WJ Dying Battery
 

Hi, First time poster, sent over here for help on this guy from my home at Miata.net. Anyway, I own a 2000 WJ 4.0 with about 150k, and my father first bought it in 2003 ( so I've known it for most of it's life, and I took it over because Miatas don't like salt that well) Anyway, I'm about 100 miles from most of my toolkit, and living in a rental as I save up for a house down here, and the complex doesn't allow "car repairs" on the property, so I take the Jeep to Jiffy lube for it's scheduled oil change last Sunday, run a few errands and park it. I don't touch it until Wednesday (I take the train downtown for work) and the battery is dead as a doornail. Locks don't work, lights don't work, etc. I assume I was an idiot and left the lights on or something, get a jump, drive around run some errands, it goes off/on just fine, I get a reading of 13.5-14 V on the dash, etc. I park it Wed. night and when I come back thursday night, it's completely drained again. At this point I think that Jiffy Lube did something to cause a dark current draw (muck with the under hood light switch, get the cigarette lighter stuck on, etc) so I drive about 15 miles, idle for 15 minutes, park, prove that it will warm start, (barely) then drive for another 5 miles and idle for another 15 minutes. At the end of this this afternoon I pulled the ground from the battery, and went inside. About four hours later I was going to run out for pizza, reconnected the ground and it wouldn't turn over.

What's the best course of action? Should I assume the battery is OK and go buy a multi-meter and start looking where the draw is? If so, what's a good stock reading for the cold draw of a 4.0 WJ? Alternately, should I just assume my battery had a catastrophic failure and replace it? (no issues last winter, this week was warmer than last and it hasn't had issues before, fine to flatline is odd)

Batteries CAN just give up without warning. I'd get it load tested to make sure it's all right. The battery in my WJ was fine one day, would barely turn the engine over the next morning. After it finally started, I drove to Battery World where they tested to and found a bad cell. They put a new Willard 850 CCA in it and it's been fine for 2 years so far.

Yeah, I understand a big drop in one day given a battery kicking it, but the unable to unlock/ unable to even give dome lights is a weird deal.

had this problem check ground strap pass firewall. went on o'really website and found part for xtra long ground strap had them order it(called in morning had it that evening for me). do not remember part#or price but i want to say under $25 Good luck.

Toss the battery on a charger overnight then check it. A full charged batt should be 12.6 vdc. 12.3 is 50%, etc. There are charts / state of charge on the net as a function of voltage. So if you charge it overnight and it sits outside the car a few hours should still be 12.6, if its down to 12 etc then pfff bad batt.

Just a tale to put batteries in perspective. My Chevy was running fine all day, then I stopped to see a friend. An hour later when I went to start it, there was a BANG under the hood, then nothing. I opened the hood to find all the battery caps had blown off! WTF? We jumpstarted it and the voltage went full scale!I put every electrical load on it I possible could and got it down to 15 volts and drove it home. Turns out a cell connector inside the battery sparked and ignited the hydrogen, blowing the caps off and, like a fuse, opened the connection between the cells. Absolutely no warning of the failure.

You want screwy? I got a jump this morning, bought a battery at wally-world (three hour wait for an install, F*** that) grabbed a multi meter at home depot because walmart didn't know if they even sold the things, then the Jeep started starting and running fine with the old battery, etc. I guess I should return the battery and the meter. (when did multi-meters jump to $40, they always used to be $20)

In case you haven't noticed, the dollar ain't worth what it used to be. You can spend over $100 on a good one.

My opinion? Return the Walmart battery and buy a GOOD one with cell caps so you can check the electrolyte levels. Battery World sold me a Willard 3 years ago and it's still good on a load test. It weighs a ton with a thick case and doesn't leak vapors around the posts so there's no corrosion problem.

Back in '78, when I drove my '67 skylark to Texas, I was giving a guy a jump in an apartment parking lot, and he let the + cable hit my inner fender, causing a spark near the 'vent cap' style battery, ....and there was this CRACK! and the battery split down the side from the hydrogen explosion.
Ever since then I supervise the battery jumper cables. :pwned:

Sounds like you may still have some troubleshooting to do. Keep the voltmeter. Check for voltage drops. Sounds like bad cable ends or connections at the engine and starter?

I returned the Home Depot one. Amazon has ones that look like they have equivalent functionality for under $15. I used to fix lawnsprinklers, and ended up concluding that unless you're doing high precision electrical work, it's easier to get the disposable ones than the nice ones (and the Home Depot one didn't look that nice anyway). Honestly one of the guys on Miata.net suggested that the grease monkey may have bumped the cigarette lighter getting it stuck part way in. I honestly think that's the most likely scenerio, since the issues stopped once I pulled the under hood light and put the lighter inside the center console.

Cigarette lighter being stuck in? Would get pretty hot.
My daily driver is a $19.95 Craftsman, and I am a licensed electrician. Is what I use in the field. Of course the ohm meter function is blown from being connected to 120vac too many times accidentally, but it does show open and most likely shorted conditions. After 4 years I know it well.
If I do need better resistance readings I also have the meter I bought at the Flea Market for $4. (It was $5, I only had $4, he took it.)

If the battery load tests OK with a full charge, check and clean battery terminals, sometimes a bit of corrosion can cause mind boggling problems, next check your alternator. With your multimeter on ohms X 1, check the rectifier for polarity function, disconnect wires to alternator, then contact common lead to alt case, and positive lead to main alt terminal, should read 0 ohms continuity good. Reverse the leads, should read no continuity, rectifier is OK. If you get no continuity either lead positions, rectifier is open no voltage, no good. if you get continuity in either position the rectifier is closed no good, as the alternator is essentially putting out charging voltage in AC whereas half the sine is charging, and the other half is discharging, resulting in net zero charge, the battery bearing the system load resulting in net battery discharge. Also, the planned obsolescence rule for batteries applies here, I had a 5 year NAPA battery in my '03 WJ and that battery croaked exactly in the last month of the 5th year.

I'm surprised it lasted that long. Usually they die in 4 years and with prorating, you're better off buying a new one yourself.

Um, well, almost. When testing an alternator rectifier bridge with an ohm meter, couple things to note. First, there are three diodes on each side of the bridge, so you may have 2 working and 1 blown, and still show ok with the ohm meter. Second, on a x1 scale, the forward resistance of the paralleled diodes will not read 0 ohms. And of course, if you put the black on the case and the red on the output, you should see an open circuit on any scale, or you have a shorted diode(s). Right idea but you have it backwards.
Best deal I found is take it to advance to test in vehicle with their little LCD screen tester, it gives a print out analysis and graph of the function of the 6 diodes in the bridge rectifier and also says whether one is failed or not.