Alternator troubleshooting voltage drain

[Dano64]

Hi everyone. Trouble with alternators hoping you can help.

1964 GMC. Newer 250.

Pulled an old alternator with a bad diode. It was draining the battery when the truck wasn’t running. It was connected with only one wire to the battery.

New alternator fits up perfectly and wired properly as best I can tell. The heavy wire goes to the battery and the two wire plug in the alternator has one lead going to positive and the other going to the generator light.

Here’s the problem. It’s still bleeding off power. With the two conductor plug plugged into the alternator, it drains full voltage. With the two conductor plug unplugged, it drains about 3 V and then quickly drops down 2.4 or so on a meter.
.

I’m stuck. I don’t know any other variables to fuss with for the alternator.


Haven’t run it. Charging up the battery.


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[James]

What are you measuring with your volt meter, can't tell from your pictures?

I found a very basic alternator circuit for your alternator. Can you tell me where you are measuring voltage?

[Dano64]

Originally Posted by James

What are you measuring with your volt meter, can't tell from your pictures?

I found a very basic alternator circuit for your alternator. Can you tell me where you are measuring voltage?

Hey James.

I’m not sure I can do it with the diagram. It’s a new alternator with no external regulator and it has the two wire plug on it.


I have a battery charger on the truck right now so I get 15 V between battery positive and the heavy red charging lead off the alternator when the two wire plug is plugged in with the right side red wire connected to the battery post on the alternator and the white wire going to the generator light.

When I put the voltmeter back on in the same exact place with the red and white wires unplugged from the alternator, I get a voltage drop that shows initially about 5 V but quickly dropped down to less than half a volt.




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[Dano64]

Originally Posted by Dano64

Hey James.

I’m not sure I can do it with the diagram. It’s a new alternator with no external regulator and it has the two wire plug on it.


I have a battery charger on the truck right now so I get 15 V between battery positive and the heavy red charging lead off the alternator when the two wire plug is plugged in with the right side red wire connected to the battery post on the alternator and the white wire going to the generator light.

When I put the voltmeter back on in the same exact place with the red and white wires unplugged from the alternator, I get a voltage drop that shows initially about 5 V but quickly dropped down to less than half a volt.




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This is what I tried to do. https://wiringall.com/10si-alternato...g-diagram.html Now I’m wondering if that white lead that goes to the idiot light it’s basically the same wire that goes to switch to power that I see on some diagrams… And, myDash cluster is out right now meaning it’s not getting power. Maybe that’s my problem.

And since one of those leads is the voltage drop since wire, maybe I’m being a bonehead to test this with a battery charger on. Oof


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[AZKen]

Lets take it easy here. We have NO IDEA what you are doing yet. We have NO IDEA why you are doing it.
You may or may not know all this, but I have to start somewhere.
1. You can't measure ANYTHING with a battery charger hooked up.
2. Blank
3. Blank
4. You show a meter reading of 13.55V and you say motor not running, so you are reading battery voltage or charger voltage. Normally a charged battery is 12.6V, So that's a little odd but batteries can be charged a little higher.
5. I see and read that you do have the alternator wired correctly as long as that white wire is in fact wired to the gen bulb. Then out the bulb to ignition switch "ON" terminal.

Hook it all back up and leave it hooked up.
6. Charge the battery, take off the charger. Test voltage across battery with motor not running and key off.
7. Start motor. Test voltage across battery again with motor running.
8. Report the two readings.
9. Stop disconnecting and testing other than no. 6 and no. 7.
10. If you have a battery drain, you don't know where it's at.
11. A bad battery will go dead on it's own. Your tests did not prove anything.
12. Do not look at any wiring diagrams or internet !!!!!!!!!!!! You have it wired correctly.
13. When you measure voltage on a truck/car motor, the black meter lead is on a good clean bare metal ground. (not on a wire). The red meter lead is on the wire or terminal you want to test for voltage.


Questions:
1. Does your charged battery (12.6V +) go dead overnight or what?
2. What is the complaint that caused you to start testing after the new alternator was installed????

IMPORTANT: When testing in No. 6 and No. 7. Place leads directly on battery posts and take reading. Then place leads on battery cable clamps and take reading. Both ways for 6 and 7. Report all voltage readings carefully to us. Meter on DC.

