Using Ammeter to test alternator draw?
Thread Starter
Intermediate
Joined: Jun 2023
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From: Central Iowa
Using Ammeter to test alternator draw?
I have a draw on my battery, running it down after several days. I have followed several of the older threads on using digital ammeter to test draw; basically following the Bill Curlee threads, I tested and found a 1.3 amp draw with everything at rest and powered down. That is a bunch! I want to do the alternator diode draw test next; his instructions say:
" "Alternator. The diodes can and do go bad. If hey go bad, the field windings will always be powered and draw current. If you suspect that the alternator is drawing current, remove the field terminal wire from the back of the alternator and insert the AMP Meter in SERIES with the circuit (between the red wire and alternator field terminal). The current draw should be ZERO. If you have current draw, one or more diodes are bad in the alternator. The last alternator that I checked this way was drawing 5 amps.""
My question is: Is this test hookup done the same as the initial battery draw test, meaning with the positive battery cable connected, and the negative battery cable disconnected?
Thanks much folks; I enjoy the forum, although I do not participate much. You have helped me on several items.
" "Alternator. The diodes can and do go bad. If hey go bad, the field windings will always be powered and draw current. If you suspect that the alternator is drawing current, remove the field terminal wire from the back of the alternator and insert the AMP Meter in SERIES with the circuit (between the red wire and alternator field terminal). The current draw should be ZERO. If you have current draw, one or more diodes are bad in the alternator. The last alternator that I checked this way was drawing 5 amps.""
My question is: Is this test hookup done the same as the initial battery draw test, meaning with the positive battery cable connected, and the negative battery cable disconnected?
Thanks much folks; I enjoy the forum, although I do not participate much. You have helped me on several items.
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From: Cape Coral, Florida
Do an AC ripple test if you suspect a bad diode...a faulty diode will let AC into the cars electrical system and cause havoc...anything above 0.2 to 0.5 AC volts is BAD...you test AT the alternator and not at the battery.
Last edited by C5 Diag; May 23, 2026 at 07:02 PM.
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From: EAST TENNESSEE
Do an AC ripple test if you suspect a bad diode...a faulty diode will let AC into the cars electrical system and cause havoc...anything above 0.5 to 0.1 AC volts is BAD...you test AT the alternator and not at the battery.
https://youtu.be/OeX7xSC5jdU?si=A15Vt3wu0_c5EfDW
https://youtu.be/OeX7xSC5jdU?si=A15Vt3wu0_c5EfDW
Last edited by C5 Diag; May 23, 2026 at 07:43 PM.
Thread Starter
Intermediate
Joined: Jun 2023
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From: Central Iowa
Thanks, I may give that a try if I continue having drawdown issues. I wanted to start at the alternator, because my draw was so high at 1.3 amps. Since my initial post realized I was testing it wrong. The 1.3 amp draw was initial reading upon ammeter hookup, I did not realize you have to wait maybe 2 minutes for systems to go back to sleep. Then my draw was clear down to .086 amps; afew more minutes and it dropped to .017, or 17 milliamps, which is within spec.
Perhaps I really have a bad battery (Optima redtop), even though it tested OK at Optima dealer. I have had it happen before on other vehicles, batteries test OK; I still suspect continued problems are battery. Replace it with new and no more problems.
Thanks for info on alternator test.
Perhaps I really have a bad battery (Optima redtop), even though it tested OK at Optima dealer. I have had it happen before on other vehicles, batteries test OK; I still suspect continued problems are battery. Replace it with new and no more problems.
Thanks for info on alternator test.
Moderator, Tech Contributor
Joined: Sep 2013
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From: Cape Coral, Florida
Thanks, I may give that a try if I continue having drawdown issues. I wanted to start at the alternator, because my draw was so high at 1.3 amps. Since my initial post realized I was testing it wrong. The 1.3 amp draw was initial reading upon ammeter hookup, I did not realize you have to wait maybe 2 minutes for systems to go back to sleep. Then my draw was clear down to .086 amps; afew more minutes and it dropped to .017, or 17 milliamps, which is within spec.
Perhaps I really have a bad battery (Optima redtop), even though it tested OK at Optima dealer. I have had it happen before on other vehicles, batteries test OK; I still suspect continued problems are battery. Replace it with new and no more problems.
Thanks for info on alternator test.
Perhaps I really have a bad battery (Optima redtop), even though it tested OK at Optima dealer. I have had it happen before on other vehicles, batteries test OK; I still suspect continued problems are battery. Replace it with new and no more problems.
Thanks for info on alternator test.
This is the CORRECT way to do a draw test...we don't pull fuses !!
Le Mans Master
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what amp meter are you using?? its got to be pretty precise reading .040 to .025mA??? Are you talking about a clamp or meter inline.
Last edited by helga203; May 24, 2026 at 06:31 PM.
