MemCals Needing a Good Home...

[David Papworth]

Look at the schematics for the 7730 on Ludis's page. U9 (the 16054995 chip)
likely contains a couple of op-amps that make injector pulse
width a linear function of TPS, MAP, and CTS,
when in limp mode. The netres forms the feedback resistor networks
in these analog circuits, such that the relationship of fuel to TPS/MAP/CTS
is appropriate for the engine size involved and the sensors used in
that application. Hence the variability of the netres across engine types.

The early Bosch fuel injection in Volkswagen Type 3 worked this way
all the time -- there was no digital computer -- it was all done
with analog circuits.

I expect they then lock the timing at some fixed value -- say 10BTDC
by passing the ignition trigger right on through as the signal to fire
the coil.

The engine should now be able to start and run, probably not very well
and not with good emissions or power. But the prom can be bad, the
microprocessor bad, and most of the sensors bad, and you can still get the
car out of the fast lane of the freeway or out of that bad neighborhood
you just found yourself in.

If you study Ludis's data really closely you will see
the the netres is the same for the 2.8 and 3.1 V6, so the values
could not be all that critical. (They are different for the V8).

> Has anyone been able to come up with a pinout and/or internal connection
> diagram for the netres?  And while we're at it, could someone explain HOW
> a resistor network provides a malfunctioning ECM with the ability to "limp
> home"?  Maybe I'm just misunderstanding the function of limp home mode,
> does it require that the hardware is OK, but the data in the PROM is
> corrupt?
>

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