Bench ecms

[T Hergen]

I'd try tying the two ecms power supply grounds together at a single point
(keep the ground wire between them short)  and leave the sensor grounds
unattached.  Tying the grounds together at multiple points results in
ground loops which may cause problems.  If you do tie the grounds together
in multiple places, make sure you like the resulting problems better than
the ones your solving:)

The couple tenths of an ohm in resistance should not make much difference
since the sensors draw very little current (for the TPS pot: 
I=V/R=5v/5k=5ma). 5ma across 0.2 ohms isn't much.  So, the sensor ground
should be very close to the same potential as the ecm ground.

Measure the voltage between the two ecm sensor grounds and the two sensor
+5v supplies.  Any significant difference means the two ecms may come up
with different answers for the same sensor. 

Since you're driving LEDs, not high current loads like injectors, the
ground potential of each ecm should stay close.  This is a good thing
since it's then more likely that both ecms will come up with the same
answer for sensor voltage measurements.

That's my opinion.  There are other ways of doing it.

Tom


On Thu, 6 Aug 1998, Bruce Plecan wrote:

> I had been told the sensor grounds on a gm ecm floated in reference to the
> chassis ground, well that doesn't appear to be correct, everything is just a
> couple tenths of an ohm from true ground.
> With no displayed voltage drops across grounds.
> While still seperated, does it really matter if sensor grounds go to
> chassis, on an actual car?.    If so then where Have I strayed to having
> had sensor ground problems????....
> 
> 
> For coolant temp, and intake air temps, I was just going to share
> resistances.  Tieing grounds together.
> 
> For TPS, and MAP I was going to tie the grounds together, use the
> +5 from one ecm, and then share the sensor's output.
> YES/NO??????..
> 
> Cheers
> Bruce        RPM signals applied to both ecms, and powered up
>                   leds flashed and no problems today.
>                   The cat has grown boarded with the flashing LEDs,
>                   but the staff is acting like their inna stupor.
>