O2 sensor (Ford)

[M. Brede Moe]

It has been interesting to follow the O2 sensor thread. I've got a
problem with my '89 Ford using the 3-wire type HEGO sensor and the
EEC-IV that perhaps some of you are able to comment.

Earlier this year, I replaced the sensor because of a code 41 (always
lean). This because I coudn't find any other plausible reason for the
error, the sensor was just black from soot and did not have the
miscolored tip indicating some kind of contamination. Worked fine for
some months, now I am back with the code 41. The 'Check Engine' will
start flashing on each trip after engine warm-up.

This system depends on signal return via the exhaust system, which
obviously makes room for errors if you have bad electrical grounding.
When measuring the sensor signal with a high-impedance analogue meter, I
occasionally see some negative swings of the needle. I also measure some
.25V between battery negative and the sensor casing.

My question is really if these kind of voltages in the ground path can
actually kill the sensor?

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.

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

The 3-wire (Bosch) replacement sensor from Ford cost about $200 here,
which makes it more economical to pay for the extra gas in open loop
mode rather than replacing the sensor 3-4 times a year... The exhaust is
still within spec's.

Regards,
M. Brede Moe
-- 
- mamoe at aft.sn.no