There is a break somewhere in the circuit. If you have a cheep ohm-meter you could first test the coil connection on the heads to see if the coil is any good then if you get a reading of any sort other than what ever the ohm-meter is showing when the contacts are not touching anything. Place the head in the tank and check the 510 connection(should be same reading here).
My first guess is that the coil isn't connected properly in the head or there is a break in the coil. That would be verified by the first test failing. If the first test passes and the second test fails, there is a connection problem between the coil and the tank. If both test pass then there is a connection problem between the tank and the unit.