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Crankshaft speed fluctuation vs crank reference

EFI Wiring Fundamentals

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Discussion and questions related to the course Motorsport Wiring Fundamentals

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My car has a crank reference in the distributor and crank fluctuation on the crank pulley. I am a bit confused on why my car has 2 crank related sensors, one on the distributor and one on bottom end? If I am switching to ems does it matter which I tap into for crank signal or do I need to utilize both?

I think you may be a little confused because they're both "crank position" sensors. Because the 4 stoke engine uses 720 degrees rotation for one combustion cycle the ECU needs to know if the crankshaft is, say, on TDC for the compression-power revolution or the exhaust-intake revolution. The distributor* sensor rotates at half the crankshaft speed and 360 degrees there is 720 crankshaft, so by using a sender there it will tell the ECU which crank' revolution it is on, and so when to inject fuel and fire the ignition.

You may notice sometimes people will mention, when checking the ECU TDC setting, being 360 out and adding/subracting that from the timing offset for crankshaft position.

*Some engines don't use a distributor, and so a camshaft sensor is used instead, as it/they also rotate at hald crankshaft speed.