Originally Posted by ottos
OK the large component is a piezo buzzer - I think it is - (PKB6-5A0)
Is there a connection from the other side of the rheostat? I can't see one.
There is also a small Motorolla component which I can't identify.
Anyway FWIW - holding tank alarm seems very likely based on your observations of where the wire goes. The button is probably a temporary reset of the alarm.
Take a close look at the red wire from the rheostat. It appears very close to shorting out against the fuse terminal...
The rheostat has only the one connection and I suspect that it simply controls the alarm volume. The angle of the picture makes it look like the red wire is close to contacting the fuse, it isn't. Could there be an electrical pump in the tank that pumps the waste out when the valves are configured for such? Is there such a device? I understand why there is an alarm, you cannot see how full the tank is. Flipping the switch on the fuse panel marked "Holding Tank" and then pressing the button does nothing. I sealed the boat up so that it was quiet and could not detect any evidence of a pump operating when the button is pushed. Could be that it is simply non-operational at this point...