O2 sensor (Ford)

[RABBITT_Andrew at mv8.orbeng.com.au]

>It would also be interesting to know how the sensor signal is treated
>inside the EEC-IV. The shop manual says that pin 49 is HEGO return,
>which is connected to engine ground externally. Does anyone know how
>this is related to chassis ground or battery negative internally? If 
>pin 29 (HEGO input) and 49 makes up a differential input, it would be
>tempting to replace the sensor with a 4-wire unit.

The EEC-4 in some European Fords use a 4-wire sensor.  I can't quote 
pin #'s off the top of my head but I can't see why this wouldn't work.

>
>Are these sensors mechanically and electrically compatible except for
>the extra wire?

Mechanically yes, (they will screw into the same hole).  Electrically, 
most probably.