I spent this weekend fixing the O2 sensor in my RX7 (TII, 87, 155K) and here are the findings (standard disclaimer applies). The problem was that the wire between the O2 sensor and loom got broke at too places, one near the loom (about 1" long) and the other at its end near sensor. A year a go I 'fix' the break near the loom using a spade. Today I found that this wire is actually shielded (it shows in the wiring diagram, and you can verify it by carfuly examining the cut wire) and that the spade I connected just shorten the shield (ground) and the signal. (About 6 month ago I visited Sunnyvale Mazda for another issue and the slick guy at the front desk came back and about my o2 sensor and who 'fixed' it but he did not tell me anything about the short circuit). LESSON #1: the wire from the loom is shielded ! if you naively reconnected it, chances are your close loop is not functioning. Mazdatirx have an O2 for about 150$ and local Kragen has one (Bosch) for less than 30$ so I decided to go with the Bosch (#11025) and as afar as I can tell, it works just fine. I also had to get an O2 socket and found it at Kragen with not problem (around 5$). LESSON #2: Bosch O2 sensor (seems to) work just fine. (Note: the Bosch sesnor comes with anti seizing grease, if for any reason you use your own grease, make sure it is 'O2 sensor safe' and that it specified for high enough temperatures. Also, the wire connected to the O2 sensor gets very hot so use a wire that can sustained high tempertures (I used the one from the old sensor)). I also monitored the signal from the O2, when is connected and disconnected from the ECU. I used an oscilloscope (with 12VDC/110AC convertor) and DMM (TekDMM150, very handy, about 60$ at local store). Note that an electromechanical voltmeter will not work because its impedance is too low (usually 20K/V or lower). With open circuit, the O2 generates DC voltage in the range 0 - 850mv. (I also got negative reading of about 100mv but this might be due to difference in ground potential and/or noise, I used a 5' non shielded wire to the scope on the passenger seat and used ground from the ignition key). With throtle released, the voltage was close to zero (+-100mv), and when accelerated, it jumped to about 750mv. I could not make stabilize it on any value in between. More acceleration raised the voltage up to 850mv (this time the change was more or less continuous). It seems that the sensor has some kind of threshold and a very high gain around this threshold so more or less it signal is binary (yes, I know that this may sound strange). (If you want to repeat the experiment, note that the o2 sensor functions only when it is hot so you need to warm up the car). Then, I repeat the experiment with the O2 sensor connected, and now it had three states. With throttle released, it had low voltage close to zero. When accelerating, the voltage jumped to 750-850mv as before. When cruising (with or without cruise control) the signal changed alternated the high and low states in a rate of about 1 Hz. Roughly, it looks like a sine wave, and I guess that this it shows how the close loop tracks the optimal level with small errors above and below it. The error in the A/F is probably small but the sensor has very large gain around the threshold and that why the output goes between the two extremes. The DMM makes a sample about once every second and the values changed all over the place, I guess it depends what was the value at the time of the sample. However, the DMM has also an analog scale that is updated five or ten times per second and on that one, I could see the one hertz fluctuation. Probably, this is similar to the LED A/F meter sold by K&N. LESSON #3, even when the close loop is on, the voltage on the O2 sensor is either high or low (with a DMM with long time constant, this might look like a stable 400mv but it isn't realy). LESSON #4, if you want a cheap A/F meter that can do many other things, get TEK DMM 150 (its main disadvantage when used as an A/F meter is its automatic shut off). This summarizes my experience with the O2 sensor. The close loop works and I am happy. I cound't notice however significant differences with the O2 sensor connected or disconnected. I hope though that it will increase the MPG. If you have any comment/addition please let me know, I might compile everything and submit it to the Turbo II site. Spelling and syntax style comments are also welcomed. Tal
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