Is this the battery or alternator? Have to work in 2 hours!

So I just got off my first job and I had started my jeep as usual, went back in to grab my food and went back out, nothing of unusual. But as I drove home I flip the heat control to defroster and like 5 sec I had a check gauge light come on so I looked and my battery gauge showed it at 9 and below! Didnt notice any stuttering of the engine though which made me believe its the battery and not the alt but I haven't a clue with electronics so I don't know.
I got home and decided to shut it off, waited a few sec and turned the key, fired up just as normal without any slow start up or more than a turn or two over. But I saw on the gauge this

When I turned the defrost on it the battery gauge would stutter and the gauge light came on as the needle dropped. To me this sounds like battery but I thought if a battery died, your car would run bad or "chug" because the alt can't supply all the power? Or if your alt goes out, same out come. This is why I'm confused as to which is the culprit.
Spec:
97' SE 4.0l Cherokee
I've checked the wires going to the battery, none are loose enough to lose connection and power.

If your battery is more than 3 years old or if it has been completely drained a few times and recharged, I'm going with the battery.

Since your Jeep is OBD2 I think it might throw a code for a bad charging circuit.

Clean the battery terminals and go from there

Ya now not sure entirely what the heck it is, so I go to work, it stays at like 11-12 for a bit then drops and throws the check gauge, get to work, turn off the jeep, head in and grab my papers and head back out and open my doors and start folding/sorting papers. I sorta knew leaving the doors open and the lights inside shining could drain the battery if it was so I took the risk, doors were open for about 20-30 min. Got in turned the key......... Nothing, dead. Got a jump from another person and this is where it gets weird and confusing for me, the gauge jumped to 13-13.5 and for my entire route(4h,37mi) stays at that???? Turned the defroster on, high beams, heater, never budged. At this point the only thing I could possibly think of is wire connection? But I've never heard of wires being bad in a way you lose half power or something. Always hear it as on/off situations, insight?

Sounds like alternator is going bad.

Like said, make sure all the positive/negative terminals on the battery/block/firewall/fender are clean.

I got a "check gauges" on my 98 when the alternator was over-charging, but also got it when I busted my oil pressure sender/sensor. I'm to assume if something was wrong with the temp sensor, OBD II would tell me to "check gauges".

Sounds like dirty corroded, oxidized connections, probably at the battery connections.

Get yourself a voltage tester and start the vehicle and see if it stays above 13 volts or when off if the vehicle holds 12.6 volts with no draw. If it holds 12 volts it's your alternator, if it won't stay charged to at least 13 volts while running its your alt. my jeep personally when cold started it stays in the 14.4-14.8 range with low beams 2 driving lights heater and dome lights on I still charge at above 13.5 volts with a stock alternator and a group 34/78 duralast gold battery (800ca)

X2 about checking it with a voltmeter. You check the voltage with it off, IE no alternator charging, then check it running, IE alternator charging. Obviously the engine off voltage should be around 12-13v, and running it should be 13+v.

Sounds like an alternator to me, but this test will help you know for sure. Since you mention this being your first job, I assume you dont have many tools, if you do buy a voltmeter/multimeter, stay away from the super cheap models. Theyre junk and dont last. Grab a craftsman.

First take the battery connections apart and clean them. Indicated by fluctuating voltage more than anything. My Harbor Freight $3 voltmeters work great, and I have one in each of my three cars.

Borrow a car or call for a ride to work. Fix it later. Buy a second vehicle for when your XJ is down because these sort of repairs are how the life of a Jeep owner is lived.

xcm said:

X2 about checking it with a voltmeter. You check the voltage with it off, IE no alternator charging, then check it running, IE alternator charging. Obviously the engine off voltage should be around 12-13v, and running it should be 13+v.

Sounds like an alternator to me, but this test will help you know for sure. Since you mention this being your first job, I assume you dont have many tools, if you do buy a voltmeter/multimeter, stay away from the super cheap models. Theyre junk and dont last. Grab a craftsman.

