Battery drain?
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Fleet Mechanic
Joined: Aug 2009
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From: Western Washington
Battery drain?
Ok, I've got a strange battery drain issue. My alternator is draining my battery current, but only when the ignition is on or in the accessories position. I disconnected the big black alternator wire, and poof! Battery drain gone! (According to my volt meter) The thing thats got me is, why does it only discharge with the ignition on?
This is a four month old rebuilt alternator, and the drain has been driving me crazy. The truck's battery will be almost completely dead after a ten mile trip! I can almost jump it with my eyes closed I've done it so many times. I'm glad I finally chased it down.
Looks like time for a 3G swap!
Sam
BTW: Is the ignition coil suppose to get hot? After I ran the truck for a while I noticed it was pretty warm, a lot warmer than the intake manifold below it.
This is a four month old rebuilt alternator, and the drain has been driving me crazy. The truck's battery will be almost completely dead after a ten mile trip! I can almost jump it with my eyes closed I've done it so many times. I'm glad I finally chased it down.
Looks like time for a 3G swap!
Sam
BTW: Is the ignition coil suppose to get hot? After I ran the truck for a while I noticed it was pretty warm, a lot warmer than the intake manifold below it.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
First, just to make sure - can you specify how you're using your voltmeter to check for a battery drain? I assume you're using it as a series current meter between the large alternator wire and the B+ post on the alternator (with the truck off of course)?
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Fleet Mechanic
Joined: Aug 2009
Posts: 1,771
Likes: 9
From: Western Washington
I was checking for general battery drain at the battery, I pulled off the negative cable and then put one lead on the negative cable and the other on the battery. With the ignition switch off I got nothing, but with it on I got a pretty decent drain (can't remember exactly, but it really made the needle move) So I decided to try to just pull wires off of the positive side of the solenoid until I found the source of the drain. As luck would have it, I pulled off the big, black alternator wire first, and when I checked there was no more drain registering on the meter (all the other wires were hooked up) Then I put one tester lead on where the wire starts on the back of the alternator and the other on the positive battery terminal, and it was draining electricity, and it gave me the same reading as the drain when I tested at the battery.
I hope this helps!
Sam
I hope this helps!
Sam
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
Thanks Sam! A couple items to note:
1. It makes sense where you're putting the leads of your voltmeter, but how is the voltmeter set up? To use the meter this way, you need to use it in an ammeter (not voltmeter) configuration. On most handheld meters, that means switching it over to Amps and changing where the leads are connected to the meter itself.
2. Checking for a drain with the key on can't locate a drain, because this applies power to the ignition module. In this case, you're supposed to see current draw from the battery, so nothing here would indicate a battery drain.
Nonetheless, you do have a charging problem so we'll need to solve it, but I'm asking about the test setup first so I know we're on the same page. Having the meter act as a voltmeter vs. an ammeter in this case will make a drastic difference in when you do or don't see a reading (maybe you already do have it set up as an ammeter; I just want to make sure first). Let's close on #1 and then we can move forward.
1. It makes sense where you're putting the leads of your voltmeter, but how is the voltmeter set up? To use the meter this way, you need to use it in an ammeter (not voltmeter) configuration. On most handheld meters, that means switching it over to Amps and changing where the leads are connected to the meter itself.
2. Checking for a drain with the key on can't locate a drain, because this applies power to the ignition module. In this case, you're supposed to see current draw from the battery, so nothing here would indicate a battery drain.
Nonetheless, you do have a charging problem so we'll need to solve it, but I'm asking about the test setup first so I know we're on the same page. Having the meter act as a voltmeter vs. an ammeter in this case will make a drastic difference in when you do or don't see a reading (maybe you already do have it set up as an ammeter; I just want to make sure first). Let's close on #1 and then we can move forward.
Thread Starter
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Fleet Mechanic
Joined: Aug 2009
Posts: 1,771
Likes: 9
From: Western Washington
Thanks, to be honest, I not sure what I have the meter set up as, and its just the cheapest one radioshack had, analog type. There's one place for each lead. The alternator charges, I think its just draining away half the charge it makes. It makes sense about what you said about the ignition module, but why does it stop draining current when I disconnect that big black alternator wire? I would think that it would still register at least some drain.
Sam
Sam
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
To answer your question, we can't determine yet if the alternator really is draining the battery, because what you were seeing on the meter at this point isn't valid. Unfortunately you can't use a voltmeter to locate a battery drain. If your meter can't measure current, then you need to use a test light and repeat your tests.
A voltmeter measures a voltage potential between two points - for example, when you measure the voltage of a disconnected battery, you put one lead on the positive terminal and one lead on the negative terminal. The meter will read 12 volts, even though there's no current flow between the two terminals. That's because a voltmeter measures voltage potential - it's the potential for current to flow, not the amount of current that is actually flowing.
That's why it's much more helpful to use a test light or ammeter for this kind of testing, because you're looking for places where current is actually flowing. Note that none of this proves that your alternator isn't the problem - it very well may be. But we can't say for sure until the test is performed correctly.
A voltmeter measures a voltage potential between two points - for example, when you measure the voltage of a disconnected battery, you put one lead on the positive terminal and one lead on the negative terminal. The meter will read 12 volts, even though there's no current flow between the two terminals. That's because a voltmeter measures voltage potential - it's the potential for current to flow, not the amount of current that is actually flowing.
That's why it's much more helpful to use a test light or ammeter for this kind of testing, because you're looking for places where current is actually flowing. Note that none of this proves that your alternator isn't the problem - it very well may be. But we can't say for sure until the test is performed correctly.
Thread Starter
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Fleet Mechanic
Joined: Aug 2009
Posts: 1,771
Likes: 9
From: Western Washington
Ok, thanks. Some far more serious issues have come up, so I don't think I'll be able to solve this problem anytime soon....The engine is knocking on death's door, I think it might be the wrist pin in the No 2 cylinder. I don't have the money to fix that engine, so as much as I hate to do it, I think I'll have to park the old rig...
Thanks for your help, hopefully I'll be able to fix that problem sometime in the future.
Sam
Thanks for your help, hopefully I'll be able to fix that problem sometime in the future.
Sam
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