Interesting as I have almost the same issue but my car barely wants to start in the morning. MY battery is normally fubar'd after every 4-5 months. Im going to pull the defrost fuse and see if thats my issue as well

in phil's case i think this was a happy accident. He said he heard a relay click when he pulled it. the only thing I cant think happened was the relay was stuck closed and pulling the fuse opened it back up.

his defog fuse WILL always have power.

Me too. I've pulled all the engine bay fuses and it still does it. I need to pull all the cabin fuses as well.

Car started right up for the 2nd day in a row. So that must have been it.
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Another epic win for e-diagnosis

It was a fluke one.

I told him to go out and test random fuses and wires for power. The Defog fuse is supposed to be hot at all times.

the fact that it just so happened that the defog relay seems to have been stuck closed is a crazy epic coincidence.

...says Mr. "taking credit for Joe's suggestion." :D

This is why testing for voltage isn't a useful diagnostic technique for looking at this sort of problem- you need to test for current. When levnubhin stuck that test light in between the two fuse terminals, and it lit up, that meant that not only was there voltage at the fuse, but that something downstream of it was actually trying to pass current through it, and that's what was flattening the battery.

It's still lighting up now if he puts the fuse back in. I have offline info!!!!

That you got online!

Sorry my battery was dead again this morning after 2 days of no issues. Fml
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:(

Do you have access to the alternator plugs? Unplug them and check again.

If the battery was continuously discharged, it might have killed the battery in the process.

So this is really weird. It was good for 2 days, then Saturday morning it was dead, then Sunday morning it started right up. Went to my sisters house on Sunday, car sat for about 6 hours and then was completely dead again. Looks like I'm going to have to get a multimeter.

Now, when I hook up the multimeter to the battery what exactly am I looking for before and after pulling fuses?
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Current.

Most meters will have a seperate + contact (often two of them) for measuring current as opposed to voltage, resistance, etc. This is because the current mode basically presents a short circuit at the probes, and is intended to be placed IN SERIES with the load, rather than in parallel. So if you have the meter setup to measure current and you then put it across the battery, you'll blow the fuse in the meter.

So, what you're gonna do is disconnect one of the battery terminals (we'll say you've pulled the + terminal) and then place the meter, configured for current reading, in series with the open circuit. IOW, put the maters + terminal on the battery's + terminal, and the meter's - terminal on the wire that you removed from the battery. Be extremely careful while you're doing this.

Start out at the higher of the meter's current settings. For instance, on my Fluke 77 I have two I+ terminals, one for up to 300ma, the other for up to 10A. I'd start with the 10A terminal. If I got a reading of 0.3 or less, then I'd switch down to the 300ma terminal for more precise measurement.

So, if in this mode you find that you do have more current flow than you ought to, you can do 1 of 2 things:

A: Have someone start pulling fuses while observing the meter (difficult and imprecise)

or

B: Put the battery back the way it's supposed to me, and then start pulling one fuse at a time and sticking the meter probes into the fuse hole to see how much current is trying to pass through that specific circuit.

I wonder if a bad ignition switch could be an issue...if it's keeping it sitting on ACC even when the key is out.

Anyone know how many ma should be passing through the room fuse? I pulled the fuse, put the multimeter in and it's showing over 5.00 ma. I even noticed that my radio was trying to turn on and the key isn't in the ignition.
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5mA is not a lot.

phil, put the red lead in the 10A port. then pull the positive battery cable and test the amps inline back to the battery. what is the reading?

The weird thing is that when I first plug the multimeter into the room fuse port the multimeter reads into the high teens low 20's and while it's that high the radio tries to power on. After a few seconds it settles down to low 5's.
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This is not unexpected. I'd imagine that the radio has some filtering capacitors on the power input (even the keepalive input) and those caps will suck down a bit of juice when they're first energized. And 5ma seems very reasonable to me- that's 5/1,000 of an amp. A typical Miata battery with a rated 30Ah capacity could supply that load for a little over for six and a half years before dropping below rated voltage.


A couple of thoughts:

1: I like the idea that maybe the keyswitch is continuing to supply ACC (or IG1 or IG2) after key-off. It's easy to test, too: get down there and probe the R/B, B/R and G/R lines for +12v (relative to GND) after you've switched the key off. Repeat several times.


2: I just remembered the part where the wiring has been hacked. Could it be that some relay is getting switched on and is then subsequently being held latched (by its own contacts) which are being supplied by a non-switched source, only to release (and thus thwart troubleshooting efforts) as soon as you remove its power supply by pulling a fuse, disconnecting the battery, etc?