What's a good test for an alternator, I'm trying to determine if it is bad? 1990 xj 4.0l aw4 TC 242.
It's my understanding that with the motor running if I hook a multi meter up and it reads say 12.2 my battery is really good and I need to replace my alt?
Been having a problem with my blower motor fuse,blowing when I try to switch on the ac.?
I'm not trying to over think this but looking for a good place to start here. I've done cruisers tips on most of my grounds but some of this stuff in the engine bay looks like a brittle mess to me.

 

Connect a voltmeter across the battery terminals. Hopefully 12.5 to 13.5 volts with the engine off.
Turn the key to "on", (engine still off) and turn on the headlights and heater blower.
Note the battery voltage reading. It should have dropped.
Now start the engine and check the battery voltage reading again. It should have risen back to 12.5 to 13.5 volts. If it does not increase there is a problem. Alternator, regulator, wiring...

 

Thanks I'll try that and see if something is up with the alt.

 

If you turn on the jeep and you disconnect the battery. If the jeep keeps running its good. If it shuts down alt. Is bad.

 

I just checked my friend's 1990 two days ago. I wrote down the static voltage, as well as the idling (with headlight on)voltage. I then took it to an alternator/starter shop to have the double check (Thought maybe a diode was leaking). They said it is fine.

Static voltage measure 12.64v (next day 12.32v), and at idle with lights on 13.91-13.95v.

 

Actually I cannot vouch for this test. After reading around the internet quite a bit - it seems that most 4.0's will NOT stay running in this condition. I had done this test earlier in the year and it did stay running, though I did it again recently and it died almost instantly - but the alternator/battery test perfectly fine and there are no other issues (the issue I was looking into turned out to be the wiper motor draining power due to a possible interior short and it was causing the dash/head lights to flicker, thought it was possibly the alternator - it wasn't)

So no...this is actually not a valid test from what I read. It also is not safe to do on newer vehicles, although sure, with out renix's I'm sure it's no big deal.

Again, if you read around a bit - most people say the 4.0s will draw too much juice to stay running without battery connected, good alt/electrical system or not.

 

Well, just an update my alternator seems to be good.. battery was replaced not too long ago ..
I'm going to see if my (resistor) is bad and my be the cause for the blower motor fuse blowing everytime I go to switch the ac or defroster on.
Fan works fine as long as I don't switch the ac/ def on.
Thanks again