Disabling the O2-sensor
 

My question
Why is it not possible to have the lambda-box disregard the O2-sensor by unplugging the wire to the 15 degree switch? This is on a '80 SC with CIS.

Background
Earlier this fall I had a problem where the engine began to run poorly when reaching operating temperature. After some troubleshooting it turned out that the duty cycle went to 98% as the temperature went up. My suspicion was a faulty O2-sensor (or a temp related vacuum leak) so I wanted to disconnect it to see how the car ran without.

Since the sensor has been removed by a PO and then reinstalled by me there is no connector in the engine bay that I can use to disconnect it. But, knowing that the system only goes to closed loop when the 15 degree switch grounds (right?), I figured that I could make it stay in open loop (thus, disregarding the O2-sensor, right?) by removing the wire to the switch (and covering the wire with some tape). That, however, did not make any difference.

I eventually disconnected the O2-sensor by getting the O2-sensor wire out of the big black connector at the lambda-box which made the car run well again. I now have the time to do a proper troubleshooting but I'm still curious as to why the sensor still mattered with the 15 degree wire disconnected.

Thanks for reading!

/Björn

CHT sender needs to be working. you also need to check control pressures and set the mixture for all this to work right.

if someone removed the O2, i dont see why they would remove the wiring or the connector it plugged into.

The O2 sensor will improve your gas mileage. I am willing to bet that your problem was not the O2 sensor itself, but the O2 sensor RELAY. The relay is the silver cube that sits next to the ECU. It's a cheap fix.

The O2 sensor itself doesn't "ground". After it heats up, it produces a voltage in the range of .1-1V. This voltage indicates whether the mixture is too high, too low or just right. Remember that it is a narrowband sensor. It's basically a yes/no kind of input to the ECU.

When the O2 sensor is disconnected it runs in "open loop" which means that the ECU assumes a default duty cycle. As you're driving with the O2 sensor connected, the ECU is making slight corrections to the mixture based on the information received from the O2 sensor.

Ketronic not Motronic......
 

Ty,

You probably missed it. The OP has a CIS engine (Lambda) and no CHT. He has ECU not DME.

Tony

Yes, I'm aware the the sensor itself doesn't ground. What I was attempting to do was to keep the system in open loop by removing the signal that the engine is warm, namely disconnecting the 15 degree switch. My question was why that didn't have any effect.

I guess it could be the relay but apart from the high duty cycle it runs fine, also I've ran it with a broken relay before and then it ran much worse.

The PO tried to switch to an earlier model CIS but was unsuccessful. I then reinstalled the O2 sensor and built a new harness (as the PO cut the old one to pieces) to make it all work again. At the time I didn't see the point of having the option to disconnect the O2-sensor so I ran the wire without a connector.