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.
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.
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.
INSPECTION & REPLACEMENT
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.
To check the oxygen sensor, connect the dwell meter to the pick-up and ground and run the engine to normal operating temperature. Start the engine and wait 10 seconds. If the oxygen sensor is defective (or cold), the duty cycle will be 54° for the B21F and 40-50° for the B27F and B28F.
The late model non-maintenance type sensors can only be checked by measuring emissions at the tailpipe with a CO (emissions testing) meter. If the oxygen sensor in the manifold is obviously damaged, it can be replaced by simply disconnecting it and unscrewing it from the manifold. When installing a new sensor, the threads must be coated with anti-seize compound. The unit should be tightened to 40 ft. lbs. (54 Nm). Use care in handling the oxygen sensor and its wire; it can be damaged by impact or dropping
On all gasoline powered U.S. and Canadian Volvos through the 1983 model year, the oxygen sensor must be replaced at 30,000 mile (48,309 km) intervals. For 1984, Canadian models with the B21A engine were exempt, but all U.S. models require a new sensor every 30,000 miles (48,309 km). In 1985, the only models requiring replacement were the 760 GLE (B28F engine) and the 240 Turbo (B21F-Turbo engine). The 1986 760 GLE (B28F engine) was the last to require sensor replacement, again at 30,000 miles (48,309 km). All other 1986 and later models have sensors which are considered "life of the car" and do not require replacement unless they fail to function.