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Old 06-19-2018, 10:49 PM   #1
dave`12
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Alternator draining battery?

I just completed a change over (long thread here) from external to internal regulated alternator. Well, I got sick right after, and didn't start the truck for several weeks. Dead battery when I tried.

I checked with a test light, and I have a battery drain that goes away (light goes out) when I unplug the 2 pin from the alternator. It's a 12si alternator.

Alternator definitely charging, and the ammeter in the dash is working properly. But I must have screwed something up, or got unlucky and got an alternator with a bad diode? Past history, I'm leaning towards "I screwed something up."

Does anyone know what likely is going on?

Thanx.
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Old 06-20-2018, 01:57 PM   #2
VetteVet
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Re: Alternator draining battery?

Check the 1 and 2 pin wires to be sure they aren't reversed. IF the key is off the 1 pin wire will not have voltage, and when it is turned on,the wire should have 12 volts. The 2 pin wire should be hot all the time.l If they are correct then the alternator diodes could be shorted to ground and will drain the battery over time.

Sometimes the engine will continue to run with the key off when the two wires are switched.

I'll check one of my SI alternators and see what readings I get on the no. 2 connection between it and ground and then reverse the leads on the meter and see if I get a reading to see if it's flowing both ways.
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Old 06-21-2018, 11:59 AM   #3
dave`12
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Re: Alternator draining battery?

Test light tells me I have a drain.

First I did a diode test from a utube video. battery disconnected.
.810 with + on case and - on btty post
ol with - on the case and + on the bty post

So, I'm thinking the diodes are good?

The other tests:

battery 12.45v

Key off

Btty post of alt 12.45v
post 1 12.45
post 2 .7mv


Key on

Btty ost of alt 12.45
post 1 12.40
post 2 11.98

Just for fun, tried it with the engine on:

btty 14.4
btty post of alt 14.9
post 1 14.9
post 2 14.5

I also removed the - lead of the btty with the engine running and the engine did not die.

So does this mean that I simply got these 2 wires reversed (I hope)?

Oh, the engine has been turned on some 15 times since the conversion and has always shut right off with the key.

thanx again
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Old 06-21-2018, 12:29 PM   #4
VetteVet
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Re: Alternator draining battery?

I'm working a similar post where the engine is running after key off but no battery drain. I think you might have the same type problem but a different diode in the alternator is shorted through and allowing battery current to go to ground. The other on is a diode that allows current to go back to the ignition switch.

The test you did on the charging post to ground on the alternator body is correct but you may have a bad diode on the small wire terminals 1 or 2.

My alternators tested 40 to 50 ohms both ways on the no 1 terminal and showed varying resistance on the no. 2 around 18 ohms one way and higher the other. This is with all wires to the alternator disconnected. If you get zero either way on the no. 2 you've probably got a bad diode. It can still charge at 14.5 volts but will drain the battery over time. Did you test for drain between the negative cable and the battery post? You should not get over 3 milliamps. That's .003. Make sure you have all the lights off and the key and no other draws on the system.
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Old 06-21-2018, 01:11 PM   #5
dave`12
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Re: Alternator draining battery?

I was hoping I had the 2 wires wrong to the posts and that I could could swap them and put this behind me. Guess not? Anyway:

I don't have time to unhook alternator, I have to leave for work now.

Here is a quick test of how I THINK I should test amps and the draw. If correct, I have a big draw. Please look at the picture as I'm not sure how to test for amps.

The draw goes to 0 when I unhook the 1 and 2 post connector to the alternator, so the problem must be the alternator or the wires to it.

I will try the ohm test tomorrow morning.

thanx
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Old 06-21-2018, 06:54 PM   #6
dmjlambert
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Re: Alternator draining battery?

Quote:
Originally Posted by dave`12 View Post
...The draw goes to 0 when I unhook the 1 and 2 post connector to the alternator, ...
Do you mean when you unplug the 2-pin plastic connector, or when you unscrew the terminal lug nut on the back of the alternator, when the draw goes to 0?

Are you testing for draw with the key off?

Does terminal 2 of the 2-pin plastic connector lead to the existing junction (solder by factory) by the headlight?

Does terminal 1 of the 2-pin plastic connector lead to the brown wire that goes to the cab firewall connector?

If you think things are wired OK, I believe most car parts stores will test the alternator for you.
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Old 06-22-2018, 02:59 PM   #7
dave`12
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Re: Alternator draining battery?

Quote:
Originally Posted by dmjlambert View Post
Do you mean when you unplug the 2-pin plastic connector, or when you unscrew the terminal lug nut on the back of the alternator, when the draw goes to 0?


Yes, the 2 pin, I have not taken off the btty post (terminal lug?)

Are you testing for draw with the key off?

on and off, numbers for both are above.

Does terminal 2 of the 2-pin plastic connector lead to the existing junction (solder by factory) by the headlight?

I was pretty careful putting it in, and it's all taped up now. If worst comes to worst, I will untape and check.

Does terminal 1 of the 2-pin plastic connector lead to the brown wire that goes to the cab firewall connector?

If you think things are wired OK, I believe most car parts stores will test the alternator for you.
Before taking it off, I was thinking about swapping the 2 pin wires and doing the btty test drain. That won't hurt anything will it?

thanx
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Old 06-22-2018, 08:00 PM   #8
dmjlambert
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Re: Alternator draining battery?

I'm not clear about what is going on. The picture you show of the multimeter shows it set to the 200 mA range and displaying 2.9, so that is a trivial 2.9 mA. That does not seem consistent with conditions that would cause a test light glowing when hooked up in place of the multimeter. To hook up the multimeter or test light, are you disconnecting a terminal of the battery and connecting the leads to the loose battery cable and the battery post? With key off? Then while monitoring with the light or multimeter that is when you disconnect the plastic terminal from the alternator and notice the change? If it were me I would just unwrap and double check the wiring visually, because it would be rolling around in the back of my mind driving me crazy to not see it. Then I would take the alternator up to the parts store and have them check it.
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Old 06-28-2018, 07:57 PM   #9
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Re: Alternator draining battery?

Ok, finally got to this, been out of town and working a lot. Anyway, I reversed the wires going into the alternator 2 pin.

Started the engine. Seems to charge normally. Test light no longer lights up when I connect it from the neg battery cable (removed) and the neg btty post.

This hopefully means I no longer have a drain!

It's 12.84 now. Letting it sit overnight with the battery hooked up and gonna check the voltage in the morning. Fingers crossed that I was just stupid and somehow managed to get the wires backwards despite double and triple checking.

thanx
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Old 06-29-2018, 05:12 PM   #10
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Re: Alternator draining battery?

24 hours and it's at 12.5 volts. Hoping that switching the wires fixed her.

Last edited by dave`12; 06-29-2018 at 06:06 PM.
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