Pin 1 is ground and is supplied by the PCM, it should have no voltage on it. Pin 2(aka signal) is not going to be a constant 5v. When you disconnect the coolant temp sensor it should display 5v at pin 2. when you jump pins 1 and 2 together you should read 0v at the pcm.
2 wire temperature sensors are as simple as it gets. The PCM supplies 5v through a fixed resistor in the PCM. The temperature sensor is also a resistor but its resistance changes as the temperature changes. As the resistance of the sensor changes, the voltage also changes because the 5v source is fed through the fixed resistor in the PCM(its really just a series circuit with 2 resistors). the PCM monitors the circuit between the fixed resistor and the variable resistor(the temp sensor).
You really need a scan tool to see what the PCM is seeing as far as coolant temp sensor signal voltage. When the sensor is unplugged the PCM should read 5v. When the 2 wires are jumped together the PCM should read 0v.
Im a little confused how you performed your measurement. How did you determine you had .9v at the pcm?