Battery is going dead

I hate electrical problems.

When I started looking for the reason for the battery drain, I figured there would be an ohms reading when measuring between disconnected battery cables. The 74 had a high 5 ohm reading, the 76 had infinity under the same conditions. So then I proceeded to disassemble the dash and disconnect stuff to find the problem, but the ohms reading never went away even with everything undone inside the car.

Then I read this thread and figured if it is draining through the main battery line at the alternator, then disconnecting it should get rid of the ohms reading. It didn't. Took out the sense and tickle wire plug, and the ohms went to infinity just like my other car.

Deduction on my part is that there is a problem with that part of the alternator. I doesn't make sense to me that measuring ohms there with my meter would damage anything due to that circuit being subject to battery voltage all the time.

Can you make sense of this and tell me if I did something wrong?

bigredlaguna wrote:When I started looking for the reason for the battery drain, I figured there would be an ohms reading when measuring between disconnected battery cables. The 74 had a high 5 ohm reading, the 76 had infinity under the same conditions. So then I proceeded to disassemble the dash and disconnect stuff to find the problem, but the ohms reading never went away even with everything undone inside the car.

Then I read this thread and figured if it is draining through the main battery line at the alternator, then disconnecting it should get rid of the ohms reading. It didn't. Took out the sense and tickle wire plug, and the ohms went to infinity just like my other car.

Deduction on my part is that there is a problem with that part of the alternator.

Logically, yes.  However:

Infinity just means that there's no longer a path for the electricity to get from one meter probe to the other.  When there is a path, you get a reading, but the ohm reading you get is simply the resistance value of that entire path, which could pass through any number of circuits, devices, or even shorts.  To my mind that's not really helpful for something like this because while it tells you electricity can travel that path, it doesn't tell you how much, and that's what you need to know.

As an example, an aftermarket radio that has constant hot for memory for the presets will always pull a trickle of juice, even when the car is off.  (I realize you've disconnected yours, but the concept is the same.)  An ohms reading will tell you you have near zero resistance, which would make you think this could be a huge drain.  However, if you actually measure the current, you'll see it's only flowing a milliamp or two (i.e. .001 of an amp), which is not nearly enough to be a significant battery drain.

Assuming you were testing between the the two battery cables, I followed the circuits in the factory diagram (which essentially matches the one you posted).  It looks like the current flows from one meter probe to the starter, then through the main feed (circuit #2) to the horn relay post, then to the alternator plug (still circuit #2), then through the internal wiring to the case ground, which then grounds via the negative battery cable.  The big question is where's the path inside the alternator, and it looks like the juice flows though a few resistors (including a variable resistor) before getting to the ground, but at no time does it encounter a diode that would block it entirely.  I marked the path in red in the diagram.



My guess is that while yes, electricity passes through there, it's a super small amount like the preset memory for the radio.  But the only way to know for sure is to measure amps.

My problem is the one I marked in blue.  The rectifier bridge (series of diodes in the lower right) was fried, and there was nothing to stop the juice from gushing down the main alternator supply wire right through to the ground.  When I measured it I was losing several amps through there.  Unfortunately, the rectifier bridge is very easy to blow -- I have done it twice now.  This is why I'm putting my 250 amp relay back this summer.

Have you measured that one?

bigredlaguna wrote:I doesn't make sense to me that measuring ohms there with my meter would damage anything due to that circuit being subject to battery voltage all the time.

I can't say the alternator is specifically one of those (again, the manufacturer would know best), but I have seen many electronic devices and/or diagrams that specifically warn not to test for resistance/continuity or damage will result.  The meter outputs its own voltage to measure those, and if the wrong amps/volts are applied (or in the wrong direction), delicate circuits can easily go poof.  And it doesn't need to be a lot. Need an example?  Take an ordinary LED (not one for automotive use with the necessary resistor already built in), hook it to a single AA battery, and watch the LED explode.

Ok, I just got schooled.

Measuring amps between the positive post and the disconnected cable is 0.01 amps. Have yet to reconnect most of the dash, but I did put in a new light switch, opened it to the park light setting and got about a 2-3 amp draw.

I did the same test on my 1992 pickup which has first gen computer controls, and it was drawing more but only about 0.02-3 amps. It doesn't lose charge, so I'll just retest when I plug in something else.

That sounds about normal.  Anything 0.05 amp or less is a trickle that you don't need to worry about unless the vehicle won't be started for months.  Then you should have some kind of maintainer on it.

Keep testing it as you add the fuses back, but leave the car and all the electrical items off.  If a sudden significant draw shows up while everything's off, that's your culprit.

I think I figured out where my drain was happening. While rewiring the aftermarket gauges, I tapped into the blade labeled IGN on the fuse box. There was suddenly a 0.7 amp drain on the battery. I didn't note what was plugged into where when I took it apart, I bet the PO thought like I did that the IGN blade was a switched source. I moved the connection to the ACC blade and the drain went back to 0.01-2 amps.

I think maybe the gauge and tach were both hooked up to the IGN blade because the battery would be gone in only a day before.

That's still not making sense to me.   The BATT port is the one that's hot all the time. The IGN port is a switched source -- it's supposed to be hot only when the ignition is on (like you'd expect with that name).  It physically draws its power from the backup/directional lamp fuse, which is fed from the ignition switch via the pink wire with a double white stripe -- do your turn signals work when the car is off?

Set the meter to volts and test between IGN and ground.  You should only get 12V if the ignition switch is on.

This car is strange. Today I plug the volt gauge back to the IGN blade, and it works like it should with no draw on the battery. Gauge shows 0 volts with key off and battery voltage with key on.

I really should have taken a picture before I started because I am sure these things were plugged in to the wrong place. The top blade is labeled ACCY and does have full voltage at all times. I am pretty sure that this spot was used by the PO for something