A neighbor has a '90 legacy with automatic transmission and the tranny is shot (metal shavings in the front diff oil). Anyway, would the 4 speed automatic out of a '98 Impreza work? Would it just be a bolt in, or will special modifications be needed? Will the old ECU get confused with a 4 speed auto rather than the 3 speed? Worst case, can he use the center diff in his old 3 speed? Are the front ratios the same or will he also need to swap in the rear diff to match?

unless im confused here the front dff. and the trans is two seperate items.the front diff should have gear oil,is this where you found the shavings?

unless im confused here the front dff. and the trans is two seperate items.the front diff should have gear oil,is this where you found the shavings?

Yes

I don't know if it helps but the 90 Legacy has a 4 speed automatic, not a 3 speed.

The trans has 3 major sections.

 

Your problem lies in the front differential.

 

The main case contains the pinion shaft which is a matched gear to the ring in the front differential. You must keep the ring and pinion a match to each other.

 

If you are not afraid of taking the trans part way apart then you can do this:

 

Remove the rear section (tailhousing) with the 14MM bolts that go all the way around.

 

Remove the front differential.

 

Remove the 3 bolts that secure the pinion to the backside of the main case and drive the pinion gear shaft out of the main case.

 

Insert the same gear shaft from the Impreza into the Legacy trans and re-bolt the sections together, provided that the gear shaft is the same.