Is it a sensor causing the fan not to turn on when the water temp. climbs? It works when I turn it on manually, but it won't come on automatically, with or without heat on in the cab. It's a '93 379 3406C 3ZJ. Is it the sensor at the thermostat housing, or does an oil temp. sensor activate it? I would think it's the water temp. sensor, but I'm not sure. Happy Easter to every one.