DWBRubi

Just seen this and thought I would add my .02. I used a SPST switch, wired to a relay under the hood, next to the fan wiring by the radiator. I cut the ground wire to the electric fan, and wired the chassis side to input 30. The fan side of the ground goes to output 87a. The switch is wired to 86, 85 is grounded (can be connected to 30 or 87a, and 87 is left alone. I used a Hella weatherproof relay. This way everything operates as normal, until you hit the switch, which basically just interrupts power to the fan. This was the easiest way I could come up with to accomplish it.

JK-Ford

Excellent idea for simply deactivating the fan. But completely turning off the fan still has me concerned. I would still prefer to see some type of automatic control over the circuit to prevent the accidental failure of restoring the circuit. Probably the easiest way would be some type of water level switch. Or another thermocouple switch set at the top end of the temperature range.