That voltage comes from the automatic shutdown (ASD) relay which is turned on by the Engine Computer. The computer also turns on the fuel pump relay at the same times. That initial one second is to insure fuel pressure is up and ready for starting in case it bled down over a few days or weeks.
After that first one second, the computer turns the ASD relay on again when it gets pulses from the camshaft position sensor and crankshaft position sensor. The purpose of the system is to turn off the fuel pump in the event of a crash that ruptures a fuel line. The engine will stall with no fuel pressure so there won't be any pulses from the cam and crank sensors. With no pulses, the fuel pump is turned off to prevent dumping raw fuel on the ground. This system is WAY more reliable and effective than Ford's inertia switch. The alternator field, injectors, ignition coil(s), and oxygen sensor heaters all get their current from the same source so all of those systems go dead when the ASD relay doesn't turn on.
Since you had stuff apart, be sure both sensors are plugged in. Also, a thick paper spacer must be used to set the air gap of the crank sensor. Without it, the sensor can be pushed in too far where it will hit the flex plate and break. If it's spaced out too far, the pulses will be weak and can cause intermittent stalling. If you reinstall an aftermarket sensor that had a thin plastic rib molded to the end to set the gap, you are to cut the remaining part of that rib off and use a paper spacer.
Monday, July 11th, 2011 AT 7:23 AM