Please help !!! alternator not charging

[buckwheat1]

I can not get the forum search to work so I am going to ask.

I am working on a 69 p-10 with 3 wire internally regulated alternator and can not figure out why the alternator is not charging.

Here is what I have done if someone could please tell me what I am doing wrong.

Original wiring was cut and spliced so many time I decided to run a new wire to the field terminal on the alternator because I was not getting any voltage to the original wire. I do not think the alternator has ever worked since I have owned the truck. We always kept it on a trickle charger but have been stranded a few times.

Ran a new wire from the ignition switch run position to a LED on the dash then then from other side of LED down to the alternator terminal. As far as I understand the led should light when the ignition is on but the truck is not running then go off when the alternator is charging. Or the terminal on alternator should be ground until the alternator is excited the should go to positive? Is this true? I am not getting ground continuity to ether of the terminals on the alternator?

[buckwheat1]

Found out it is a 12-SI alternator...anyone else had this problem?

[ClemsonChevy]

Did your truck originally have an idiot light for the alternator? If so, and you replaced that circuit with your new wire and LED, then that is your problem. You have to use a regular filament type bulb to add some resistance to the line that goes to the alt. If not, it will not charge.

See this:
http://67-72chevytrucks.com/vboard/s...d.php?t=473839
http://67-72chevytrucks.com/vboard/s...d.php?t=465260

and very bottom of here:
http://www.chevelles.com/techref/tecref14.html

[VetteVet]

Originally Posted by buckwheat1

Found out it is a 12-SI alternator...anyone else had this problem?

Only about half the members on this forum.

I take it that you have the charging light dash instead of the battery gauge dash with the temp and oil pressure gauges.

If so then the wire from the key switch goes to the fuse panel and provides power for the dash cluster, including the charging light. From the charging light on the cluster plug, there will be a brown wire running back to the inside of the firewall terminal plug. This wire supplies power to the No. 1 terminal on the alternator which is the exciter wire to "turn on" the alternator.

On the original trucks this wire runs to the external voltage regulator to the F terminal. The regulator then jumps this wire and the white wire to send power to the alternator. This is how the wiring looks on the original trucks.




This is how it will look when the regulator is bypassed.



The wire with the resistor is used fot the trucks with the battery charging gauge (ammeter). It might be included as a back-up in case the charging light bulb blows.

Now you mention "LED" bulb as the charging light. If you do have an LED bulb in that position you need to know that the original incandescent bulb had a resistance of 10 ohms, and this is needed to turn on the alternator regulator to allow it to charge. You may know that the LED bulb has very low resistance and is not sufficient to turn on the alternator. This may be your whole problem or you may not be getting any voltage to this wire at the alternator. Turn on the ignition and check for 12 volts on this wire, if you don't get 12 volts then the alternator will not charge.

When doing the original conversion most guys just jumpered around the external voltage regulator, and changed the plug for the alternator so you may have to sort all the wiring out.

[buckwheat1]

Swapped out the LED for an incandescent bulb and it works perfect now.....thanks for the help.

I am still not sure how it works but I am glad it is.....must be F@#kin Magic!!!

[airdale94]

VetteVet knows his sh*t! Even if he's ret. AF.