Thread Starter
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From: Central Iowa
My question was, do you leave the battery cable connections on, or leave the negative cable off, like all other amp draw tests are done: Most posts on the subject say to connect it as you suggest, but in the same posts they are doing all the draw tests on the negative line, between cable and battery post. That is really my question.
Probably a moot point now, as I have my resting draw down to .017 amps, (sometimes .022), which is the proper spec.
Probably a moot point now, as I have my resting draw down to .017 amps, (sometimes .022), which is the proper spec.
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From: Portland OR
You can measure current either on the ground side or the positive side. Just a little safer on the ground side.
most DMM’s have a 10A limit which is fused. They quite easily measure in the milliamp range.
What I mentioned above was to just put the current meter inline with the batt leads on the Alt. That would have just tested to see if the Alt was drawing current.
I went through the whole process on my C5, watching it go to sleep. Mine ended up having a stuck position switch on the drivers seat. Easy fix.
Glad you got yours measured to be 17mA. Right in the range.
most DMM’s have a 10A limit which is fused. They quite easily measure in the milliamp range.
What I mentioned above was to just put the current meter inline with the batt leads on the Alt. That would have just tested to see if the Alt was drawing current.
I went through the whole process on my C5, watching it go to sleep. Mine ended up having a stuck position switch on the drivers seat. Easy fix.
Glad you got yours measured to be 17mA. Right in the range.
Thread Starter
Intermediate
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From: Central Iowa
Q. So to check if any alternator draw, you would have both + and - battery posts & cables hooked up? then just remove the red charging wire from alternator and test the amperage draw between red wire and alternator lug?
This is for future reference, as my total draw is fine, so I right now have no need to test alternator anyway.
Thanks much
This is for future reference, as my total draw is fine, so I right now have no need to test alternator anyway.
Thanks much
Advanced
Joined: Jul 2023
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From: SW CO
Thanks, I may give that a try if I continue having drawdown issues. I wanted to start at the alternator, because my draw was so high at 1.3 amps. Since my initial post realized I was testing it wrong. The 1.3 amp draw was initial reading upon ammeter hookup, I did not realize you have to wait maybe 2 minutes for systems to go back to sleep. Then my draw was clear down to .086 amps; afew more minutes and it dropped to .017, or 17 milliamps, which is within spec.
Perhaps I really have a bad battery (Optima redtop), even though it tested OK at Optima dealer. I have had it happen before on other vehicles, batteries test OK; I still suspect continued problems are battery. Replace it with new and no more problems.
Thanks for info on alternator test.
Perhaps I really have a bad battery (Optima redtop), even though it tested OK at Optima dealer. I have had it happen before on other vehicles, batteries test OK; I still suspect continued problems are battery. Replace it with new and no more problems.
Thanks for info on alternator test.
What are you using to measure the 0.017 amp draw? Clamp-on meter or series-wired?
While 0.017 amp is a very good number, sometimes modules "wake up" periodically and drain the battery. I use a multimeter with max-min-avg capability when doing parasitic-drain testing, and I leave the meter connected overnite. The average draw over time is important.
Thread Starter
Intermediate
Joined: Jun 2023
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From: Central Iowa
What are you using to measure the 0.017 amp draw? Clamp-on meter or series-wired?
While 0.017 amp is a very good number, sometimes modules "wake up" periodically and drain the battery. I use a multimeter with max-min-avg capability when doing parasitic-drain testing, and I leave the meter connected overnite. The average draw over time is important.
While 0.017 amp is a very good number, sometimes modules "wake up" periodically and drain the battery. I use a multimeter with max-min-avg capability when doing parasitic-drain testing, and I leave the meter connected overnite. The average draw over time is important.
I did try putting the C5 key in a remote place, as well as leaving it in ignition and doors unlocked (which I do often here on the farm). After a while and sleep took over, it did not seem to matter where the key was or locked vs. unlocked doors. I thought it might.
Moderator, Tech Contributor
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From: Cape Coral, Florida
An amp clamp's readings will drift over time so it's better to use your ammeter...if the battery lasts for an hour and your drain is still within limits I wouldn't worry about it...modules on these cars rarely wake up after going "to sleep".
Last edited by C5 Diag; May 25, 2026 at 12:52 PM.
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From: Central Iowa
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From: SW CO
My series meter has auto off feature after a few minutes, but I see now I can disable it, but not sure if the digfital meter battery will last the night, but maybe several hours anyway. I might try to find a clamp-on meter; it would be handier for sure, although the meter's own battery usage would still be an issue I suppose.
I did try putting the C5 key in a remote place, as well as leaving it in ignition and doors unlocked (which I do often here on the farm). After a while and sleep took over, it did not seem to matter where the key was or locked vs. unlocked doors. I thought it might.
I did try putting the C5 key in a remote place, as well as leaving it in ignition and doors unlocked (which I do often here on the farm). After a while and sleep took over, it did not seem to matter where the key was or locked vs. unlocked doors. I thought it might.
A quality clamp-on should, however pick up periodic current fluctuations, and these would be captured by the max-min-avg function of the multimeter.