Alternator is not charging (I used the search before asking)

[Turambar]

I pulled the 250 from my '79 and dropped in a rebuilt 350. Everything seems to be running great... except the alternator. I am at a complete loss... I have searched the forum for at least an hour before posting, so I am hoping to get some help.

A few pics...



The two wires in my hand were in the original alternator plug. The truck ran (barely) when I got it back. I didn't check the voltage gauge the few times I started it to see if the alternator is working. The plug was completely messed up, so I cut them and spliced new wire on to a new plug going into the alternator.

With the truck off or on, the red wire is not getting any power. Neither is the black (I think that is the color) wire.



I couldn't show clearly in a pic where the black wire goes in, but the one on the top left is where the red wire can be traced back.



This is the second of two alternators I had tested at O'Reilly's. Both tested working. In this pic, you can see one of the things I have tried. I went from the 2 plug on the alternator to the battery screw on the back. The white wire is connected to the "black" wire from the earlier pic, but like I said before, it is not getting any voltage with the truck on or off. The voltage from that screw reads the same as the battery (about 12.3 v). My battery is well grounded. The truck starts up just fine, but the alternator is not working. So I am on the 2nd alternator, and I still am not charging. My understanding is that I should be at 13-14 v if the alternator is working. I did remove the positive battery cable while the engine was running, and it did immediately die.

So wiring gurus, please help me. I am at a complete loss.

[Just call me Sean]

If you're getting 12v at the bolt and the red wire is connected to the bolt then you should be getting 12v at the red wire, but you said you aren't. The bolt connects to battery positive. The other red can go to the bolt, the brown keeps the alternator off while starting by being grounded by the ignition switch.

[Turambar]

Well, I think I made some progress. Thank you, Sean, for replying. When I jumped the red wire to the bolt, it was getting the 12v. Spliced into the previous red wire that had run to the alternator in the 250, it was getting no power with the ignition on or off.

I was kinda worried about the white wire, but we saw that it had power in another vehicle. So, I jumped the wire coming from plug two to the battery terminal on the back of the alternator like I showed in the previous picture. I started the truck, and there was still no charge from the alternator. With the truck running, I put 12v to the white wire, and VICTORY! The alternator kicked on.

So, long story short, I need to splice the white wire from the plug to a wire that gets power once the ignition is on (not always on), and I should be good. I think. I hope. *crosses fingers*

[Just call me Sean]

No. From memory I'm going off an idiot light system, not sure if there is a difference when it has a gauge. All bulbs in the cluster get power, and are triggered by ground. The ignition switch grounds the brown wire when starting, making the charge light glow and keeps the alternator off because it takes horsepower to turn it when charging. If it charges on start up the engine is harder to start because of the extra horsepower needed to turn the charging alternator. When the ignition returns to run ground is removed and power passes through the bulb on the brown wire to allow the alternator to charge. So a blown bulb can make the alternator not charge, but they put a resistor or diode, something like that, to pass the power through if the bulb blows. The charge light is the only one that gets it, and some may not notice it and put in a regular socket when changing the bulb.

[Turambar]

Originally Posted by Just call me Sean

The ignition switch grounds the brown wire when starting, making the charge light glow and keeps the alternator off because it takes horsepower to turn it when charging. If it charges on start up the engine is harder to start because of the extra horsepower needed to turn the charging alternator.

So from what you are saying, it wouldn't be good to just find a wire that instantly has power when the engine starts. That would make the alternator try and start charging immediately. It needs to be activated after the engine has run for a few seconds, but not before. Correct?

[Just call me Sean]

I would try to find out why it isn't working correctly first. It's only one wire to troubleshoot.

[Turambar]

I somehow missed this with the forum search, but I think this is very helpful and explanatory: http://67-72chevytrucks.com/vboard/s...d.php?t=604881

Sean, you have been a huge help, and this has me thinking in the right direction.

[willett]

If it's a 12 si you can unplug the connector and run it on 1 wire

[Turambar]

Originally Posted by willett

If it's a 12 si you can unplug the connector and run it on 1 wire

I thought on the three wire you have to have something "excite" the alternator to make it charge.