Hi dirtrider501. Welcome to the forum. First of all, understanding how the circuit works will help in diagnosing it. When you first turn on the ignition switch, the Engine Computer turns on the automatic shutdown relay (ASD) for one second to insure fuel pressure is up and ready for starting. Besides the fuel pump or pump relay, the ASD relay also sends voltage to the ignition coil, injectors, oxygen sensor heaters, and alternator field. If you hear the hum of the fuel pump for that first second, you know the ASD circuit is working.
The Engine Computer turns the ASD relay on again when it sees engine rotation, either cranking or running. It knows that by the pulses coming from the crankshaft position sensor and on some engines, the camshaft position sensor. Both of those sensors run on 5.0 volts or 8.0 volts supplied by the Engine Computer.
The fuel pressure you measured could be from the one second initial burst. The missing voltage at the coil can be explained if you measured it when the engine wasn't rotating. If the fuel pump is running and the coil has no voltage, there has to be a break in a wire or splice since both are on the same circuit. To double-check, measure the voltage to the coil for that first second and again during engine cranking. If it's there only for that first second, suspect the crankshaft position sensor.
The rest of the system operation has to do with a crash that ruptures a fuel line. The engine can't run without fuel pressure so it stalls. Loss of pulses from the crankshaft position sensor tells the Engine Computer to turn off the ASD relay. That removes voltage to the fuel pump so it also turns off to prevent dumping raw fuel onto the ground. The circuit is very reliable but often misunderstood. A lot of people try to diagnose the first thing they find missing, spark or fuel pressure, but it's important to check for both. The most common failure is the crankshaft position sensor. The individual coil and pump circuits account for very few problems.
Tuesday, July 20th, 2010 AT 1:26 AM