ECM for 4cyl &Altitude (was 747 & 4cyls)

[Patrick G. Moore]

Hi All,

Sorry, this is WAY more interesting than work
right now.

I am not an electronics expert, but one job as
an aerospace engineer in the early 80s was looking
at our dual redundant avionics on a spacecraft.
We looked at the circuits for all the sensors
and controls to determine if it was safe to switch
back to a primary system after it had found a fault
and switched to the backup.  We had several cases
of cosmic ray hits in the chips that made something
look out of range temporarily.

Seems like a good look at what inputs are
absolutely essential for the ECM to run
the engine is in order.  (ie. nothing happens
without crank sensor signal)  Then, what
can the ECM will deal with by using some
default if out of range.  This will, of course,
ultimately depend on which bin you choose.  I think
most all of them use similar strategy, but I would
feel better with one which was completely known
for this analysis.

Then of course there is the ECM itself.  I have had
mine go bad in my 88 Astro (747?).  It would run ok
when cold for a few minutes, then the check engine
light would come on, and it would run bad, and
sometimes stall.  I hooked it up to turbolink on
my laptop and I saw some really strange stuff.
The TPS was all over the place, the MAP was way
off, everything looked crazy.  It would shortly
set quite a few codes, seemed like different ones
each time I restarted it.  Unfortunately, I couldn't
watch it at startup because my laptop would reboot
as it was connected to the radio power (don't ask).
Finally, I saw it set a code for the D/A or A/D
self test.  This convinced me it was the computer,
along with it's occasional loss of ALDL sync.

The point is, it seems like it may not be hard to
use the check engine light output to signal a switch
to a redundant ECM.  At 5000' yeah, you've got some
time to mess around, but at less than 1000', you're
landing (spray pilot in WAY previous life).  An ALDL
monitor like turbolink on a small dash display would
be cool too.  You might add your own input limit
watching code in the ECM for safety.  Or the ALDL monitor
could warn you to manually switch.

I know you want to avoid redundant ECMs for the wiring
nightmare.  But how else ya gonna fire those plugs? Still
need some sort of crank sensor even with fixed timing.
I think a redundant ECM, whose critical sensors were
duplicated, and non-critical possibly shared, may
ultimately be best if not easiest.

Couldn't you make a fairly short adapter that would
just passively splice most of the non-critical inputs.

Some questions for the list.

1.) How long does it take for most of these units
to perform self test and proceed to run the engine?

2.) Will it even recover properly if powered up
to an already spinning engine? (I think yes.)

3.) If an unpowered ECM is seeing injector pulses
on its drivers from a passively connected "other
ECM" will it hurt them?

4.) Would it be possible to run both at the same
time on the same sensor inputs and just switch
the 4 injector outputs and the DIS output line
with some sort of relays? (This would be best
because you would know if your backup had a
problem before you needed it.)

I'll shut up now.
Pat
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