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!!
1N4007 LED 220 ohm |
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
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.