What is the deal with my electric fan???

[soule64]

This thing is driving me nuts. I have gone through 3 temperature switches in the last month, which is actually 20 miles and about 45 minutes of running time since I am still in 'shakedown' mode after the rebuild. I got the fan and control/relay from Summit and so far they have replaced the temp switches for free. I wired it exactly per the schematic and have double-checked all of the connections. If I understand this thing right, the thermal switch is an open circuit (when cold<180deg) to ground for the relay. A bi-metallic strip or thermal compuond reacts to heat and closes the circuit at 180deg and the relay closes and connects the fan to the battery. So far so good? I installed the switches in the water jacket of the intake manifold just below the thermostat and used a small amount of teflon tape on the threads of the switch, but checked each installation for ground with a multimeter - all were good. They have all worked at first, for at least the first cycle, but die at some point after that. I have taken the lead each time and grounded it to the engine block and the fan comes on. Next, I have taken each of the bad probes and soaked the ends in boiling water - each time there is no continuity (there should be, right?). The last time I talked to Summit, the guy said that I must have too much voltage going through it...Huh? Next he tells me that I should bypass the temp switch and control it manually with a switch in the cab. I told him that I paid for a system that would run on it's own without me having to rely on me remembering to turn it on (I am going to add a manual switch in addition to the one in the manifold) and it should work as advertised. SO... probe number four is on it's way. Have I jacked this thing up or is there a possibility that I got 3 bad probes in a row? Thanks.