I will wait for tests results of 6 and 7 and the answers to the questions. I will respond if I get all that.

[lizziemeister'sV6]

Shouldn't this truck have an external regulator originally? Need more information if it's got the correct alternator. Then more attention on the "wiring". I bet my last dollar that the problem lies right before ones' eyes.

[AZKen]

Yes probably had a Delcotron with external voltage regulator. Always better to get rid of that, all it's nonsensical wiring and the VR..... and install a 10si as he did.
I believe he will be installing new harnesses.
The alternator wiring is very simple. He has it correct. I believe the problem is not in the alt wiring or the alternator. I'm guessing a bad battery, bad connection or bad ground, but need the answers and test to begin troubleshooting in an orderly fashion. All electrical begins with making sure the battery is OK and charged. Failing cells can be hard to catch.

All we really know is his battery is or was running down. I am going one step at a time and have my suspicions. He has now come to the right place. We will reset and delete all the confusion and testing he is doing.................someday.

[AZKen]

Edit:

Originally Posted by azken

yes probably had a delcotron with external voltage regulator. Always better to get rid of that, all it's nonsensical wiring and the vr..... And install a 10si as he did.
i believe he will be installing new harnesses.
the alternator wiring is very simple. He has it correct. I believe the problem is not in the alt wiring or the alternator. I'm guessing a bad battery, bad connection or bad ground, but need the answers and test to begin troubleshooting in an orderly fashion. All electrical begins with making sure the battery is ok and charged. Failing cells can be hard to catch.

All we really know is his battery is or was running down. I am going one step at a time and have my suspicions. He has now come to the right place. We will reset and delete all the confusion and testing he is doing.................someday.

[biscaynebirdie]

Sounds like Dano just needs to perform a good old test as follows, then share his results:

1. Record the battery voltage with the key turned off.
2. Record the battery voltage with the key turned on (engine not running).
3. Record the battery voltage with the engine at idle.
4. Let engine reach operating temperature. Record the battery voltage with the engine at around 1,500 RPM. The voltage should obviously increase a bit, to somewhere between 13.4-14.9 volts... 15.3 volts maximum. If it exceeds 15.3 volts, let off the throttle immediately.

This information will help all of us better determine what your problem is. In other words, it will help us help you!

AZKen, am I missing anything here?

[AZKen]

Originally Posted by AZKen

Lets take it easy here. We have NO IDEA what you are doing yet. We have NO IDEA why you are doing it.
You may or may not know all this, but I have to start somewhere.
1. You can't measure ANYTHING with a battery charger hooked up.
2. Blank
3. Blank
4. You show a meter reading of 13.55V and you say motor not running, so you are reading battery voltage or charger voltage. Normally a charged battery is 12.6V, So that's a little odd but batteries can be charged a little higher.
5. I see and read that you do have the alternator wired correctly as long as that white wire is in fact wired to the gen bulb. Then out the bulb to ignition switch "ON" terminal.

Hook it all back up and leave it hooked up.
6. Charge the battery, take off the charger. Test voltage across battery with motor not running and key off.
7. Start motor. Test voltage across battery again with motor running.
8. Report the two readings.
9. Stop disconnecting and testing other than no. 6 and no. 7.
10. If you have a battery drain, you don't know where it's at.
11. A bad battery will go dead on it's own. Your tests did not prove anything.
12. Do not look at any wiring diagrams or internet !!!!!!!!!!!! You have it wired correctly.
13. When you measure voltage on a truck/car motor, the black meter lead is on a good clean bare metal ground. (not on a wire). The red meter lead is on the wire or terminal you want to test for voltage.


Questions:
1. Does your charged battery (12.6V +) go dead overnight or what?
2. What is the complaint that caused you to start testing after the new alternator was installed????

IMPORTANT: When testing in No. 6 and No. 7. Place leads directly on battery posts and take reading. Then place leads on battery cable clamps and take reading. Both ways for 6 and 7. Report all voltage readings carefully to us. Meter on DC.

I will wait for tests results of 6 and 7 and the answers to the questions. I will respond if I get all that.

It's been a week so he's too busy or figured it out.