Volvo Coupes/Sedans/Wagons 1970-1989 Repair Guide

Oxygen Sensor Feedback System


See Figures 1 and 2

This is a self-tuning engine control system (sometimes referred to as the Lambda Sond system), designed to reduce emissions and improve fuel economy. An exhaust gas sensor (oxygen sensor or lambda sensor) is located in the exhaust manifold and monitors the composition of the exhaust gasses leaving the engine. This analysis is fed into a closed loop feedback system. By reading the oxygen content of the exhaust stream, the system continuously adjusts the air/fuel mixture to provide optimum conditions for combustion and efficient breakdown of pollutants by the three-way catalytic converter.

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Fig. Fig. 1: Oxygen sensor location-B21 and B23 engines

The major components of the system are: the oxygen sensor, the electronic module and the frequency valve. The oxygen sensor is a platinum coated ceramic tube. It is located in the exhaust manifold. The inside is vented to the atmosphere while the outside is connected to the exhaust gas flow. The output from the sensor is fed to the electronic module. This device supplies a control current to the frequency valve.

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Fig. Fig. 2: Location of the B27F oxygen sensor. The sensor is mounted above the catalytic converter on B28F models

The frequency valve alters the flow of fuel in the injection system by activating a diaphragm in the fuel pressure regulating valve. By altering the air/fuel flow in the system according to the signal received from the oxygen sensor, the frequency valve keeps the air/fuel mixture within the narrow band needed to allow the three-way converter to operate efficiently.


See Figure 3

The frequency valve (so called because it operates on a set frequency) functions during what is called its duty cycle. The duty cycle corresponds to the ratio of closed-to-open circuit impulses from the electronic module. The cycle can be measured in degrees by using a high quality dwell meter which reads up to 70° or more. The dwell meter is connected to an instrument pick-up connector located on a wire coming from the electronic module. The pick-up connector is located on the firewall, in the engine compartment to the left of the master cylinder. The dwell meter should be attached to the pick-up and ground.

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