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Mikael,
Wow, that was quite some letter, especially for someone who doesn't think their English was good! Your English is a lot better than my Danish.
Anyway, hopefully I can make some sense of all this.
As you said, the O2 sensor only has three wires. Two of those wires are for the preheater - +BAT and Gnd - and if you measure +BAT and Gnd, you don't have to worry too much about those ones.
The third wire, the thick green one, is the important one. You already found out it is "shielded"; in other words, it is a wire totally surrounded by another conductor. This type of cable used to reduce noise pickup on low-voltage signals, and it is commonly used on our cable TV systems in North America, as well as DS3 (E3 in Europe) telecomm signals. The outer shield is grounded at the LH computer (to Pin#5), while the inner conductor carries the signal from the O2 sensor (Pin #24).
If you used an 'insulation-displacement' connector - the kind that look sort of like a 'V' and cuts through the insulation and grabs across the wire - you most likely shorted out the shield to the signal conductor. That is not good. It would be like putting a nail through your tv cable wire. Pull the connector from the green O2 wire, and measure the resistance to ground; it should NOT be 0 ohms.
You must remove that connector, and make sure the outside shield does not touch the inside conductor. You might have to cut off the insulation around your 'T'-connector with an Olfa knife (hey, isn't that a Swedish invention?), and check for a short circuit. Since you made your connection at the computer, you will also have to make sure the shield is still intact all the way back to the connector in the engine compartment. If it isn't, the low-voltage signal from the 02 sensor will get lost in all the noise.
As for the reference signal, it comes from the LH computer. You can only measure it when the O2 sensor is disconnected. Remove the connector for the green wire in the engine compartment, and turn the ignition on; you should see 0.5 volts. In fact, if you connect the Halmeter up to the green wire while the O2 sensor is disconnected, you should read mid-scale (Lamba=1).
When the connector is back on, and the engine is running, the signal should fluctuate between about 0.2 and 0.9 volts (0.2 is lean, and 0.9 is rich - the voltage swings should average out to 0.5, which is Lamda=1); it should not stay at 0.5V.
Disconnect the Halmeter for now, and make sure you get the correct signals from the O2 sensor.
You're right - if your O2 sensor wasn't working up til now, you wouldn't have passed any emissions test.
-- Ottawa, Canada
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