Anytime you are looking for electrical problems, you need to
keep in mind Ohm's Law. Voltage is equal to resistance times Current.
Reading voltages at various places in the airplane gives you little
information other than that the conductor is not entirely
disconnected, unless you remember that current flow affects voltage
drop.
I once forgot this principle and spent money that didn't need
spending. The strobe on the tail wasn't working, so I checked to see
that there was power available to it by pulling the feed wire off,
measuring the voltage there, and finding battery voltage. OK. Must be
the strobe's power supply shot. Bought another ($500) and installed
it. It didn't work, either. Then I remembered: E=IxR. With the strobe
turned on (but not working, of course) I took voltage measurements at
the bus (OK), at the circuit breaker's output input terminal (OK), at
the breaker's output terminal (not OK!) and realized that the breaker
was internally defective. There was full battery voltage there with
the power supply disconnected, because the meter requires only a few
microamps to drive it and the hundred ohms or so in the breaker's
corroded contacts wasn't enough to drop the voltage to it. Connecting
the power supply introduced a much lower resistance, increased the
current demand to two or three amps, and caused a huge voltage drop so
that the strobe was dead. The airplane needed a $15 breaker, not a
$500 power supply.

Dan