I'm not sure where the ground is but you can do the same test on the ground side. Put the negative lead on the negative battery terminal and the positive one on the alternator case.

 

I merged the threads together.

Like others said check the grounds. There should be one from the engine block to the battery.

 

I did some checking today and all the grounds that I could find looked OK. And I used my Fluke meter to check resistance (ohms) in the cables. My thinking is that IF they were bad they would have a lot of resistance in them? All of the cables tested had 1.5 ohms or less resistance. I would think that is acceptable? So what's next? I HATE electrical gremlins

 

Checking resistance with no load on the cable won't tell you much. You need to check the voltages with the charging system operating.

 

key word there is voltage. 1.5 ohms is on the way-higher-than-I'd-like end of things. 20 amps through a 1.5 ohm cable is a 30 volt drop through the wire...umm...we're working with a 12 volt electrical system.

four feet of 4 AWG wire should be somewhere in the 0.05-0.1 ohm range, or less.

The way you check with the engine running, you treat the cable in itself as a voltage source. one end of it is positive, one end is negative. What the meter will show is the voltage drop through the wire.

 

More troubleshooting via FLUKE:
With engine shut off resistance across the ground of the alt and the ground of the fuse box is at 1.5
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With engine running at idle it's -99.6 fluctuating around -100???
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Voltage across those same points was 12.50-12.75 AT IDLE. As was all the other points and grounds of the alternator and battery + & -, block ground and fuse box + & -
So what does all that tell me?

 

tells me that you're lucky you're not buying a new fluke. NEVER measure resistance on a voltage load.

also, was that the ground of the positive at the PDC you have the probe in? I thought the exposed stud was positive. Reading 12.75 still sounds like you're reading system voltage, not cable voltage drop.

 

The stud I had the meter lead on was the negative stud. There is a + and a - stud on that side of the PDC. There is on my truck anyway. SO how do I read VOLTAGE DROP across a cable? I'm obviously missing something here....

 

With the meter on VOLTS you put a lead on EACH end of the cable.

 

Lets try this.

Never measure resistance when a voltage is present, it can damage the meter.

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