Re: UrQ FTCU ground on the intake - What the #@$%.

[ejfluhr@austin.ibm.com (Fluhr)]

Lucky ur-q owner Andrew Finney rants:
>Question: WHY THE $%&* DID AUDI GROUND THE FTCU
>ON THE MANIFOLD? Did they think "Lets run the most
>important ground wire on the car through a small gauge wire,
>through the firewall to a very hot and hostile environment where
>it's bound to fail and cause the owner much inconvenience and
>potentially kill him/her"

Because the intake manifold provides the best (?) compromise of a 
good reference for the "ground" or "lower-potential" voltage point of
the oxygen sensor.

It is cool to hook up a DMM to the oxygen sensor from inside the car,
using the dash wiring harness for a ground refernce, and running a wire
through the shift-rod tunnel to the oxygen sensor.  Due to wiring
resistances and poor contacts in the dash wiring, you might get to see
negative voltages on the DMM when in off-throttle deceleration.  BTDT.  :-) 

>Solution - Can we just create a new power source and ground
>using audio quality distribution blocks and connectors and
>ground the whole mess to the body? Is there a reason for
>grounding on the engine? Is it safer? Is it less susceptible to
>reverse voltage spikes (did I make that up?)

You can probably do this, as long as you have good quality (and preferably
multiple) ground straps from the engine to the body, and you ground the ECU
at the body at one of these straps.  On some (many?) car these ground straps
are left off by people who do not understand their use.  If the ECU sees the
same "ground" that the oxygen sensor sees (e.g. intake manifold ground point),
then it doesn't matter if the engine is really at +0.05v higher potential
than the negative battery of the terminal.  If, however, the ECU sees "battery
ground" and the oxygen sensor sees "battery + 0.05v ground", then the ECU 
will run the engine just a tad too lean (the ECU thinks the oxygen sensor
is really putting +0.55v out when the oxygen sensor is trying to tell
it +0.5v, a.k.a. ~stoichiometric).

Later,
Eric
---
Eric J. Fluhr                                Email:  ejfluhr@austin.ibm.com
630FP Logic/Circuit Design                   Phone:  (512) 838-7589
IBM Microelectronics Div.                    Austin, TX