DME clarification
 

After coming back from the shop, I'm a little confused. Under my seat, are there seperate boxes for the DME and the O2 sensor? Or does the O2 sensor plug into my DME box. If my tech says that the O2 isn't working correctly, what exactly would the problem be?

This is on a 84 euro that has added a cat. Do not know if it is the original DME or not.

Under your seat, if it's like my '88, you'll see 2 boxes. The larger one is the DME unit itself and the other smaller one is the DME Relay. There's also a round metal thingy that, I think, makes adjustments for ambient air pressure or something.

A US 3.2 has a DME, a DME relay and a round altitude sensor under the driver's seat. The O2 sensor is wired into the engine to DME harness so yes, it is directly wired to the DME. Euro cars were availble with or without 02 sensors and cats (they go together).
Depending on the DME, support for an O2 sensor can be added by using a jumper in the harness to enable it.

-Chris

is the O2 sensor necessary at all if you aren't worried about smog legal?

Removing the O2 sensor from the DME will make your idle and part throttle mixture run richer and *potentially* affect your mileage.

AFAIK, the O2 sensor makes the DME maintain a 14.x:1 A/F ratio at part throttle, or anything under 60% throttle and ~3KRPM's. The O2 sensor does not affect the mixture at anything above those points.

what's the easiest way to tell if my O2 sensor is working properly?

An O₂ sensor generates a voltage in the range 0.1 - 0.9Vdc after it is warmed up. Induce a lean mix by loosening the oil cap, the voltage should drop.

the yellow harness wire connects to the O₂ sensor heater (+) white wire, brown harness wire connects to the other the other white O₂ sensor heater (ground), the green harness wire connects to the black O₂ sensor (output voltage)