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Lurker, your question is on how the EDU knows when (plunger position) to energize and de-energize the Solenoid Spill Valve?
I scanned the wiring diagram and noticed that you're right, the EDU seems to only receive pressure demand and injector control signals from the ECU.
I think there is a piece of the puzzle we are missing or not seeing currently. For the EDU to have to derive pump cam position from injector control doesn't seem like an efficient route to control the valve when the engine already has a cam or crank position sensor (which you could easily determine pump cam position from).
So I don't have an answer to your question, but to add to the discussion, I am familiar with a diesel high pressure pump by Denso that uses this same spill metering to control fuel delivery. The pump is much larger than this one used on the FA20, and features a cam position sensor to measure the position of the pump camshaft. I would think Denso would take a similar approach for this pump as well, though the sensor may be elsewhere or the signal derived from another sensor (potentially cam or crank position sensors).
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