The following video will give you a good overview of how various engine sensors work together to help provide spark and fuel to the engine to make it run.

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Comments about the video by SeanR, member of the AACA forum.

18X interrupter rings on Harmonic Balancer detected by crank position sensor:

  • Indicates engine speed. Required for engine to start.
  • Signal sent to ignition module terminal G.
  • Spaced at 10 degree intervals.
  • If 18X signal is lost, engine will immediately die and cannot be restarted.

3x interrupter rings on Harmonic Balancer detected by crankshaft position sensor:

  • Used to sense position of #1 and #4 piston. 
  • Signal sent to ignition module terminal H.
  • Required for engine to start.
  • Voltage at terminal H should be 9.5 volts while cranking.
  • If signal is lost, no trouble code is set.
  • If 3X signal is lost while running, engine will continue to run but cannot be restarted.

18X and 3X interrupter rings:

  • ECM also separately monitors both 3X and 18X signals to fire in "simultaneous fuel injection mode" aka (fast start ignition) until engine reaches 600 RPM. Then switches to Sequential Fuel Injection Mode.
  • Both signals required to start engine to trigger Ignition Module to send spark to fire plugs.

Cam Position Sensor:

  • Cam magnet sensor signal senses when #1 cylinder at 25* after TDC.
  • Only responsible for triggering to SFI injection mode once 18X and 3X signals are present past 600 rpm.
  • Reduced engine performance due to ECM trying to run injectors on default injection mode.
  • If signal is lost, Code 41 is set but the engine will still run and can be restarted.
  • Cam sensor signal is NOT needed to start the engine.

I wanted to write this out to show that when the engine wont start. We should troubleshoot here instead of to the cam sensor which is not required to get the engine to start and for high level explanation, is only used to trigger sequential fuel injection delivery mode.

Thanks to SeanR, member of the AACA Reatta forum, for this information and video.

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