First lets clear up a few things. You might have a separate fuel pump relay, but if you do, it is turned on at the same time as the automatic shutdown, (ASD) relay. Those relay(s) will turn on for one second when you turn on the ignition switch. THAT is why you will have fuel pressure, and that proves the relays are working, the Engine Computer has control of them, and their fused circuits are good. You might hear the hum of the fuel pump too.
The Engine Computer turns those relays on again when it sees engine rotation, (cranking or running), and it knows that by receiving signals from the crankshaft position sensor and the camshaft position sensor. The next important tidbit is the air gap for the crank sensor is critical. If you installed a new sensor, it either had a thick paper spacer on the end to set that gap, or it had a thin plastic rib molded onto the end. Some aftermarket sensors use that plastic rib. OEM parts use the paper spacer. As soon as the engine is cranked, that spacer slides off and is done doing its thing. If you remove, then reinstall it, you must stick a new spacer onto the end of it. If you don't, it will usually go in too far and be broken by the ring on the flex plate. If you reinstall a sensor with the plastic rib, you're supposed to cut that rib off, then use a paper spacer. That's because the rib gets worn down over time and will not properly set the gap a second time.
Once the signals are received from those two sensors, the Engine Computer turns the ASD relay on, (and the fuel pump relay, when there is a separate one). The ASD relay sends current to the ignition coil pack, injectors, oxygen sensor heaters, alternator field, and possibly the fuel pump.
The easiest way to tell if the ASD relay is being turned on is to measure the voltage at any of those items. The switched 12 volt wire will be the same color at the coil pack and every injector. For most models that is the dark green / orange wire. A test light works best for this because digital meters don't respond fast enough. You should see the light turn on for one second when you turn on the ignition switch, then it will go off. What is important is if it turns on again during cranking. If it does not, the signal is missing from one of those sensors.
If you're lucky, you'll get a diagnostic fault code for the sensor with the missing signal, but they often don't set just from cranking the engine. Then you'll need a scanner to view live data. Both sensors will be listed with some type of indication showing if the signals are there. I have a Chrysler DRB3 scanner for my vehicles. That lists each sensor with a "No" or "Present". Aftermarket scanners should provide the same information.
Sunday, December 18th, 2016 AT 1:59 PM
