Before approximately 2001 to 2002, the Engine Computer needed to see signal pulses from the crankshaft position sensor and the camshaft position sensor during cranking, then it would turn on the automatic shutdown, (ASD) relay. That relay sends current to the ignition coil(s), injectors, fuel pump or pump relay, oxygen sensor heaters, and the alternator field. If either sensor failed while you were driving, the engine would stall.
In later models, if a sensor failed while driving, the computer would keep the engine running on the one good sensor, but only until you turned the ignition switch off. After that it wouldn't restart because it had no way of knowing which piston was coming up on top dead center on the compression stroke.
In still later models, the engine would restart with a failed sensor, but it would only run up to a certain speed, as you described. Below the specified speed, injector pulse timing isn't terribly critical, so it is synchronized to one of the sensors. Once the magic rpm is reached, injector timing is switched to the other sensor. If that second sensor has failed, the injectors cut out at that speed.
The place to start is by reading and recording 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
To see the definitions, or I can interpret them for you. Be aware fault codes often do not set for these two sensors just from cranking the engine, so if the battery was recently disconnected or run dead, there may be no codes to read. Those lost signals often need more time to be detected, as in when a stalled engine is coasting to a stop. For that reason, don't assume both sensors are okay just because there's no fault code related to them.
When there are no fault codes to direct you to the circuit that needs further diagnosis, you'll need a scanner to view live data. I have a Chrysler DRB3 for all of my vehicles. That one lists each sensor with a "No" or "Present" during cranking to indicate whether those signals are showing up at the computer. When one remains on "No" or is sporadic during cranking, that is the circuit that needs to be diagnosed. About half of the time you'll end up with a cut wire or corroded connector terminal. The other half of the time the sensor itself has failed. Aftermarket scanners usually have a similar way of indicating whether those sensors are working.
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Thursday, January 9th, 2020 AT 2:31 PM
