It suggests the field circuit is ok. What I think is happening is the circuit is switching on and off about 400 times per second. The percentage of on-time varies to vary average current flow through the field circuit. That's the way vcr power supplies work. Digital voltmeters take a reading, analyze it, then display the reading while taking the next measurement. Subsequent readings might be taken while the field circuit is turned off or turned on. This all happens multiple times per second and is why the display jumps around.
What we need to do is to perform a "full-field" test to bypass the voltage regulator. That involves grounding the dark green wire between the alternator and the voltage regulator in the Engine Computer. The best way to do that, if you can get to it, is to ground that wire at the back of the alternator while the engine is running, and measure the voltage on the output wire. That dark green wire can also be grounded at the Engine Computer but I don't know how hard that will be to access the connectors so you can back-probe it. It's in pin 8. The trouble is both connectors are black, but there is no pin 8 in the wrong connector. Each one has four rows of pins with ten pins in each row. Pin 8 is in one of the outer rows, third from the end. Just be sure it's a dark green wire.
If the two wires are plugged into the back of the alternator, it will be safer to ground the dark green wire there. The wrong wire is dark green with an orange stripe. On some models those two wires go through a small black plastic block so you can't tell which wire goes to which terminal. If that's the case, I can paste a copy of a procedure that might help identify the right terminal.
When you ground that wire, watch the voltmeter or the brightness of the head lights. You'll hear the alternator strain. If the voltage doesn't go up significantly, suspect a shorted diode in the alternator.
Wednesday, February 2nd, 2011 AT 1:01 AM