Battery Draining, Alternator Bad?

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Forum Name: General Discussion
Forum Discription: General Mobile Electronics Questions and Answers
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=57911
Printed Date: May 15, 2025 at 5:26 AM

Topic: Battery Draining, Alternator Bad?

Posted By: reynolds-wrap
Subject: Battery Draining, Alternator Bad?
Date Posted: June 17, 2005 at 4:00 PM

1999 Ford Crown Victoria P71, all aftermarket equipment presently disconnected.

A couple weeks ago, my car's battery began discharging over a period of 18-36 hours to where it would no longer start (the starter would just click-click-click).

I did the following:

Battery voltage, fully charged (car off): 12.4V

System voltage, battery charged (car on but cold): 14.5V

System voltage, battery charged (car on, regulator warmed up): 14.1V

Car off, battery disconnected, ohm meter connected between + and - terminals: 2000+ ohms

Car off, + clamp connected, ammeter connected in series with negetive clamp and post: 40mA (50mA or less is acceptable)

Battery load tested: Good but weak, replaced.

So I've yet to find a parasitic drain from any of the electronics. Even with a new battery (with higher CCA and RC), the drain is still present, though slowed maybe 10-20%.

So I'm thinking...leaking diode on the alternator?

But would this cause the draw amperage with the car off to read more than 40mA?

(btw, I've used a digital multimeter and an analog multimeter to verify all readings)

I'm quite the newbie when it comes to charging systems (I'm quite the newbie when it comes to anything car related, really, but my CV is a lot easier to work on than any car I've previously owned, so I'm going out of my way to learn how to do as much as I can by myself)... posted_image

Replies:

Posted By: Mad Scientists
Date Posted: June 19, 2005 at 5:14 PM

While measuring current draw (key off, engine off, doors closed, etc..) disconnect the alternator.. if the current draw drops, you might have found a problem..

Posted By: maglin
Date Posted: July 07, 2005 at 10:57 PM

best way to test a bad alternator is to start the car, then disconnect the BATTERY. (agh... can't remember if you disconnect teh ground or the power wire to keep it from sparking now, sorry) if the car stays running, the alternator has the balls to keep it running. now if you have all sorts of aftermarket crap going, you may have trouble even with a good alternator.

not sure why the guy in the last post said to disconnect the alternator... it'd just drain teh battery, and quick at that.

~~Vinn

Posted By: Mad Scientists
Date Posted: July 08, 2005 at 5:05 AM

maglin wrote:

best way to test a bad alternator is to start the car, then disconnect the BATTERY. (agh... can't remember if you disconnect teh ground or the power wire to keep it from sparking now, sorry) if the car stays running, the alternator has the balls to keep it running. now if you have all sorts of aftermarket crap going, you may have trouble even with a good alternator.

not sure why the guy in the last post said to disconnect the alternator... it'd just drain teh battery, and quick at that.

~~Vinn

Disconnecting the battery while the engine is running isn't the best way to check the alternator.. it's the best way to smoke the vehicle's electronics. You could do this decades ago when cars had no electronics and generators, but with cars build in the past 20 years there's too much electronics to risk by disconnecting the battery while the engine is running..

Use a friggin' voltmeter..

The mention about disconnecting the alternator refers to doing a current draw test with the engine off.. if there's a bad diode in the alternator it can cause a current draw when the engine is off.. measuring engine off current draw, if the current draw drops when the alternator is disconnected there's a possibility that there's a problem with the alternator.

Jim