747 meet 730, 730 meet 165

[T Hergen]

I think  the sensors that get gnd and +5v such as the map and tps (not
the [heated] O2) can be shared.  I don't think they rely on a resistor
divider arrangement like the coolant temp sensor.  The map on my '92 GM
truck even appears to have an internal amplifier/buffer (can't measure
resistance, but give it gnd and +5v and a voltage that changes with
vacuum comes out [no pullup needed]).

Using an external pullup on the coolant sensor followed by an op amp may
or may not work.  Others on the list indicated that some ECMs switch in
different pullup value depending on the temperature (improves ECMs
measurement resolution over particular ranges).  If you're just using
potentiometer to simulate the coolant sensor, adjust the pot for the same
voltage in the dual ECM case as in the single and they should indicate the
same temperature.  The only time the pot may be a problem is when your
near the range where the ECMs want to switch the pullup value -- anyone
know about what temperature this happens?

What are you currently doing for the O2 in the single ECM case?

As others have stated, digital inputs should be okay.

Tom



On Wed, 6 May 1998, Orin Eman wrote:

> It probably wouldn't work.  Some sensors are fed thru a resistor from
> a voltage source (eg coolant).  You would end up with half the source
> impedance which would affect the voltage at the sensor.
> For example, when the sensors resistance is the same as the source impedance,
> the voltage across it would be half that of the voltage source.
> Half the source impedance and now the voltage across the sensor
> would be 2/3 that of the source... quite an error.  I wouldn't expect
> smoke from the ECU in this case tho'.
> 
> Some digital inputs might be OK, but you would have to reverse engineer
> the input circuitry to be sure.
> 
> For the resistor fed sensor, it wouldn't be too hard to make an
> op-amp plus transistor circuit to feed the second ECU.
> 
> > Has anyone any experience with running 2 ecms off the same 
> > inputs?.  ie would they have enough "impedance" to share inputs?
> > or would they do something strange?.  The outputs would be 
> > loaded independently of each other, as far as driving any loads,
> > it would all be run off a small motorcycle battery.  Was just
> > going to use some resistors (like 20 ohm for the injectors),
> > and hook up a ignition module with a 20 dummy load on it also.
> >   If this is unknown ground can anyone give me a list of things
> > that might go wrong.
> >   I can almost smell the smoke now
> > Bruce       Glad I scotch guarded all my Cone Shaped Hats 
> >                  the other day.    
>