Testing ECU/wiring, unsure about some voltage...
 

When expected voltage is below 1.5V, and my multimeter reads about 2.5 mV, is that normal, or is that too low? Reason I ask is that when it says expected voltage should be 0V (as in ground) and it reads 85mV, then I don't really understand the difference between "below 1.5V" and "0V". Should 0V be actually no voltage at all? Or is my "below 1.5V" voltage too low?

well is 2,5 milivolts less than 1.5 volts, but not less then 0 volts?

Yeah, but 85mV isn't less than 0V either?

As said above, if it's in milivolts, ignore it on the whole UNLESS it's the O2 sensor input that is being looked at.

Just what item are you looking at ???????? What wire in the ECU plug?

I'm looking at everything, checking whole ECU harness on my S3. Want to determine if anything at all is bad. O2 sensor actually shows about 0.4V, even though it should read 0V. Maybe I didn't warm the car up enough.

Pretty much everything that's marked as "below 1.5V" or "0V" in the FSM is reading in mV. All the other stuff is close to the numbers mentioned in the FSM, except for the pins related to the switching and relief solenoid valves, since I don't have ACV and they're disconnected. And lastly my neutral switch is sending "in gear" voltage - I don't think I even have a neutral switch, 12A tranny.

O2 Sensor output being 0.4 vdc is normal for a car without an airpump/acv.

NORMAL series four/five cars 02 will read close to zero vdc BECAUSE the airpump air goes thru the ACV to the EXHAUST PORTS

Could that skewed reading on O2 sensor confuse ECU and cause poor mixture ratio?

incase you didnt know, althoough it seems like you do, s3 is 84-85 GSL-SE( the only one with an ECU) s4 is 86-88 and s5 is 89-92. an S3 of coarse is a 1st GEN.

Yeah, since GSL-SE is fuel injected, it uses ECU in a manner similar to S4. Actually carbed FBs have an ECU as well, it's just different and it's purely for emissions and it's called Emissions Control Unit instead IIRC.

So are there any actual methods for testing the functionality of the ECU itself? Or should I just pull it open and do a visual inspection?

K, I popped the ECU open, looks damn fresh on the inside. Probably not a source of any problems. Hmm...

Not at all. The 02 sensor only interfaces with the ECU when your actually driving the car. Or put another way, the car only uses the 02 sensor when driving and then it will let the ECU go into what they call closed loop to bring the afr down to the 14.7afr range and only then IF your more or less cruising along. Starts into closed loop at approx 1500rpm if crusing and goes out of close loop when over approx 3500 rpm while crusing.

Foot to the floor and the 02 has no influence. Idle the car in the driveway and the 02 has no influence.

Each pin on the ECU can be checked using the FUEL AND EMISSONS section of the FSM, subsection called CONTROL UNIT. That section shows both INPUTS and OUTPUTS of the ECU.

Mixture at idle can be changed on a series four using the Variable Resistor. This VR has NO influence while driving the car.

Why are you suspecting fuel mixture problems??????? I almost forgot, I usually see afr/voltage outputs from the narrow range 02 sensor, of about .7vdc or .8vdc when at idle with the engine fully hot and no airpump/acv working on the car. You seem to see 0.4vdc which is real close to 14.7afr and to me that is a bunch tooooo lean at idle. Idle being 750-850 rpms more or less.

Cool. I was testing it with the car warmed up, but turned off.

Some results are with key to just ON, engine OFF and other results with engine at idle if I read the page that is attached correctly.

oh, for the love of crop! :D