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I recently replaced my generator with an alternator in my ’53 half ton pickup (converted to 12V negative ground) and botched the job. I created a short that was draining my battery. I think I’ve found and fixed the problem, but before I mark this case closed, I want to find out if a couple of unusual readings I’m getting indicate another problem.

I’ve disconnected the battery cables and turned everything off. The truck has no clock so there should be no continuity through the electrical system. But, when I put an ohmmeter between the battery cables I get a 1.75 ohm reading. When I move the ohmmeter to the new alternator and take a reading directly across the alternator I see the same 1.75 ohms. So, I have continuity through the electrical system even when everything is turned off and the current is going through the alternator.

Next I reconnected the + battery cable and inserted an ammeter in series between the battery’s minus terminal and the ground strap. The ammeter showed 2mA. Everything was still switched off so the current must have been going through the alternator.

Is this normal or is there something wrong with the alternator? Are these readings acceptable or should it be showing no continuity through the circuit?

I don’t drive the truck every day and the whole reason for switching to an alternator was because I was tired of charging the battery every couple of weeks. How long will a 2mA draw take to kill my battery?

And another thing . . . not that it really matters . . . just out of curiosity . . . but, why is the current so low? If I take 12 volts divided by 1.75 ohms I get 6.86 amps.

The battery will self-discharge long before a 2MA load will drain it. (one month or longer) The resistance value you're getting across the cables with them disconnected from the battery has to be an error or a meter set to an incorrect range. 1.75 ohms would cause a noticeable spark when the cables are connected, and that amount of current flow would drain the battery in a couple of days. It's more likely 1.75 K-ohms (thousands) if you're using an auto-ranging meter.

Try leaving the ground cable disconnected and putting an ice pick type test light between the negative battery post and ground. If the light glows, you've still got a parasitic drain somewhere.
Jerry

Thanks for the advice. I'll give it a try. I'll also double check the resistance reading.

If it turns out that it really is a 1.75 ohm resistance through the alternator, would that indicate that there's something wrong with the alternator?

It's a new alternator, but it's taken me a couple of months to get around to installing it. I want to make sure that it's good in case I have to try to return or exchange it.

Thanks!

Disconnect the main charging wire at the alternator and see if the resistance changes. I would tend to ignore resistance numbers and believe the measured current draw- - - -too many ways for an ohmmeter to get confused when it's used the way you're doing it.
Jerry