 AM by nvu

[Stevo F]

Update- I sourced a used OEM alternator off eBay and it worked for about 2 minutes then stopped charging. Back to the drawing board. Thr only alternator they had worked all thr time is the POS Duralast unit. Just wondering if I should get an old style plug, and wire it in and use the unused ‘98 alternator or might that cause issues with the electrical system?

[nvu]

It should work.  Only models I know need all 3 pins on the alternator were the sti and XT models with the 32bit ecu, electronic throttle.  If yours is cable throttle, it's the old style and only uses 2 of the 3 pins. 

[Stevo F]

I went ahead and ordered the older style 2 pin connector for my Subaru refurbished older tyle alternator. I just want to make sure I wire it in correctly. Based on this diagram- which pin on the 3 pin connector correlate to which pin on the 2 pin connector. I found this diagram of the rear of the alternator and the 2 pin connector.

[Stevo F]

OK, I was able to match up the L "light" and S "Sense" wires from the 3 wire plug to the 2 wire plug. I temporarily attached female spade connectors to the wires and hooked them up to the alternator and it was charging at 13.8- 14.0 volts, right where I wanted. Having a built in voltmeter is new to me, and I noticed on some occasions it would go down to around 12.7 volts for a few seconds, then back up to upper 13's- I assume there's no issue there, as the Duralast alternator I was running temporarily was doing something similar.

[nvu]

Odd, it should not kick off from charging at all for these models.  There's no ecu control if it's 2 wire so no way to tell it to power off.  Do you think there's a corroded wire in the harness causing intermittent charging?

If you have a spare connector, dash lightbulb, and a switch, you could make a dongle from the alternator to the battery + and manually force the alternator on to test.