I have a Ford engine temp sensor in the head along where the head studs are, took out one of the larger plugs. The sensor has a 1.5V signal coming from the Ford PCM, and a ground, as well as the sensor body is grounded thru the block. If I ground out the 1.5V signal it goes to hot on my gauge cluster in the cab. I took a soldering torch to the sensor and got a reading from it, the hotter it gets the more it grounds the 1.5V signal and the more the needle on the gauge cluster increases in temp.

Problem is when normal driving the gauge sensor doesn't work, stays below "C". I know there is coolant where the plug is, just no reading. Any ideas?

 

Soldering irons are really hot (500º+), far higher than an engine block will ever be. Perhaps you have a bad sensor?