Chrysler Repair: 87 JEEP CHEREKEE FULE SYSTEM, electric fuel pump, jeep cherekee

Question
   I have a 87 Jeep Cherokee 4.0 MPI. It is very hard to crank. It's only getting 20psi fuel pressure. I've replaced the electric fuel pump, filter and regulator. I've also replaced all the sensors. I'm sure that I need around 30 psi of pressure. What's happening is that when the switch is turned on the fuel pump comes on and shut off at 20 psi. I know for sure the pump and filter is good. But at 20 psi the fuel pump relay opens up. What confuses me is that the coil on the relay stays energized but the switch part opens up. Other people has fool with this so it could be a wiring problem. Wil, please tell me what you thing, It's driving me crazy...If I already sent this to you then I'm sorry. It's been a long morning.

Answer
This is where I would look! I might not be right.First off the fuel pump relay will deenergize if it does not see a engine cranking reference.By removing the ground to the relay.I would look at the crank position sensor.It is mounted in the trans bell housing at the top left by the engine.It has a 3 wire connector.A way that I used to test them was with a dvom set in the AC volt range.Crank the engine to see if it generates voltage.That is with it disconnected and I don't remember which 2 pins to check.So its trial and error.