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O2 weirdness

JJacobs

NAXJA Forum User
Location
Fort Collins CO
This is on a 96 4.0. Throwing codes for high signal voltage both front and rear O2. Checked with a scan tool, both are indeed reading 1.0V steady, computer is disregarding and forcing open loop. Unplugged both sensors pid still reads 1V. Backprobed pins A24 and A25 to find 4.2V. It does make sense that the pid only goes up to 1V as that's the max that would normally be read.

So 4.2 is pretty close to 5V vref, checked and no continuity to pin A17. With C101 unplugged the circuit drops to 0V, however pcm power, ground and vref all come from that connector. So I removed the vref pin from the socket, reattached C101 to the pcm, still 4.2V. A junkyard pcm has been tried with no affect. The signal wires were ohmed out from pcm connector to o2 connector, no shorts to adjacent wires, the circuits check out. With C101 unplugged, inducing 12v to A17 does not cause voltage to either A24 or A25.

Working through the pinpoint test it says if under 4.95V (damn near vref) to replace the O2 and if 4.95 or over to check for a short to vref. I am indeed under that number, but not by a lot. Checking on iatn.net found mention of the pcm shooting 4. something volts down to the o2 to check the sensor. I can see this happening on the heater side, but not on the signal wire.

Anyone run across this before? It's probably just the sensors, they are both junkyard pieces as far as I know.. just hung up on why the pcm seems to be shooting 4.2 volts down the signal wires.
 
Hallo. Are you not mixing up two kind of O2 sensors?
There are Zirconium and Titanium sensors.
Zirconium ones are generating Voltage by themselves and Titanium need 5 volt from the ECU. Both will give a Voltage between 0.1 - ! volt.

'92XJwim
 
4.2 is probably the open circuit float voltage of the signal input. I bet if you tie it to ground with a 100 ohm resistor or so, it will go down to a normal-ish value. I'd suspect an open circuit in the harness between the PCM and the O2 sensor... you checked for shorts, but did you check for opens?

Also, I wouldn't be surprised if it actually does send 4.something volts down to the O2 sensor via the signal wires. In an 87-90 XJ apparently a resistive titania O2 sensor was used instead of the more common voltage-generating zirconia sensors, on these model years I would actually expect to see that kind of operation under normal conditions. I can still see it happening during a system test on other model years though.
 
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