Are you completely sure you're measuring the signal line? I think you're measuring the 5 volt feed (power supply).
Measure ALL 3 pins with the map-sensor connected and disconnected (of cource with key on). And report back with some reference to which measurement is which. 1,2,3 or a,b,c or left,middle, right.

Do you still have the standard 2-bar map sensor?
Connect it electrically and measure all 3 pins just like above. Of course you cannot start the engine.