Thanks Larry. That makes sense.

I discovered when I put the thing together that just turning power on and
off to the circuit board would result in its always going into flash mode so
I just cut the wire to the original switch. A chattering reed switch could
certainly be doing this though.

Brian's observation about reed switches jibes with my other reed switch
experience. I installed an AquaLarm raw water sensor in the boat three
years ago. All was fine the first season and it did save me once from
overheating my exhaust hose and or engine. Last year, however, it began
sounding every time I started the engine and not going off until I went
below and moved the adjustment back and forth or banged on it. Big pain.
It's been better this year but is starting to do it again. This unit has a
magnetic reed switch in it.

Too bad. The ideas of a switch hermetically sealed from any moisture is
conceptually attractive but I'm not sure thay belong on boats in critical
applications any more.

--
Roger Long