If you have the gray square relays with four terminals, it can be plugged in either way and work just fine.
It sounds like the two ground circuts are ok. One is the ground for the relay coil itself. The other one is reading to ground through the fuel pump motor.
That next test where you found battery voltage, if it is there all the time including when the ignition switch is off, that is the voltage supply for the fuel pump motor. The conclusion is the high-current circuit including the motor is ok. What appears to be missing is the 12 volt turn-on voltage for the relay. Since this is intermittent, a good suspect is a stretched terminal in the relay socket. Pushing the relay sideways while a helper cranks the engine might get it started, but it should also cause the pump (and engine) to quit while it's running.
The next thing is if the Engine Computer is intermittently not supplying the turn-on voltage. The question then becomes is the problem in the computer or is a signal to the computer missing. GM used a number of different circuits over the years. ALL Chrysler vehicles use one circuit that is very similar to one used by GM, and that involves the Engine Computer. Signals must be received from the crankshaft position sensor and camshaft position sensor. That tells the computer the engine is rotating and the fuel pump needs to be turned on. Other GM systems run the pump from a terminal on the oil pressure sending unit. (Low oil level can cause a stalling problem before the warning light turns on on those systems). With the Chrysler-type system, an intermittent sensor is a common cause of a no-start, but it is WAY more common for the problem to show up after the sensors get hot from normal engine operation.
What you must do next is test for voltage on that fourth terminal. It should appear for a second or two right after turning on the ignition switch, then it might go away until the engine is cranked. The first few seconds is to be sure fuel pressure is up and ready for starting. The voltage must be there again during cranking. Here's the key. If the voltage appears for the first two seconds, the computer is working. If it doesn't appear again during cranking, suspect the cam or crank position sensors.
Monday, April 26th, 2010 AT 10:37 PM