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    Oxygen Sensor Earths - The theory explained Archived Message

    Posted by Mike Walker on April 19, 2007, 8:43 am, in reply to "Oxygen sensor earth?"

    I hope the following helps.
    Every earth point on a motor vehicle is at a slightly different voltage due to the various current return paths used by all the other equipment on the vehicle and will also have interference on them as the current flow to each component changes. This means that using the body as an earth return produces errors in the measurement of the lambda signal and also the lambda input circuitry in the ECU needs to be filtered to remove the interference pulses. This slows down the response of the lambda system.

    There are three ways used to avoid these problems.

    1. Earth the Lambda back at the ECU and not at the vehicle earth point this way the ECU can measure the exact voltage without errors.

    2. Earth the sensor at the body but take an additional wire from the same point back to the ECU so it can then compare the difference between the two wires and cancel out any volt drop effects. In this case the second sensor wire is a 'reference wire' and NOT an earth. (e.g. Rover)

    3. Do not earth the sensor at all , just take the two wires back to the ECU which then monitors the difference between them. In this case the voltage compared to the vehicle earth is irrelevant. (e.g. some Fords)

    Mixing these systems and trying to use so-called 'universal' sensors causes these problems. We get ECUs of types 2&3 sent for test with a fault 'Lambda Earth Fault' because the sensor has failed or the wrong type fitted. Also we find people try earthing the sensor in type 1. This means that a proportion of the starter motor return current is carried by the ECU lambda earth terminal during starting often leading to seriously damage to the ECU. Also watch out for some cars (notably Peugeot/Citroen) where the sensor type fitted to a particular vehicle model may vary depending upon the Engine Management system fitted e.g. Bosch systems using one type and Marelli systems another.
    I hope this helps everyone rather than makes matters worse.

    Link: http://www.atpelectronics.co.uk

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