low voltage on fuel pump

hallo i've got a chevy G20 converson van with a 350/350 but i've got problems with the TBI , i've got only 8,1 volts on the fuel pump and i don't have a wiring diagram for it,it is also a LPG (liquid propane ) conversion i got the Van this way because it cost 1.30 USD for one liter of gasoline in Belgium it's agood reasen i think.
Can anyone tell me where i can find the fuel pump relay ,thanks Joebar