And if you're still lost
The VR is constantly looking for battery voltage to fall into a very narrow window, and continuously asks the alternator to to put out power, to make that happen. The asking, is done by varying the field strength in the alternator.The VR does this by grounding the one brush. While the other brush is supposed to be seeing battery voltage. If the VR case is not grounded at all, then the VR cannot work at all. It might as well not be there.But if the case ground is a poor,the VR has to work harder to achieve enough ground time, to punch up the battery voltage; and the poorer the ground ,the harder that VR has to work. On the other side of the VR,as mentioned, it is looking for the battery voltage to fall into it's window. If for whatever reason, the input side of the VR is below that window,again the poor VR goes into overtime trying to get that voltage up, meanwhile it could be cooking the battery. If the input voltage to the VR is above the window, then the VR is supposed to open the ground circuit to the field coil, and that shuts the alternator off.
Now back up a bit, remember the alternator field coil is supposed to be seeing battery voltage at the one brush? Well if it doesn't see close to it, then the field strength may not be sufficient, no matter how hard the VR works, to pump electrons into the battery.
Also, all this sensing and grounding and pumping electrons is going on continuously.
If you have read this far, you have probably had the idea that you can just go out and manually ground that brush on the VR side and drive the alternator into overdrive,right? And you can. This is called full fielding. But the connection between the alternator and the battery, has to be in good shape, or you can set your car on fire! (This is where post #12 comes in) Also, all your measuring equipment should be previously set up, and turned on, because you only want to full field for as long as it takes to read the battery voltage, and to compare that to the charge voltage. If they are the same, well then the alternator is hibernating. But if the voltage difference is 6 or more, as in the alternator is putting out 18 volts, then stop the test. Switch your tester to A/C volts and try again. If the a/c voltage is also way above battery voltage, then the alternator is BAD, get rid of it, one or more of the diodes is ruptured and they are not easy to replace. Ideally there should not be any A/C component in the charge circuit at all.
At idle the alternator can or should be able to put out a lot more volts than what is in a fully charged battery. I don't think 15 to 18VDC is excessive during full-fielding. But removing the full fielding, voltage at the alternator should very nearly equal battery voltage. (see again posts 2 and 12)
So to recap, full fielding should send about 15 to 18VDC into the battery. In turn that 18 vdc or very near to it, should be seen almost everywhere on the car, especially in the ignition circuit and at the input brush on the alternator it self. Removing the full-field wire,the ground jumper, should collapse the voltage back to battery voltage.And there should be NO A/C component to the charge voltage. And the battery voltage should be very near to 13.2 before the test starts.
If you get these, then there is a very good chance that your entire charge circuit is good. If not, then the hunt begins.
Line 44,So that then leaves the VR as suspect. But you have to prove a couple of things first. 1) that the circuit between the VR and the field brush is continuous,and 2) that the case to battery ground is continuous, and 3) that the VR is seeing battery voltage or very near to it. If you got those, then throw out the VR.
Short-cut; you could just start at "line 44", maybe you'll get lucky.
But,be advised that post #12 is always on the table with our A-bodies, and should be inspected periodically, or just do the MADD bypass.
