I fly a 2009 C182T with CAP. We record the "TACH" time from the Engine/System page where ENG HRS appear. On the steam gauge Cessnas I fly TACH time is read off of the tachometer and generally understood to record the time that the engine is run above approximately 1000 RPM which produces the requisite oil pressure. The POH for the 182T only describes that the HOBBS requires oil pressure of 20 PSI to run. However, I have not been able to find what sensor or stimulus runs the ENG HRS. Anyone know where this information can be found?
Your understanding of how tach hours work is not correct, you are conflating some elements of Hobbs time and tach time with each other.
Tach "time" is not electrical, it is driven solely by the rotation of the engine. In my 182, the tach "odometer" is calibrated so that 1 "hour" on the meter means 144,000 rotations of the engine... That's 2,400 RPM * 60 minutes = 144,000 RPH. If the engine makes 72,000 rotations, the engine hours meter will read 0.5. I could be turning the prop by hand and take two full days to spin the prop 72,000 times, the tach will still read 0.5.
I often cruise at about 2,300 RPM, and thus after a one hour flight, my "tach time" will be
2,300/2,400 or 0.96. Fly for an hour at 2,000 RPM, and tach time will be
2,000/2,400 or 0.83.
So "engine hours" aren't really hours at all, but rather "equivalent hours if the engine was running at typical cruise RPM". It's most likely based on your POH's published cruise numbers from Section 5. It's intended to be an accurate reflection of engine use, and thus is the basis for engine maintenance intervals like TBO, SBs, and oil changes. The Hobbs is more oriented to an accurate reflection of aircraft block time, more useful for rental rates, etc.