If you have an HP 5971 or Agilent 5972 MSD and you can't get the diff pump to turn on there are only a few possibilities.  1)  You have a massive leak and the diff pump controller board won't allow the diff pump to turn on.  2)  The DC power supply in the MSD is dead and can't run the pump controller board,   and 3)  The diff pump heater is dead.  

HP 5971 and HP 5972 Mass Spectrometers have difussion pumps and on the frame of the diffusion pump are two sensors.  A   Too Cold Sensor, the bottom (white wires) and the Too Hot Sensor, the top (red wires).   The signal from the sensors report to the startup and run electronics of the Power Distribution Board.