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Fortunately I have Quite a bit of tools lol, after my dad was a mechanic and I was given his tools after he passed away.
If I recall, there should be one in there but again I'm not electronic smart so anything with electricity confuses the crap out of me lol
The oddest thing is that's its been at 13.5-14 now since that night and has only dropped maybe 1v once when I turned on the defroster. I'm so freakin confused on how that's possible or happens lol

Kevin108 said:

Borrow a car or call for a ride to work. Fix it later. Buy a second vehicle for when your XJ is down because these sort of repairs are how the life of a Jeep owner is lived.

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I'm seeing this now lol

Demonoid369 said:

I'm so freakin confused on how that's possible or happens lol

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Simple corrosion, and loose contacts at the battery clamps on the posts does that. Nothing complex about it at all. The connection can arc and reconnect, the jeep moves the cables move, and so the contact comes and goes, that simple. Heat and cold shrink and cool metal, the contact comes and goes. It is called a loose, corroded, connection!!!

This can also happen with the carbon brushes in the alternator, or the diodes in the alternator. What you saw was early warning signs that will return!!! Batteries can short out internally, and that can come and go, until they finally die. A loose ground connection can move, and come and go, and thus can do the same. Finally the voltage regulator on yours is in the PCM, and they are electronic and can and do fail, so the PCM internal voltage regulator can be going out.

Lastly, something in your wiring may have shorted and drawn the voltage down, like the large starter cable from the battery, or the blower motor may have been drawing excess current.....

You need to take it to a store with a tester and have the battery, alternator and starter load tested first. Clean the battery posts, clamps and main contacts at the PDC center (Fuse box) and do the same for the main grounds, and look for any damaged wire insulation under the hood.

You can also watch youtube videos on how to do very simple battery and alternator and starter tests with a volt meter.

Best tool in my kit for electrical issues: a $2 Harbor Fright battery terminal cleaner.

I had similar issues. pulled the battery and alternator, had each bench tested and both showed good..so I was like wtf.

Cleaned all the connections to the alternator, and all the grounds.. Never had the issue again.

Once while wheeling about 3 hours from home I had the ground fail that goes to the block on the passenger side. Instead, the starter grounded through the woven cable between the firewall and the block, setting the plastic wire loom for the main harness on fire. Moral of the story: check and clean your grounds! lol

Here's a perfect example of how off the gauge can be...

LOL, we call that a non-linear gauge in the math world, LOL.

Ecomike said:

LOL, we call that a non-linear gauge in the math world, LOL.

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Hey! Don't use smart people words with us... I had to utilize google for non linear then I had to use it again for a word they used to describe non linear :gee:

Come back to our world and speak our language ya immigrant lol

MaxxXJ said:

Hey! Don't use smart people words with us... I had to utilize google for non linear then I had to use it again for a word they used to describe non linear :gee:

Come back to our world and speak our language ya immigrant lol

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I'm glad someone said it.

I'm pretty dang smart but I ain't no Alvin Einstein :lecture:

Late model volt gauges are much more accurate because they use the value the ECU measures, much closer to the battery. The 96 and earlier gauges are at the end of a long gangly harness built from the cheapest, thinnest gauge wire Jeep/Chrysler could possibly use without things breaking, which is why turning the radio up, the headlights on, and the HVAC blower to high will make it droop a few volts.

Measure voltage at the alternator B+ terminal (the big stud with the plastic shield around it) to ground, the voltage from the alternator +B terminal to the battery positive terminal, and also the voltage from ground to battery negative and the voltage from battery positive to the fuse box supply terminal.

When I say battery positive or negative, I mean the post itself, not the terminal clamped to it. Make sure you scrape through any corrosion when measuring, the probe should touch bare metal.

I would bet on a dead alternator but without those measurements you can't really draw any solid conclusions.

You should see:
approx 13-15 volts (preferably around 13.8-14) from alt B+ to ground
approx 0-0.5 volts (definitely no more than 1V) from alt B+ to battery + (meter should read positive voltage with red probe on alt B+, if it reads negative voltage this way, your alternator is not charging anything)
approx 0-0.5 volts from ground to battery negative - any more indicates bad grounding issues
approx 0-0.5 volts from battery positive to fuse box supply terminal - any more indicates crummy wiring in between or heavy charging current (extremely unlikely given your problems)

Post up your measurements and I'll take a closer look at the charging system schematic in my 97 FSM.