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 June 27 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.

[Stevo F]

So I believe I discovered thr original issue of a delay of charging was due to the battery indicator bulb being burned out, so it wasn’t signaling the alternator to start charging upon startup. I’ve replaced the bulb and so far, it is charging within a couple seconds of starting.