Lambda Sensor

The Injection Minis use a Bosch Lamba Sensor Part No. 0258003193

Testing the Sensor
You need to test the sensor while it is in circuit. You also need a digital volt meter that can read milli volts.

The engine needs to be at operating temperature.

The Blue/Red and Black wires (loom end)are the sensors heater supply, this is controlled through a relay, witched by the ECU.

The grey and the green/grey wires (loom end) are the sesor signal wires. You need to strip back a little to connect the volt meter. Grey +ve, Green/Grey -ve. Best to use the clip on probes for the multi meter.

With the engine running, you should see the voltage go high to around 500mv and then drop to around 200mv. The time period should be at LEAST once a second (sensor is getting tiered at this point). The voltage numbers are not important as long as they go hi/low.

If the voltage is stuck high, using a hose pipe clamp, gently pinch the fuel supply ‘flexi’ line (orange), momentaraly, until the engine can be herd to alter tone, too much and the engine will stall. This gives a lean condition and the sensor should read low volts. The lambda sensor is working correctly and the problem lies elsewhere. If the voltage stays high, the sensor may be faulty, or there may be another problem, such as a faulty lambda sensor heater relay.

If the voltage is stuck low, using a hose pipe clamp, gently pinch the fuel return ‘flexi’ line (green), momentaraly until the engine note can be heard to alter tone, too much and the engine will stall. This gives a rich condition and the the sensor should read high volts. The lambda sensor is working correctly and the problem lies eslewhere. If the sensor voltage stays low the sensor may be faulty.

This method relies on the accuracy of the measuring equipment and the competancy of the person carrying out the work.

The best method to carry out these tests is with a diagnostic unit where the voltage reading that the ECU is using can be seen. This is the most deffinative way do determine the lambda sensor operation.

How does the ECU use the Lambda Sensor

An average ECU using a narrowband sensor will generally only use the lambda sensor’s output during two specific conditions:- (a) during idle, ie. when the engine is under no load apart from keeping itself running, and (b) during part-load conditions (which we usually term ‘cruising speed’) where the engine is keeping the car at a constant speed. These two conditions account for the majority of the car’s operating time.

A car equipped with a wideband sensor is able to usefully use the lambda signal over a wider range of operating conditions but it is still mostly utilised around stoichiometry or during lean-burn operation.

The ECU will generally ignore the lambda sensor’s output during three further conditions:- (c) when the car is accelerating – the ECU will spent much of its time deliberately enriching the mixture to avoid hesitation and to provide extra power, and (d) when decelerating or ‘engine braking’ (which we usually term ‘on the over-run’), when most ECU’s will shut off the fuel completely to aid economy.

Finally condition (e) is a special case – that of cold starting, where the ECU deliberately runs the engine rich all the time to promote quick starting and ease cold-running. Once everything is up to temperature, normal operation will resume. Depending on manufacturer, a special type of Lambda sensor may be specified that has an extra quick heat-up time which means as soon as rich running is not needed anymore, the lambda is ready for operation.

During conditions (a) and (b), the ECU is said to be operating in ‘closed-loop mode’, and in conditions (c) and (d) the ECU is said to be in ‘open-loop mode’. A correctly functioning lambda sensor will ease the transitions between these two modes.

That means the ECU will be operating as designed with accurate feedback as to the combustion efficiency. If the sensor is worn or damaged, it will not be ‘set-up’ properly prior to transition, so hesitation may occur. Also a rough or inconsistent idle may occur, or a loss of power at cruising speed.

This is because the ECU is acting upon false information from the worn sensor, which will not be a true measure of combustion. As the resulting air-fuel mixture is inaccurate, the engine will not run properly.

A wideband sensor will measure the varying mixture over a much wider range, so the distinction between closed and open loop is not a clear one. However, in most road-going vehicles the ECU does not use the wideband sensor output during rich running conditions, instead using its advantage to control fuelling during lean running conditions, which results in improved fuel economy and lower emissions.