Dead battery- drain or bum alternator?

[BuffaloRider]

So last year my warning chime went to shit and I left the lights of my 1991 Grand Marquis on like 6 or 7 times draining my battery. So this friday when I went to drive my car home and the battery was dead I thought "it might be that stock 2g alternator" but figured it was just from me killing it and reviving it all those times lol.

I got a new battery, put it in, then started doing a coolant flush this weekend. Half way through the job I went inside and passed out last night. This morning I realized that I left the hood of my car open, oops. I don't have an underhood light so I didn't consider that it would drain the battery. I continued with the flush, went to start the car and the battery was stone dead, absolutely nothing.

Just last weekend I picked up a 3g alternator and was planning on doing the swap soon as I noticed that my alternator got super hot within ten seconds of starting the car, and my stethoscope was revealing some loud noise from it.

So, did my battery die from some kind of drain, like for example if the hood was open and a switch for an underhood light that is not present drained the battery (idk if this is possible, that's why I ask)? Or would it be more likely that a bum alternator caused my battery to drain after the last time I ran it during my coolant flush?

All I know is I've got to figure it out today so I can finish flushing the water out of my cooling system. THANKS!

[gadget73]

Only way to tell if its got a draw short of getting out the meter and checking is to charge the battery and leave the car sit a day or so to see if it drains. If it does, charge it, unhook it, let it sit and see if it goes.

An under-hood light left on overnight might do it.

Also check the voltage on the battery with the engine running and with it not running. Not running should be 12-12.6 typically, running should be 13-14.5, depending on particulars. Basically you want it higher with the motor running. If its not higher, the alternator isn't alternating.

[BuffaloRider]

After I jumped it and let it run for a while the battery read at 13.5 ish. I turned it off and checked again and it was about 12.4. I did notice that I had started to undo one of the plugs from the alternator to see if it was a direct fit with the JY 3g one i grabbed, and it wasn't fully seated. I imagine there should have still been contact but if there wasn't then that could explain it. I'll check it again in about an hour when I go for another drain and fill.

Oh and I left something under my hood when I slammed it, so now I've got a new dent in my hood son of a bitch!

[BuffaloRider]

I went to start it and it was dead. Voltmeter reads 11.3. I hope this isn't going to ruin my new battery. I'm trying to jump it again now but both parking spots next to my car are full. Then I'll pull the battery cables and see if it dies again, which I'm betting it won't. Would that mean that there is a draw?

[IPreferDIY]

Yup. Does your voltmeter have an ammeter setting? With the battery connected and engine off, you can measure how much of a draw by taking off the battery ground lead and putting an ammeter between the lead and the battery terminal. (You can probably do it on the positive side too, but taking off my ground lead is more convenient.) Start at the highest setting on the meter and go down as needed. My computer etc takes about 25mA when it goes dormant, and my audio stuff when connected takes another 85mA.

For an alternator with a separate charge wire, you can check to see if you have bad diodes allowing current flow by taking off the charge wire (with battery connected and engine off) and putting an ammeter between the charge wire and it's connector. From what I've seen of the 2G's, the charge wire is in a plastic connector, so that would complicate things.

If there's an under-hood light circuit with no bulb or a burnt out bulb in it, I doubt it would cause a problem since the circuit would still be open. FWIW, I've actually removed bulbs from my trunk, glove box, and front foot areas to avoid unwanted current drain.

If the battery has only gone down to the 11+ volt range, it should be okay, though I'd use a charger to top it up once you get everything sorted out. Using the alternator to charge it when it's really dead is a big no-no.

[GM_Guy]

Quicky test2, jump it, and let the battery charge up for a short while, hook up the jump vehicle for a minute or two (with the jump vehicle running, slightly off idle depending on what it is or you will have two dead batteries). Hook your voltmeter to your battery so you can easily monitor it, disconnect your existing alternator (pull the regulator plug first) while keeping an eye on your voltmeter. If the alternator is the cause of your drain there is a good chance you will see it react on the voltmeter (higher static voltage with the alternator removed from the cicuit).

Alex.

[gadget73]

I can tell you from recent experience that unless you drive the car for a bit, its not likely to fully recharge. I jumped the Mark VII a couple weeks ago and let it idle for about 20 minutes. Next day it was flat as a pancake. I jumped it again and took it for a drive, and the battery was still holding fine a week later. Idle speed output on the stock alternators is pitiful at best. Without enough time and current available, you'll get a surface charge on the battery that will bleed off quickly.

[BuffaloRider]

Yeah, I don't think it charged enough last time I jumped it. I took the battery to the parts store and had them fully charge it. I've been disconnecting the battery while continuing my coolant flush (drain and refill method because I have no hose) and it seems to be doing fine as far as keeping a charge. The volts do go up to 14ish when running, although very slowly.

I'll be doing what this video explains to see if there is a parasitic draw (which is pretty much the same thing IpreferDIY said)

https://www.youtube.com/watch?v=eXX8TfzRAKk

Thanks for the help guys!

[IPreferDIY]

When I had alternator issues awhile ago, the alternator shop had a chart on the wall indicating charge times for various degrees of battery depletion. IIRC, being somewhere in the mid-11V range required about one hour of running time to get the battery to a decent level.

[BuffaloRider]

Well well well... looks like we got ourselves a parasitic draw!

While touching the negative cable and negative lead with my multimeter probes I get a reading of 2.7 amps. I tried removing all of the fuses under the dash and got no change (or so thats what my wife said while looking at the multimeter). Idk if there's a fuse box under the hood or not, which is something I should know by now. I'll check my manual and I might check the under dash fuses again... this time without my wives help. I'm not saying that she can't handle looking at a screen and telling me if a number changes, but maybe I should just see the multimeter readout for myself lol. You know, just in case.

I was going to do the 3g alternator yesterday but figured it would be smart to figure this shit out first before diving into that.

[sly]

If you have or can borrow a clamp meter for inductive current reading (or feel brave enough with a cheap digital in-line on the charge cable - don't use an expensive one just in case something goes wrong), check the current on the charge cable with the car off. It should be zero. If there is ANY current on that line when the car is off, the alternator IS the issue.

clamp meter use:


regular current meter:

[BuffaloRider]

( i posted this before seeing slys post)

I've been reading and I heard that a bad diode in the alternator can cause it to draw power from the battery but still charge. Given the general condition of the alternator (25 years old, noisy, gets really hot really fast) and the fact that pulling fuses yielded no result I would say that this is a promising lead.

I'll do another parasitic draw test with the alternator disconnected tonight and see what I get. Might even have the parts store check it too...

[sxcpotatoes]

I also thought you could feel the alternator, if the engine is off/cold and the alt is warm or hot, it's sucking juice.

[VicCrownVic]

To go with what Sly said, an alternate way to test if the alternator is the issue is to completely disconnect it then recheck the current draw at the battery. It will not be zero with this method since you will still have some draw for things like radio and clock memory.

[gadget73]

Unplug the alternator. If it stops drawing, that will answer your question. 2.7 amps, if it is the alternator, might not be enough to really heat it up in an obvious way. By the time it cooled off enough after running, the battery would probably be dead anyhow.


You could undo all of the mess at the starter relay and figure out which bundle of wires has the draw, then attempt to figure out which things that feeds. 2.7 amps is a fair bit, more than a simple stuck-on light bulb somewhere.


Some small draw is normal, but we're talking a few milliamps. If its more than 100ma, something is on. I'd expect maybe 20ma under normal conditions to account for clock, ECM, and radio memory.