When you purchase a DeWitts Radiator & Fan Combination, we install the temperature switch (195on/175off) into the radiator inlet tank. When the temperature hits 195, this switch contact closes and the fans will come on. In order for this to function correctly, the radiator much be properly grounded. A ground strap is provided on each package and the installer must connect this wire to a good chassis ground or the fans will not function. If your fans were operating and then stopped working, a good test is to remove the wire from the switch and touch it to a good ground. If the fans come on, then the problem is likely the switch. If the fans do not come on, then check the main fuse for each fan.
Black Ice MSDS (updated)