Parasitic Draw
 

Something is draining the battery in my '67 GTO. I'm reasonably familiar with electrical circuits but I'm definitely not an electrician.

With everything shut off, the neg. cable off the battery and a meter between the cable and battery post I show full battery voltage. I've recently done some electrical work so I looked there first but unhooking that stuff made no difference. I next disconnected the hot wire from the back of the alternator and the draw dropped from 12.7 volts down to about 5.5. As I held the probes in place that number kept dropping at the rate of a tenth of a volt about every 10 or 15 seconds until I got tired of watching it at around 3.8 volts.

And that's as far as I got today. I recently had surgery on my right hand so I'm doing all this mostly one handed which slows things down considerably.

Any thoughts... alternator? Voltage regulator (external)? Next steps? I would appreciate any advice on what to do next. Thanks in advance.

A clamp-on DC ammeter would be more helpful in tracking down a parasitic current leak. Since it appears you don't have one, use the ohmmeter setting.
Assuming the alternation is the original 10DN type, disconnect all the cables connected to the BAT terminal and disconnect the F-R connector. With the ohmeter positive lead on the BAT connector and the Negative lead to alternator ground, see if the reading is > 100KΩ. (Note that there is a 0.5uF capacitor in the alternator and it may take some time to stabilize.) If lower, one of the alternator diodes or capacitor is leaky.

With the BAT terminal still cleared off cables and the F-R connector removed, set the ohmmeter positive probe on the R-terminal and the negative probe to alternator ground. This checks the diode powering the regulator field solenoid. See if the reading is > 100KΩ.

Yeah, I probably should have sprung for the more expensive meter but at the time I didn't think I would have much use for it.

With BAT terminal and F-R connector both disconnected meter reads 193.1 KΩ between BAT and ground. Between R and ground, with everything still disconnected, it reads 142.3 KΩ. So, what is that telling me. Sorry if I seem a little dense here; like I said, not an electrician.

I certainly appreciate the help, thank you.

Just covering bases here. Does the car have an EFI system on it?

The parasitic draw from the alternator is less than a tenth of a milli-amp based on your ohmmeter measurement. I'd say the alternator diodes are fine and the problem is elsewhere.

Nope, quadrajet.

Just a note of caution if using an analog meter-
When measuring resistance they typically apply "backward" voltage- positive to the black test lead and negative to the red test lead. I suspect that all digital meters aren't "backward", but don't know for sure.

Back to your problem- if you leave the battery ground cable connected and disconnect the positive cable, what resistance do you measure from the positive cable to ground? Knowing this resistance value might give a clue about what items might be responsible. Also, if this value varies with movement- of wire harnesses, bouncing of car, etc.- that would be a clue.

I use a very simple method of chasing parasitic draw. At night when dark. Disconnect the device you want to test. Remove ONE bat lead at the bat & touch the lead on/off onto the bat post. If you get no spark, you have found the problem. If you get a spark, something is still drawing current.

How did you discover this? How long does it take to drain your battery after charging it? Basically, how much current is flowing? A little or a lot?

Regardless, I don't think there's an easy way to guess at what is drawing current.

If it were me, I would first try to isolate the circuit that is drawing power.

I would do this by systematically pulling fuses while monitoring current. It may be possible to isolate your problem with voltage readings but that's complicated and access to "test points" is a bear. Current measurement will be more direct and easier IMO.

The only risk is if the current draw is high enough to damage your meter, but your description makes this seem unlikely.

What is the maximum current setting on your meter?

If you cannot ballpark the current level and you are worried about damaging your meter, find a fuse that is the same amp rating as your meter's maximum current reading. Disconnect the positive cable from the battery.
Set one end of the fuse on the positive battery terminal and touch the positive cable terminal to the other side of the fuse. If it doesn't blow, you have gained confidence you won't damage your meter.

Then put your meter leads between the positive terminal of the cable and the positive terminal of the battery. You are completing the circuit with your meter, which is the way to measure current.

At this point, you should read a current if your load is always present. If not, the challenge becomes bigger because you are dealing with an intermittent or "situational" draw.

Assuming you can read a current, start pulling fuses until you find the circuit that is drawing power. You can replace them immediately if you don't see the current go away and move on to the next.

If you go through all the fused circuits and don't see the current drop, you have still made progress, as you've eliminated a LOT and can start following the few wires and connectors between the battery terminal and your fuse block, starter, etc.

Mike

For reference I am using a Fluke 115 digital multimeter.

With the alternator completely disconnected as per my last test, I show no continuity between Pos. cable and ground. With alt. reconnected I get a reading of 55 K ohm. If I then unplug the R-F connector only, it jumps to 222 K ohm. Again, I'm not sure what that's telling me.

I'm not entirely sure how long it takes to discharge since I'm not driving it that often. The first time it happened the battery wasn't completely dead but wasn't strong enough to crank the starter enough to start the car. My best guess is that was after a week or so. The second and last time the battery was completely discharged after 2 or three weeks.

Max current setting for the meter is 10 amps but since the fuse is already blown for some reason it's effectively 0 right now. No replacements to be found in this small town in very rural Kansas so I had to order some. I may know where I can borrow a clamp on meter but I'm not likely to be doing much fuse pulling etc., until I get the use of both hands again. No hurry, it's not a daily driver and cruising season's pretty much over.

If and when I find the problem, and I will eventually, I'll let everyone know what it was. In the mean time, thank you all for your help and advice.

How old is the battery?

Battery is brand new. Got it in June to replace the 6 year old one in the car that would no longer take a charge.

If it took 2 or 3 weeks to drain, the current flow is well under 1 amp.

Assuming your battery has an amp-hour capacity of 60 (just a guess) the current draw from whatever is causing it to run down is estimated around 150-200 mA (60 A-hr/336 hr = 0.18 A).

This should be easy to measure with your meter and there should be no risk of blowing the meter's fuse if you get it replaced.

That is not a lot of current, but enough that it could be almost anything. It could be a short in a lighting circuit, a leak through corrosion products at your fuse block, or even a powered load (like a light bulb) caused by a wiring error or failed switch.

Mike

I am hoping the OP is not offended if I ask a question regarding the same topic.

Measuring the disconnected negative cable to negative battery terminal on a 10A multimeter setting reads 0.38A pn one of my cars. That gets me a week on a fully charged 60A-hr battery.

Looks like I have a draw somewhere too so the next plan would be to pull the fuses and see if the problen goes away. Are there other checks that might make my life easier?

You didn't state what year car, but removing wires from the alternator (generator?) and voltage regulator (if you have one) would eliminate or confirm these devices as problems.

The car is a 1980 Firebird a series 10SI. I disconnected the alternator and my amp draw went away. Looks like a diode or possibly a regulator issue. My voltmeter gauge needle would bounce at lower RPM with my high beamsl so I knew there was a problem.

You are pretty good Bill, thanks!

What do they say about a blind squirrel?

I would be the blind one on this Bill. The alternator must have been on its way out as my volt gauge is steady now even under heavy load.

The next culprit on the list was the car stereo but I didn't have to go that far.