On 14 Aug 2018 16:01:59 GMT, dpb wrote:

Well, there's _always_ voltage at the switch; the only question is does
it close and are the contacts any good to get to the other side...

It's pretty clear now that the two outside (black) wires are the two 120VAC
phases coming in, and the two inside (blue) wires are the two phases going
into the motor.
http://www.bild.me/bild.php?file=1169657flow04.jpg

Well, did you not check for 240V _across_ the two blacks??? One would
presume nobody's moved anything and got both on the same phase, but...

Yeah. 118VAC RMS on each to the green ground when the motor is running and
236VAC RMS to each other when the motor is running, as measured on my DMM.

Interestingly, I originally checked each black incoming phase to the steel
water pipe expecting that to be grounded - but it wasn't all that good as
the voltage was around 20 volts so I used the green ground instead.

Sure, just take a screwdriver with non-conductive handle and push down
to close the contacts...let go and it'll reopen if pressure cutoff or
run 'til builds pressure if drops while closed.

Something weird happened, but wasn't repeatable, but it could be the
pressure built up.

a. I turned the circuit breaker back on
b. The motor ran
c. I checked all the voltages (which were as expected)
d. With a stick I pulled the contacts open
e. Obviously the motor stopped
f. But when I let go with the stick, I expected the contact to slam back
g. They didn't. They stayed open. Huh?
h. I pushed them closed, expecting them to stay closed
i. They didn't. They opened when I let go of pressure.
j. Huh?

What I'm "thinking" (but need to test) is that there is an electromagnet
which keeps the contacts closed (something kept them closed in step (b)
above. Maybe it was just the lack of pressure though, and not an
electromagnet.

Contacts pushed open:
http://www.bild.me/bild.php?file=4628253flow06.jpg
Contacts pushed closed:
http://www.bild.me/bild.php?file=1733078flow07.jpg contacts closed

Anyway, I'll have to test again as there seems to be three states:
a. Contacts automatically closed and they stay closed
b. Contacts pushed open and they stay open
c. Contacts pushed closed but they won't stay closed

It could depend on the pressure though... but "something" keeps the
contacts closed which didn't do it once I opened them manually.