You have to check for voltage while cranking the engine. Voltage will appear at the coil, injectors, alternator field, and oxygen sensor heaters for only one second after turning the ignition switch to "run", then the voltage will go to 0 volts. You might hear the hum of the fuel pump for that one second. The voltage will come back during engine rotation, (cranking or running). It comes through the automatic shutdown (ASD) relay. The Engine Computer, (PCM), turns the ASD relay on when it receives pulses from the camshaft position sensor and the crankshaft position sensor. Somewhere around the early to mid 2000s they changed the strategy so the engine could still run when one of those sensors failed but before that signals were needed from both sensors for the ASD relay to turn on.
To test the entire ASD switched circuit, connect a test light to the positive ignition coil wire, the 12 volt feed wire to any injector, or either small wire on the back of the alternator. The 12 volt feed wire is often dark green / orange. Whatever the color, it will be the same on all injectors and the coil. Watch for the test light to light up for one second when you turn on the ignition switch. A digital voltmeter might not respond fast enough for you to see it. If you see that one-second pulse, the ASD relay and wiring are okay and the computer has control of it. No need to check fuses.
What's important is if that voltage comes back during cranking. In the rare event it does, you have to determine if you're missing spark or fuel pressure. Usually it will be the fuel pump that fails to start up. Banging on the bottom of the tank often gets them going. Unlike GM fuel pumps that suddenly leave you sitting on the side of the highway, Chrysler pumps almost never quit while they're running. They typically intermittently fail to start up after being turned off.
There have been times no diagnostic fault code is set related to the cam and crank sensors. In that case it is helpful to have the Chrysler DRB3 scanner connected or an aftermarket scanner that can display live sensor data and has a "no-start" troubleshooting menu. It will display those two sensors with "no" or "present" to indicate if signals are being received from the sensors. Even if it does finally set a code related to a sensor, that never means "replace the sensor". While it's true the sensor is usually the cause of the problem, the code really means that's the circuit that needs further diagnosis. There could be a wiring problem such as a corroded connector pin or cut wire.
One of the strategies the Engine Computer uses to know when to set a fault code is if it's getting a signal from one sensor, it had better be getting a signal from the other one. They have their ground return wire and 5 volt, 8 volt, or 10 volt feed wire in common. If either of those wires are open circuit, (cut, or corroded at the splice), neither sensor will produce a signal. The computer will not know the engine is rotating so it won't set a fault code related to the two missing signals.
Please consider a to help us answer more questions.
Tuesday, November 22nd, 2011 AT 10:11 PM