Crank shaft sensor

The car was not starting so during diagnosis the auto electrician discovered that if the crankshaft sensor is disconnected then the car could start but when sensor is connected the car just cranks but will not start. Once the car starts then can connect the sensor and can drive to where ever you want with no problems. He said I should buy a new crank shaft sensor and I bought one from Nissan and was fitted but the car is still doing the same thing. Once you connect the sensor, the car can not start but once you disconnect the sensor then the car starts of course with an initial higher rev but later moderates to the normal idling. I went to Nissan for diagnostic check and they said its a diesel pump which needs replacement. I am confused because for two months I am driving the car with no problems except that I have to disconnect the sensor when starting the car. Please help.