f2racer

iTrader: (1)

I've had an Autometer A/F Meter tapped into the O2 sensor wire (3C) into the ECU for about 2 years. About a month ago or so, it read consistently leaner that it has ever done before. Up to this point, I was using one of the little blue "spade" type connectors as seen at David Disney's site. I figured the connector had gone bad or something, so I cut the wire altogether and wired it back together with a generic Radio Shack butt connector. The next time I drove the car, the A/F Meter didn't read a thing, with the exception of starting the car when it does it's usual "sweep". Otherwise the thing was completely black the entire drive.

Figured something was definitely wrong and broke out the old voltmeter. I measured the voltage straight from the O2 sensor (at the connector in the engine compartment, attaching the red to the connector and grounding the black) and got a reading of .537V with the car off, but ignition on. Went to the ECU wire and it read -.425 (negative reading)?!?!?! Went back to the engine and got .536V (positive reading)!?!?!?

WTF? Is the O2 sensor a straight wire right into the ECU, or does it go through some other "stuff" before getting into the ECU? Why does one read positive and the other negative (does something invert the signal)?

The was directly against the wire with the entire ECU connector I tapped into unpluged, I chucked the butt connector. If the O2 sensor is wired directly shouldn't I see the same reading from the two locations? Could grounding be bad somewhere on the car?

I proceeded to re-wire using flux and solder. Didn't have time to test drive it to see if it makes a difference... Can anybody explain what I'm seeing?

Thanks!

wstrohm

f2racer,

My manual says the wire from the O2 sensor to the PCME (ECU) terminal 3C is a black, shielded wire with a gray jacket. When testing at the PCME end of the wire were you on the black wire, or the shield? The voltage should read the same at both ends; there is no processing in between the sensor and the PCME. I cannot account for your negative reading. The positive reading at the sensor should be a function of exhaust (i.e., O2 sensor) temperature. On a cold engine the output voltage should be approximately zero volts, with the voltage increasing as the sensor heats up. With the engine running a constant rpm and fully warm, a scope trace of the voltage should show a sinewave-like voltage funcion with a period of about 3 seconds, with the voltage varying from about 0.4 volts to about 1.0 volt.

When engine is warm, under acceleration the voltage should be "0.5 - 1.0V" and under deceleration the voltage should be "0.0 - 0.4V."

This info is from page F-158 of my Mazda RX-7 1994 Workshop Manual.

---- Bill

J.S.J

It sounds like you were reading the voltage drop between ground and the 02 sensor shielded wire near the ECU. The volt meter will read in a negative state in that case. You need to get past the shielded part and in to the 02 signal.

f2racer

iTrader: (1)

I was testing at the "black" wire which has a connector into the ECU. I'm positive it's the right wire. Pretty sure the wire was not shielded though. If it was, I probably wasn't testing the right thing and soldered the freakin' thing wrong!

What pisses me off is that the darn A/F meter was reading right just a month ago, of course I had to go and screw around with it. Well at least the car is seeing leaner than it should...

When i was testing with the voltmeter, I did it right after driving, so the sensor was still likely hot, which probably accounted for the positive voltage coming straight off the O2 sensor. I guess I've got a little more diggin to do.

Thanks!