12 Volt Charging Problem...

My M715 has both 24v and 12volt. The 12 volt operates the lights, heater blower fan and small fuel pump. The 12 volt alternator is a GM unit from the late 70's/early 1980's.

Here is what I have. When I take a voltage reading off the battery without the truck running or any 12 volt accessory on, I get about 12.4 volts. When I start the truck and turn on the fuel pump, I get about 12.1 volts. Then when I put the headlights on/turn signal lights on, I get a reading of about 11.7 volts.

The reason I ask is because after a month or two, it seems the 12 volt battery goes weak/dead. The battery is about 3-4 years old and charges up well if I trickle charge it overnight.

My buddy had an extra GM alternator and I switched that in and I still get the same readings, so that rules out the alternator. It should be noted that there is one heavy red/hot going to the stud on the back of the alternator. There is also a red and white wire going into a small white plastic clip in the top of the alternator. The red wire coming out of the clip is also attached to the hot red/alternator stud on the back. The white wire is just clipped off and is hanging there.

Any ideas? I thought that the alternator would "charge" the battery while running any give me a higher reading on my voltmeter, but it goes down instead. I also do not have a voltage regulator of any sort that I see inside the engine compartment.

Just scratching my head at this point...

Could be the battery had some experiences that gave it a bad attitude. I'd pull a strong battery out of something else and try that for a day or two and see if you get the same readings.

There are one and two wire GM alternators. I can't remember the scoop on them. I think your white wire goes to ignition hot with key on but don't hook it up till I talk to a friend of mine. If I remember right some alt need to be turned on or excited by the signal from the switch. Some are one wire and self exciting. Take a screw driver and dangle it between your fingers with the tip almost touching the rear bearing cap. It the alternator is charging just above idle and higher the screwdriver will magnetically stick to the bearing cap. You should be putting out 14.0+ when everything is working. Always start at the battery but I'm willing to bet it's the way it's wired.
I'll call my friend tomorrow. He knows a lot about the GM alternator.

Yup, sounds like the alt. isn't charging.

Check that the alt has 12 volts coming in the stud on the back,were your red wire from the white plug is going to. If my mind serves me right the alt white wire has to do with the light on the dash. If it was working before I dont think the white wire is the problem left unhook. But if it was not working before you can try touching the white wire to power and see if it sdtarts charging.

The stud on the back of the alternator is definitely hot/positive and getting 12 volts...

It sounds like your alternator is not charging. Since you are loseing voltage it is doing nothing. You should be getting about 14.4 volts when it is properly working.

There are 2 major types of GM alternators. The specialized one wire is self excited, in other words you only connect one wire which is from the positive of the system. The other is the integrally regulated which has three wires to connect. The positive wire goes to the major post, one wire comes from the ignition switch to the exciter and the third is for the dash light; however it is not required for proper operation.

Here is good explanation with picture, go about half way down

http://bbs.zuwharrie.com/content/topic,38679.0.html

Galen