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Failed Oxygen Sensor or ECU? 200 80

posted by  someone claiming to be Dave Barton on  Sun Nov 12 05:51 CST 2000 [RELATED]

I'm fixing up an '80 242 DL and I do not know the cars history. I have found that the oxygen sensor, when hooked up, is not properly cycling and the car is basically running rich (I think). I believe the sensor works OK since it produces varied voltage (between 0 and 1 volt) when unhooked from the ECU. I tried another used sensor and it too varies in voltage, but does not cycle when plugged in. I am thinking that the ECU needs to be replaced, but before I buy another one from a salvage yard, I thought I would seek out any of you who have come across this before. Thanks







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    Re: Failed Oxygen Sensor or ECU? 200 80

    posted by  someone claiming to be abe crombie on  Sun Nov 12 07:18 CST 2000 [RELATED]

    The easy quick test for the lambda control unit is to warm up engine to normal temp and then unplug the O2 sensor from the wire at firewall. Now grab the wire on harness with a bare clean hand and then touch clean metal for a ground with your other clean bare hand. This should make engine run rough as frequency valve duty cycle changes (can hear this as frequency valve buzzing getting more intense) to enrich fuel mixture.

    Now grab battery positive post (won't shock you) and the engine will run bad again as the lambda control unit leans mixture, frequency valve duty cycle will get less and the frequency valve will get very quiet.

    If these two range tests are successful then the lambda system is okay and the base CO adjustment should do it given that you have determined that the O2 sensor does range 0 to 1.0 V.







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      Re: Failed Oxygen Sensor or ECU? 200 80

      posted by  someone claiming to be Dave Barton on  Sun Nov 12 10:52 CST 2000 [RELATED]

      Abe,

      I have to say I had my doubts about this "sci-fi" Lambda ECU test procedure. I've been a home Volvo mechanic for about 10 years and this was a new one for me (never too old to learn though). I performed this test and got no reaction out of the B21F motor... no change in idle or in the frequency valve (it is running very constant and fairly loud at all times). I then did this test on my 245 Turbo to see how subtle the changes should be and it did register changes exactly as you pointed out.

      As Roger mentioned the temp sensor (in my case all there is besides the gauge sensor is a thermal time switch under the #4 cylinder intake port) could cause this if is it disallowing ECU operation because it thinks the car is cold. Is there a way to test this sensor in the car (probably not). I suppose I could pull it and check resistance while heating it with a heat gun.

      As a clarification, I believe the car is running rich because it smells that way... no smoke or rough running. Actually it's running very nice, but I do want it to pass a smog test so I can register it.







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        Re: Failed Oxygen Sensor or ECU? 200 80

        posted by  someone claiming to be abe on  Sun Nov 12 11:47 CST 2000 [RELATED]

        The thermal switch that sets duty cycle to a fixed stting until warm

        (120 F +/-) isn't used on 4's in '80 (it was used on 6's in '80).

        The next thing to check would be the O2 sensor circuit into ECU. There should be .5 V on the sensor lead when the O2 sensor is unplugged. If not then go inside and check the lambda control unit plug for corrosion.

        It wouldn't be a bad idea to clean the grounds on side of intake for the lambda system. These shouldn't be on the same bolt according to an ancient service bulletin.







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    Re: Failed Oxygen Sensor or ECU? 200 80

    posted by  someone claiming to be Roger on  Sun Nov 12 06:17 CST 2000 [RELATED]

    Hi Dave,

    If I'm remembering correctly, your '80 242 has the K-Jet with Lambda for fuel injection. It modulates the mixture by modulating the frequency valve. The first basic sanity test - the frequency valve should be buzzing. If so, the ECU is getting some sort of input (maybe not correct) and generating an output. If so, I would check other things before getting a new ECU.

    Other things that could cause it to run rich:

    Temperature sensor. The ECU has a temperature sensor (separate from the gauge temperature sensor) that disallows ECU operation if below a certain temp. If the temp sensor is failed, ECU may never start working.

    Control Pressure Regulator: If your control pressure is wrong, it could cause the car to run rich.

    Fuel System Pressure: If the return path is blocked, the system pressure will be too high which can cause problems.

    (I apologize if I'm a little vauge here - it's been a few years since I worked on a K-Jet car...)

    Roger






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