David Lesher wrote:
I can't see why he can't do all the testing with the engine off. That's
LOTS safer. Yes, he'll drain the battery slightly but we're talking only
a few minutes here.

The test results with engine on vs. engine off probably depend somewhat
on where the battery is. If it's under the cowl, then engine on or off
probably won't make that much difference - all of the battery-recharge
current will be mostly under the cowl and won't affect the readings
much. If the battery is back in the tail someplace, then I would expect
more difference between engine on and engine off - there will be several
amps flowing from the generator/alternator back to the battery,
especially if the engine has just been started.

Then make a simple schematic:

JPI

1-----------2breaker3---bus4----[master contactor]5-------6Battery+

and measure two ways; from each point to a known clean ground, and
from one to another, such as 2-3, 3-5 etc.

The only thing I would add to this is to test both sides of everything,
like:

JPI1---2breaker3---4bus5---6[master contactor]7---8Battery+

This helps show if you've got bad contacts in the master contactor or on
the bus.

"To ground" will be hard to see small drops; 'across' is twice as many
good connections to make in tight places.

Tip: Get some heat-shrink tubing and cover up all but the last 0.1-0.2"
(2-4 mm) of the meter probe tips. This helps prevent shorts when poking
around in tight places. You can also do this with electrical tape, but
the heat-shrink stays put better.

If the battery is much less than 13.8v, with the master off; I'd wonder
how it cranks the plane.

I'd expect the voltage on a fully-charged battery to be something like
12.6 to 12.8 V. 12.3 or 12.4 might still crank the engine, but slowly.
12.0 or 12.1 is totally discharged.

Disclaimer: This is based on experience with ground vehicles and
equipment. I don't have an A&P; I don't even have a TG&Y. Your
mileage may vary.

Matt Roberds