I cracked the ECU box for the first time today. I feel like I've deflowered my car. :-P

Now, I'm kinda apprehensive when it comes to expensive and fragile computer modules, and as you might remember, I will have to tap into the O2 sensor wires. My question is, how do I gain access to the wiring harness, and what safety precautions must I take? I don't want to fry the ECU. I guess, you removed the ECU somehow to take this clear picture? Right now, the wiring harness is almost completely obscured.

And another issue. The little meter that I'm installing can be operated with voltage between 5 and 12 V. Of course, operating voltage may be as high as 14.5 V in our cars. Should I use a stabilized power source?

<img src="http://audi12v.home.comcast.net/pix/O2signal.JPG">

Gotta give credit where credit is due. ;-)

After you remove the cover pry up the metal band over the ecu and slide it out. You should be able to access the wiring harness now.

As for your last question, I hooked my meter to one of the posts below the relays behind the driver's side kick panel. Forgot which one off the top of my head but I *think* it was the one labled 75x.

Let me know if you have any more questions. :-)

I forgot you just borrowed it. ;-)

I used terminal 15 as ignition (usually green wires) power source. So, do you think 14.5 V will fry the meter?

For now, I'll just pry up the metal band and start looking for the sensor wires. Thanks!

off the black "tape" from around the wires.

Was the meter designed for automotive applications? If so, I wouldn't worry about it.

but only with the ignition on, so the voltage was around 12, but not higher as will occur with the engine running. The article in the magazine that first mentioned the meter, didn't say anything about using a stabilized power source. I'm pretty sure it's fine without...

Two are slightly lighter than the other two. And there is one green wire with a black tracer (or what it's called). In UrS4's pic it looks like the two light green wires are the ones I want to tap?

<img src="http://pictureposter.audiworld.com/50038/o2_wires.jpg">

is going to know which other wire is the one you need to tap. Common sense would dictate your assumption correct. However 5 minutes with a voltmeter and a pin-point probe would confirm continuity across either O2 sensor signal wires and the correct wires within the ECU harness. Remember we're all taking signals from 1 sensor and not two. So at this point you're the pioneer. You'll have to do the groundwork finding the second sensor signal lead.

I had 3 light green wires and the colors were so close you could barely tell them apart.

So you will confirm that you tapped into the lighter green wire which is one of the two O2 signal leads? I suppose I could start peeling at the O2 Sensor connectors and check wire color there.

If I want to probe the wires with the ECU connected to the battery, I'll need a better meter with decent probes. I guess I'll fly by the seat of my pants with the risk of crashing and burning, yay! ;-)

Lady luck's on my side. ;-)

no longer the same size or color wire as they are now all smaller for fitment into the ECU harnesses.

I've got a strong gut hunch it is indeed the other light green one you've mentioned. But at the same time my gut wouldn't support such a risky move as hooking it up and risking a $1k mistake.

Use alligator clips and straight pins if you don't have a fine enough set of multimeter probes. And you won't need power to check for continuity. All you're looking for is the signal wire coming from the sensor.

When I was walking MrD thru this I told him of the "light green" wire. That gave him literally two choices. The odds at-best were 50/50 he'd get it right so my sense of logic tells me the other light green wire is very likely the other sensor signal lead or I would've been more specific with him. I'm sure I knew for sure at the time I was telling him. But time has clouded my memory and I'm just not confident enough to recommend you go for it un-tested.

You could always hook a t-tap into it and the O2 sensor wire(s) to confirm with your MM then remove the t-tap if it's not showing continuity and wrap a little piece of vinyl electrical tape around where the t-tap cut thru the wiring insulation.

No need for both of us to get our hands dirty. I'm going in now! :-P

Trust me, I really don't want a new ECU. ;-) I'll t-tap.

when I get home.

I wouldn't mind getting that email, though.