My alternator has been starting to fail for the past couple weeks on my 96 5.2 so I'm buying a new one from advance. I know the stock one is 90 amps, but the 120 amp is cheaper. Are these interchangeable or should I just stick with the 90 amp?

 

I'd go for the 120A; however, do consider that it also may require upgrading the wiring a bit.

OTOH, the "Big Three" is a worthwhile upgrade ANYWAY.

RwP

 

you still won't use any more amps than the system NEEDS at a given moment though the 120 won't have to "work" as hard to put out how ever many amps that may be..... if the system is drawing 30A at the moment that is all the alternator will produce no matter how many gazillion amp it may be rated at..... got same issue right now on my Wrangler.... they say 75A is original, 90A is optional, they stock the 90 and have to order the 75 for the same money..... same housing, same hookups....

for what it is worth I looked up an alt for my Wrangler and it shows the same one all the way back to 1988, (my Wrangler is a 97) D series, W series (square body 2 and 4wd) Dodge Dynasty, Reliant, Sundance, and minivans showing the same PN according to the "buyers Guide" on Rockauto. My Wrangler alternator is locked up solid.... there is a guy on CL that I have been talking to who has a lot of auto electrical parts including 2 of these alternators, brand new in the box for $50/ea..... I had talked to him about getting one as a spare for my Dakotas since I have 2 of them but now I may buy both that he has.... one for the Jeep and 1 for the shelf since between my son and me, we have no less than 6 vehicles that would take the same alternator..... my son just put one on from the junkyard on his 90 4wd 3/4 ton which also takes the same unit.

 

The main advantage of a 120A alternator is that no alternator puts full current out at idle; if you idle at night, like say at stop lights, a bigger alternator can prevent dropping off as badly.

And as Volaredon says, it's easier to supply what the system wants (that is, if you need 75A, the 90A will do, but the 120A is running a bit cooler for the same output.)

RwP