Lets start with reading the diagnostic fault codes. Chrysler made doing that yourself much easier than any other manufacturer. Cycle the ignition switch from "off" to "run" three times within five seconds, without cranking the engine. Leave it in "run", then watch the code numbers appear in the odometer display. You can go here:
https://www.2carpros.com/trouble_codes/obd2/p0400
to see the definitions, or I can interpret them for you.
The next thing is to see what the automatic shutdown, (ASD) relay is doing. You've already been on those test points. Use the wire that is the same color at every injector and at the positive of the ignition coil(s). That's usually a dark green / orange wire. Easiest is usually to back-probe through the rubber seal at an injector. You can also use either smaller terminal on the back of the alternator. A test light works best for this because digital voltmeters usually don't respond fast enough. If you can prop the light so you can see it from inside, do that, otherwise you'll need a helper to run the ignition switch.
You should see the test light turn on full brightness for one second after the ignition switch is turned to "run", then it will go off. That tells us the ASD relay is working and the Engine Computer has control of it. You may hear the fuel pump run for that one second too. Next, the test light must turn back on during engine rotation, (cranking or running). If it does not, the suspects are the crankshaft position sensor or the camshaft position sensor. If either one fails while driving, the computer will have time to detect the missing signal while the engine is coasting to a stop. When you have the condition you described where you suddenly have a crank / no-start, that may not give enough time for the computer to detect the missing signal. For that reason, there may not be a diagnostic fault code indicating one of those sensor circuits is not working. The ASD test light test is the better indicator.
Let me know what you find. If there's no fault code related to one of those sensors, and the ASD relay doesn't turn back on during cranking, we're going to need a scanner to view live data. I have a Chrysler DRB3 for all of my older vehicles. That one lists each sensor with a "No" or "Present" during cranking. Aftermarket scanners have a similar way to show that information. If one doesn't switch to "Present" when you start cranking the engine, that is the circuit we need to diagnose. Fault codes never say a sensor or other part is defective or needs to be replaced. They only indicate the circuit that needs further diagnosis. In this case either sensor is a good suspect, but we normally want to do some other tests first before blindly replacing the sensor. We'll discuss that further once we know if one of those circuits has failed.
Monday, August 7th, 2023 AT 8:59 PM
