alternator problem/no charge at idle

[boxrocket]

Hey everyone,

 

I have kind of a weird problem I can't figure out. I have a 72 510 with an internal regulator alternator upgrade and an L20B. The alternator is out of a 82 720 rated at 50 amps. I connected the wires as per the IR upgrade diagram. Everything was fine for a while, but then...

 

I noticed my headlights were dim when I would start the car, and they wouldn't get bright until I got to about 3000 rpms or so, and then they would brighten up and stay bright, even at idle. But they would be dim until I got to that magic rpm and then they would be fine. I figured it was a bad internal regulator so I took the alternator in and it tested good. But I got another one anyway just because the bearings were noisy in the other one. So I popped the new one in and same problem.

 

This is what I've done so far. I checked the alternator output at the post at idle and its 12V. Then I would rev up the engine and once it pops up to 14 it will stay there, but it does taper off to about 13 or so volts the lower the rpms get, like at around idle. Checked the upgrade wire I added from the alt. post to the battery and I've got matching voltage at the battery...it's doing the same thing as the alt. I checked the belt and made sure it was spinning properly, I even turned the engine over by hand and visually saw the alt turning okay. If I start the car and let it idle and pull the battery negative off the car dies, but conversly if I rev it to get the 14v and take the cable off it stays running.

 

I'm thinking there may be a wiring problem associated with the field activation jumper but I wanted to get some input if anyone has ever ran into this problem before. Thanks ahead of time.

[datzenmike]

When you turn the ignition to ON (not start) does the RED ignition or charge light come on? This light has to be in working order or there will be no charge.

[boxrocket]

The car currently has no gauge cluster installed, so no, there is no charging light even in the circuit. The plugs that go to the back of the gauges are essentially just hanging there. However by that logic there would be zero charge at any rpms, but I'm getting charging voltage, it only happens after I rev the engine up.

[datzenmike]

Plug in the gauge cluster and it should work. Or even just a small bulb as a load.

[boxrocket]

Yeah I'll try that - I'm loking at wiring diagrams trying to make sense of why that would do it. Quick question, kind of unrelated. I was inspecting my external regulator plug that I left on the main harness (I jumpered the regulator side of the harness) and I noticed a four wire plug with a black, blue, wht/blu, and yellow wire color but I can't find what it's for on my wiring diagram. It comes out of the main harness very near where the six wire regulator plug comes off. Any ideas?

[DanielC]

This is similar to how a GM "one wire" alternator works. Your alternator, and a GM alternator use the charge light to "excite" the field coil in the alternator. Applying 12 volts to the field coil makes the alternator start to charge, and once it starts charging, it can maintain itself. Without the 12 volts to the field coil, it cannot start charging easily.

 

However, the iron pieces in the field coil have a little residual magnetism. Not near as much as when there is 12 volts applied to the field coil, but some. Spin the alternator fast enough, and this residual magnetism is enough to generate 12 volts, and the alternator starts to charge.

 

If the alternator was set up to just start charging without the ignition light, it would always have a constant drain, and the battery would go dead in a few days

[datzenmike]

Well put Dan.

[banzai510(hainz)]

i WOULD TAKE THE ALTERNATOR IN AND HAVE IT CKED OUT.

 

If was working before and now it doesnt then have it cked.

 

rev motor up to 14 volt. then put the lights and wipers on. and if you loose alot of voltage then its loading down and its BAD

[datzenmike]

Banzai, he said his dash isn't hooked up, so the charge light isn't in the circuit.

[banzai510(hainz)]

Yeah I notice people say that you need the instrument panel to complete the circut.

 

I cant verify this as I never had such a proplem

But My 521 charged up just fine when I had the dash out. and I verifyed it.

But my dash harness put a high resisitance or soemthing which did POp the fuse then no charge. I reseat the plug and worked fine after that

 

the 510 right up say you need a diode also but never actually seen one done if you just wire it up using the jumpers also.