A prototype for a super-simple indication that the engine is consuming fuel. An LED blinks when the injectors are actually injecting. Note that there are several conditions when the engine can still be *spinning* even if gas and spark are not being delivered. [Rotation itself can be determined from the NEO crank-sensor output, or just watching the vacuum gauge for suction.]

The signal comes from the #1 injector output, which for test purposes is tapped by yet another paper clip stuck into the ECU plugs. The other one is the IGF signal for the tach. Someday all this will come off real harness taps, but for the moment this hackery is fun and serves the purpose. It's great show-n-tell when people actually see the car, with the dash half ripped apart and alligator clips hanging all over the place.

This is the injector pulse waveform. It's an open-collector output that sits at a nominal 12V and gets pulled to ground to fire the injector. When the signal ceases and the injector closes, we also see an inductive kick around +100V from the injector coil, which is really odd since you'd think the designers would have added diodes inside the ECU to soak that up. Or maybe the ECU actually *measures* the kick to make sure that the injector is still there and still has a good coil -- coming from Toyota, frankly, that wouldn't surprise me.

[Update to that: I found out later that injector coil kick-back is left to "fly up" because trying to suppress it by shorting out would *damp* the field collapse, and not let the pintle close as fast.  Since the critical timing is that of the mechanical valve, its movement needs to match the signal as closely as possible so injector drive circuits try to mess as little with the magnetics as they can.

The circuit is idiotically simple, and can almost be determined by inspection from the picture. The formal representation was sent to the prius_technical_stuff yahoogroup:

      The simplest possible "I am consuming fuel" indicator --
      !! note: slightly updated circuit since original post!!

                 |                    |
      [+12V] ----*--|>|--------|>|----*------/\/\/\/--------
                   1N4007      LED           220 ohm       |
                                                       tap |
      ECM # E4-2 (#10, injector #1 drive)  ----------------*------>
      [top ECM connector, one pin upward                        to
      from its lower right corner, yellow wire]                injector

      which simply flickers the LED when the #1 injector opens.
      The 1N4007s are to protect the LED against the ~100V
      inductive kick when the injector closes, but still allow
      the switching artifacts to reach the ECU.
[I used a 180 ohm since it was laying around; it makes this particular LED just a little brighter.]

This is a 2-second or so time shot while waving the camera around in front of the LED, with the engine idling. This shows how short the no-load duty cycle is -- there's just a *little* bit of vertical elongation of each image, but for the most part it's playing strobe light. The LED sits down by the side of the MFD bump, off in peripheral vision, and the flicker is not bothersome at all. At higher RPM and under load it starts looking almost solid.

However it is completely invisible under sunlight, so for now it gets a little foreskin of gaff tape to block light from above. I expect that some similar sort of shroud will need to go over the final version when it moves into the real gauge cluster.

050511,060803 _H*