Possible alternator toast?

So, i drove through a good amount of dirty muddy water, splashing,2 days before this occurred.

Wheeled in that, drove home 200 miles, drove the jeep the next day fine, then, all of a sudden this problem hits.

Went to start the jeep that morning and it was dead as a doornail, wouldnt even give me dashlights, Nothing. jumped it, it runs for a minute or so then starts sputtering/low idle wanting to die, the battery meter on the gauge cluster is sporadic, hovering below the half way mark at 12, moving around a little as it sputters. sometimes itll even out and then start idling ok. but if i shut it off, the battery will be 100% dead again when i go to turn it back on, no lights, no chime, nothing. So i tested voltage on the alternator and got a reading of around 11 to sometimes 12.

Good chance this is my main problem?

Ive been told it is and have pretty good confidence in the advice i received, but id like any possibles otherwise as im short im time to solve the problem and only have 1 day to work on it basically. I am not very savvy when it comes to the jeeps electrical system as far as knowledge.

Its a 97 XJ, 4.0.

First things first. The electronics don't like to run when the voltage available to them drops below 10v. So, when you have a really dead battery, just jumping it usually won't do the trick. There isn't even enough voltage to "excite" the alternator for charging.

That said, you need to fully charge the battery. While its charging would be a great time to clean the cable ends and renew all system grounds.

Now that your battery is fully charged you have to make a decision--should I ASSUME that the battery is good or have it load tested. Personally, I would have the load test.

Once you have a good and fully charged battery you can hook up a voltmeter to the battery posts and fire your xj up. If the voltmeter reads less than 13.5 volts your alternator or regulator is not right. I'm not sure on the 97 if they went back to the separate regulator or not.

You can drive it to a parts store and have them test the system for free.

Back-check your IP voltmeter with a "known good" multimeter, but it sounds like you're on the right track. Whatever you do, do not pull the battery cable with the engine running! (I've been trying to break people of that for years. It's fine on 1973 and older cars - but solid-state control electronics started to hit the scene in 1974, and I've known of quite a few solid-state modules blown out by the voltage spikes caused by this "test." Don't do it - you or anyone else reading this later.)

Anytime you get a suspect reading from the meter in the IP, the next step is to back-check it with your multimeter - you can check at the battery terminals, and (if you want to be thorough,) leave the ground on the battery negative post and put the positive terminal on the alternator output post (the reading should be withing .2VDC or so of what you get at the battery, and will be just slightly higher if it's working properly. If it's lower suspect the alternator anyhow.)

Easiest way to quick-check the alternator - charge the battery (the slower the better) overnight. Check battery voltage. Start engine - check voltage at the battery again. If the alternator is working properly, system voltage should be at least one full volt higher (resting fully-charged battery voltage is 12.6VDC nominal, with alternator output running 13.0-14.5VDC nominal, with a norm of 13.6-13.7VDC. Anything above 12.0VDC is acceptable for the battery resting voltage, it does drop slightly as the battery ages.)

i should mention that i swapped the battery out for another and got the same result.

i checked the output at the battery and if i recall it was right in 13.something

could someone possibly point out to me the test post on the alt? im 90% sure i got it right, but i had never done it before...i did get the reading of ~11-12 on the alt on whatever i checked it on. assuming that was the right post. if i did that right, then the alt is definitely reading lower than the battery.

then yeah, you can assume you toasted your alternator... welcome to driving an xj with a low mounted alternatorl

There are, as you've probably seen, three posts on the back of your alternator. I refer you to the picture linked here - http://www.geocities.com/JeepI6Power/img/ndensor.jpg

At roughly 11:00, you'll see two terminals with a grey plastic surround. These are the connections for the field coil, and are not the terminals you want to check for output (you can check to see if the regulator signal is getting through here - as I recall, it's usually less than 4VDC.)

At about 2:00, there is a single post with a heavier surround (in this picture, it's "fenced in" as well.) This is the output post you're looking for to check alternator output.

thanks very much! ill double check on that post tomorrow.

the one i was getting a reading off of was one of the smaller ones you mentioned being at the 11 o clock position

i tested the right post today and the alternator checks out ok on the volt meter at ~14. so does the battery. so now im really confused what to do.

Ok, you have a fully charged and load tested battery, you have cleaned all cable ends and renewed all grounds, and you have 12v static at the battery posts and 14 output from the alternator.

Well, now you need to disconnect the battery and test the cables with an ohm meter for excessive resistance.

joe_peters said:

Ok, you have a fully charged and load tested battery, you have cleaned all cable ends and renewed all grounds, and you have 12v static at the battery posts and 14 output from the alternator.

Well, now you need to disconnect the battery and test the cables with an ohm meter for excessive resistance.

Click to expand...

Before you go pulling cabling to bits and testing it, test the connexions.

Take your voltmeter, put one probe on the stud and the other on the ring (for instance, you'd check the battery connection by putting the tip of one probe on the battery post, and the other somewhere on the clamp.) You should not see any more than .1VDC potential difference - if you do, clean the connection.

If cleaning the connections doesn't help, then take the cabling off. IIRC, you should see something like .001 ohm/foot of resistance. Any more than that, you're looking at needing new mains. Hit me backchannel if it comes to that...