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Winston

wrote:

(...)

So, could you do the usual electronics fault finding thing of using
the diode check function on the output lead of the alternator? - (then
reverse, repeat, see difference or none) even cheap DMM have them now

You could do that if you could access the inside of the alternator.
I guess that you could sort alternators with shorted diodes from the
outside, by comparing the forward drop of the 3 phase rectifiers.
The shorted units would have ~0.7 V less forward drop than the good
units. A unit with two shorted series diodes would have ~1.4 V less
forward drop, at least theoretically.

And I would humbly disagree re ohmmeters vs ammeter's - its all to
easy to fry an ammeter, the 10a range on most DMM's is rated for a few
seconds only, then the copper wire shunt disengages itself....besides,
10a is leakage current in a vehicle.

What vehicle normally experiences as much as 10A of battery drain,
with the key out of the ignition and the lights off? Are there any?

Note that Karl revealed the high value of leakage current to be well
within the range of his DMM:

Karl: I narrowed the problem down to my alternator. Drain of .2 amp.

And I would humbly disagree re ohmmeters vs ammeter's - its all to
easy to fry an ammeter, the 10a range on most DMM's is rated for a few
seconds only, then the copper wire shunt disengages itself...

The 10A current shunt built into the DMMs I use are 100% duty cycle
and will tolerate 20A for 30 seconds without damage.

No fear that any of my DMMs will be damaged if used properly.

The DMM ammeter in series with the negative battery terminal is a safe
and effective method for troubleshooting leakage current.

--Winston

--

Don't *faff*, dear.