The crankshaft position sensor can have a thick paper spacer stuck to the end that sets the gap. It slides off the first time the engine is started. Some aftermarket sensors have a thin plastic rib molded to the end to set the gap. It will partially wear away over time so to reinstall a used one you must cut the remaining rib off, then install a paper spacer. If you don't use any spacer it's possible the sensor could go in too far and be broken from the flex plate hitting it.
The camshaft position sensor lives in the distributor. Both sensors are powered by a regulated 8 volt supply inside the Engine Computer. It doesn't happen often, but it's worth mentioning that if that orange wire to either sensor becomes shorted to ground the computer will turn that power supply off to protect it. Once the short is gone, the ignition switch must be cycled off and back on to reset the supply.
When the Engine Computer receives pulses from both sensors it turns on the automatic shutdown (ASD) relay. That relay sends current to the injectors, ignition coil(s), alternator field, oxygen sensor heaters, and fuel pump or pump relay. Since the fuel pump and ignition coil are tied together in the same circuit it is necessary to check both of them for a no-start condition. If you have spark OR the pump is running, (which can be hard to tell), you know the two sensors are working. That's when you must troubleshoot the dead circuit. When BOTH are dead, that's when to look at the sensors. The sensors cause a lot more trouble than the pump or ignition coil.
In addition to turning on the ASD relay when the engine is rotating, the computer also turns it on for one second when you turn on the ignition switch. You will usually be able to hear the pump hum for that one second but it's hard to hear it during cranking because of the noise from chimes and the starter.
The most reliable test is to measure the voltage from the ASD relay. That's the dark green / orange wire at the coil, injectors, or either terminal on the back of the alternator. You can prop up a test light so you can see it from inside the truck or you can jump the starter relay from under the hood. I can describe two ways to do that. If you see battery voltage for the first second after turning on the ignition switch, you know the wiring and relay are okay and the computer has control of it. Whether or not the voltage returns during cranking will determine where to go next.
Thursday, February 24th, 2011 AT 6:38 AM