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I am planning/designing an OBD2 logger which should detect engine start to begin operation (detecting engine off is trivial since logging via OBD2 is already going on).

Since the device should consume little power while not logging (engine off, car parked), how can I detect the ignition event (few seconds delay is acceptable) without powering up or keeping powered the ELM327 chip with polling of the ECU?

Would a trigger based on OBD2 voltage be enough? usually when engine is on, the voltage raises above 13.3V, and when off, even when the 12V battery is fully charged, the battery drops below 13V (I may be wrong with the numbers, but that's the idea). Basically a monitor on the pin 16 voltage.

Otherwise I would need to route a wire from the fuse box from a supply which is off when the engine is off, but it involves more wiring and potential complaints from the dealer during regular checks...

Maybe electrical noise on the pin 16 would indicate operating engine, or at least it would indicate the car is "awake" (it awakes simply by opening the driver door), which would be followed within minutes by engine startup. After all, when the engine is off and the car is parked, most electronics and devices are off and the alternator is also off, so the electrical noise should drop quite a bit.

If relevant, I have a Volvo V60 MHEV (mild hybrid).

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Pin 16 on the OBD port is an always on source for power. This provides power to the reader when plugged in (including your ELM 327). You're not going to "see" much there as to whether the engine is running.

My suggestion to you is to wire in the equipment and use a keyed-on power source to power the ELM327 and whatever you are using for logging. You could use a spade terminal power tap to come off of a fuse for your power source. You will want to tap into the wires for the OBDII port rather than using the ELM327 device plugged in directly to the port, otherwise you'll interfere with getting codes read, etc. Besides, it will always be powered on if you plug it in directly. Yes, this involves wiring, but it is about the only way you're going to get done what you're looking to get done.

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