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ENash (Computer)
20 Oct 05 14:18
I have a 1996 Ford Aspire. A broken spark plug caused the spark plug wire to fall off and produced misfiring. It upset all my settings and possibly contaminated my oxygen sensors. I have replaced the unheated oxygen sensor with no correction to the 20% loss in gas mileage that has occurred.

I do not claim to have a full understanding of all of the terms but will use them as I know them.

For background, I provide the following. I have bought a 4-wire universal heated oxygen sensor(down stream). The purchased sensor has 4 wires which are white(heater), white(heater), blue(signal), black(ground). The information in Chilton's manual shows that the corresponding Aspire's wiring does not agree with the locations indicated as appropriate for new sensor's description of the Ford/Mazda connector wiring for the vehicle. In addition the Chilton's manual says that one should backprobe the sig rtn which would correspond to the ground wire to measure the output voltage. This is a common connection to other sensors according to the wiring diagram and I believe it is a common signal ground.

In addition, the colors of the wires do not agree with the Chilton's wiring diagram. At the wiring harness they are correspondingly physically B/W, B, O/B?, Y/G. The electrical diagram shows O/W(htr pos), B(htr grd), B/W(vpwr), Y/G(sig rtn). Y/G is clearly the common ground on a number of sensors on the diagram and I assume from this that it is correctly sig rtn as I believe that sig rtn is the same as "sig -".

To simplify the above I can state that I have verified that the first two wires are the heater wires. There is a 6.8 ohm resistance on the oxygen sensor which is about right. This leaves only the other two in question. If backprobing only the sig rtn gives the 0-1 voltage range, then I believe that the signal wire is the fourth wire. However, I am suspicious that it should have read to backprobe between wire 3(sig +) and wire 4(sig -, sig rtn). This would make the wiring agree with the information provided with the new oxygen sensor. It also would indicate that the fourth wire is not vpwr.

I have already wired the sensor with sig rtn connected to signal and the other wire connected to vpwr. I now suspect that this isn't correct but I am not sure.

Should I connect the wire 3(O/B? -> Blue) and wire 4(Y/G -> ground)?  The electrical diagram shows O/B should be O/W and if so comes directly from the PCM and the Y/G is a common return for sensors(throttle position, EGR, etc) as well as the oxygen sensor.  Notice the electrical diagram seems to be wrong about the heater circuit wires.

I do plan to check the voltage as a precaution to make sure that vpwr doesn't actually have any voltage applied.
   
ENash
TobyHarris (Automotive)
21 Oct 05 4:16
I can't help much with the woring but I would recommend disconnecting the battery for 30 minutes or so to clear down the PCM and allow it to "relearn" and adjust its internal maps  - its worked for me in the past.

Despite what I read about the Ford PCM only taking a few miles to "relearn" I have found it normally takes a few hundred miles.
ENash (Computer)
21 Oct 05 9:48
I have disconnected the battery for 15 minutes before testing mileages and also have reset the PCM once using a borrowed scantool. This has not helped.

I decided to rewire the oxygen sensor and install it. I made wire 3 connect to signal and wire 4 to connect to ground as I think that that is the only thing that makes sense. I did not find vpwr on wire 3 suggesting that the Chilton book is wrong. I still have to get a connector to attach to the car before I can start driving again. I plan to disconnect the battery again. I have turned on the engine and did not get a check engine light which is encouraging but may have no bearing on mileage.

The previous tests were only driven for about 120 miles but I religiously followed the rules on reteaching the PCM by driving at different speeds for specified times.  
carnage1 (Electrical)
21 Oct 05 23:27
if you wre missfiring for a long while your catalytic converter could be clogged and creating excessive backpressure or you oculd have fowled an injector if you have port injcetion. also did you replace the wire, the spark plug, the distributor cap, and the rotor? if you missed any of those you might still be missfiring. it is also conceptually possible you damaged the coil and all cyl. have weak spark. as for the wiring you need power to heater and ground to heater(not the one directly from the ecm). you need referance ground (directly from the ECM) and you need the output of the o2 sensor. the output of the O2 sensor is a positive voltage. wire in everything except the wire you think is the sense and check it with a voltmeter in relation to chassis ground. if it reads a positive voltage that is less than 10V it is sense. if you get a negative voltage it is the ground to the sensor. if you get more than 10V it is part of the heater circuit and if you get direct continuity to ground it is part of the heater circuit. hope this helps cause that was a lot of typing.

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