GB2320577A - Testing device for oxygen sensors in vehicle engine management systems - Google Patents

Abstract

The testing device 20 comprises a housing 22 supporting signal input means 27,29 into which a varying voltage signal from an oxygen sensor to be tested may be input, voltage detecting means for detecting the varying voltage signal input from the oxygen sensor, and indicator means 26 (LD6-LD12) for indicating the detection of the varying voltage signal by the voltage detection means. The testing device further includes control means formed and arranged for controlling the voltage level detection capacity of the voltage detection so that the testing device is capable of distinguishing between, and testing, different types of oxygen sensor which produce different maximum output signal voltages. The device also contains means for indicating if it has not been connected to the correct one of the input/outputs present on an oxygen sensor.

Description

2320577 OXYGEN SENSOR TESTER This invention relates to oxygen sensors

commonly known as "Lambda" sensors which are used in vehicle engine management systems which control fuel injection and air/fuel ratios in vehicle exhaust gases. More specifically, the present invention relates to a sensor tester apparatus for testing such oxygen sensors.

The oxygen sensor is usually connected between the vehicle engine outlet and a catalytic converter linked to the vehicle exhaust. A feedback loop from the oxygen sensor leads to an electronic control unit (ECU) which controls the fuel injectors injecting fuel to the vehicle engine. The ECU is also connected to an air intake sensor. The oxygen sensor is used to monitor the air/fuel ratios in the exhaust gases from the exhaust output of the engine. The sensor usually has essentially two output "states": a "rich" state indicating a high fuel/air ratio and a "lean" state indicating a low fuel/air ratio. The ECU uses the output signal from oxygen sensor to control the fuel/air intake so as to maintain the level of pollutants output from the engine to the catalytic converter within a desired range or "window" of effective operation of the catalytic converter. A "rich" output signal from the sensor tells the ECU to make the injected fuel/air mixture leaner, while a "lean" sensor output signal tells the ECU to make the fuel/air mixture richer.

-2A commonly experienced problem with such engine management systems is that failure or malfunction of the oxygen sensor may go undetected for some time, leading to increased emissions of pollutants from the vehicle exhaust. Such increased emissions, once detected (e.g. during an M.0.T. emissions test), are often wrongly attributed to failure of the catalytic converter, or the ECU, rather than of the oxygen sensor. Correct diagnosis of oxygen sensor failure can avoid the costly replacement of the catalytic converter and/or ECU.

It is therefore important that an effective means of testing the operation of the oxygen sensor, preferably in situ in a vehicle, be found.

Oxygen sensor testers have been developed in the past which depend on the detection of an electrical signal from the output of the sensor. These testers interpret the detection of any electrical signal output from the sensor as confirmation that the sensor is functioning. There are several problems associated with this assumption. One significant problem is that, at present, there are at least two types of oxygen sensor available on the marketplace, and used in engine management systems, which have different output voltage ranges and different electrical tolerances. Until the present invention, oxygen sensor testers have generally only been capable of testing one or other of the available types of oxygen sensor.

One oxygen sensor tester is known which has been designed to be capable of testing two known types of oxygen sensor but this tester must be switched manually to the correct one of -3two respective modes of operation for testing the two types of sensor. This creates difficulties for an operator using the sensor tester since it is very difficult to visually distinguish between the two types of oxygen sensor.

A further problem is that the Lambda sensor, in situ in the vehicle, will normally have exiting therefrom. a plurality of electrical wires connected to various other items such as a power source, heater, and electrical earth (all of which will usually be necessary for operation of the sensor) as well as the signal output wire connected to the ECU. It can therefore be difficult for an operator wishing to test the oxygen sensor to identify correctly the signal output wire. This can be a time-consuming operation involving consulting an operators' manual, and is prone to human error or guesswork.

Another problem is that merely verifying that the oxygen sensor is outputting an electrical signal to the EM does not, of course, confirm whether the sensor is correctly calibrated and/or is functioning in the correct manner. For example, the sensor may be outputting a different (e.g. lower, or higher magnitude) electrical signal than that which it is expected to output for a given level of pollutants present in the engine output. A common problem experienced with oxygen sensors is that the sensing capabilities and/or the sensor calibration may "drift" so that lower voltage output signals are generated than should be for given levels of air fuel mixture richness, thus indicating a "weaker" mixture than is actually the case.

-4It is an object of the present invention to provide an oxygen sensor tester which minimises or substantially avoids one or more of the foregoing disadvantages.

According to a first aspect of the invention we provide an oxygen sensor testing device suitable for use in testing oxygen sensors incorporated in vehicle engine management systems, the testing device comprising a housing supporting: signal input means into which, in use of the testing device, a varying voltage signal from an oxygen sensor to be tested may be input, voltage detecting means for detecting the varying voltage signal input from the oxygen sensor, and indicator means for indicating the detection of the varying voltage signal by the voltage detection means, wherein the testing device further includes control means formed and arranged for controlling the voltage level detection capacity of the voltage detection means in accordance with a maximum voltage of the varying voltage signal input from the oxygen sensor, whereby the testing device is capable of distinguishing between, and testing, at least two different types of oxygen sensor which produce different maximum output signal voltages.

one advantage of the testing device of the present invention is that the device automatically adjusts itself upon connection to the signal output from an oxygen sensor of one of at least two different possible types so as to be capable of testing that sensor. There is thus no need for an operator to determine, visually or otherwise, which type of sensor of the at least two possible types, is being tested.

------I,,- -5Preferably, the testing device is suitable for use in determining at least semi-quantitatively the operational effectiveness of the oxygen sensor tester. The voltage detection means is preferably formed and arranged for monitoring, over a nominated voltage range, the varying voltage signal input from the oxygen sensor. The indicator means advantageously comprises an indicator scale which indicates different relative values of the varying voltage signal, within said nominated voltage range, as the signal varies over time. The indicator scale may, for example, comprise a linear array of light emitting diodes (LEDs) which are connected to the voltage detection means. Each LED may be formed and arranged to be illuminated when the monitored voltage signal falls within a particular sub-range of the nominated voltage range.

Where the voltage detection means is formed and arranged for monitoring the varying voltage signal over a nominated voltage range, the control means is preferably formed and arranged so as to adjust the nominated voltage range by increasing or decreasing the ratio between the voltage detected and the level indicated, depending on the maximum voltage of the varying voltage signal input to the testing device from the oxygen sensor.

Advantageously, the testing device further includes signal delay means formed and arranged for slowing down rising and falling edges in the varying voltage signal input from the oxygen sensor to the signal input means of the testing device, in use of the device, so as to allow suf f icient response time for the indicator scale to respond to increases and decreases in voltage in the varying voltage signal. The signal delay means may be provided in the form of a low-pass filter.

According to another aspect of the invention, an oxygen sensor testing device suitable for use in testing oxygen sensors incorporated in vehicle engine management systems comprises a housing supporting: signal input means into which, in use of the sensor, a varying voltage signal from an oxygen sensor to be tested may be input, voltage detection means for detecting the varying voltage signal input from the oxygen sensor, and first indicator means for indicating the detection of the varying voltage signal by the voltage detection means, wherein the testing device further includes second, warning indicator means for indicating if the signal input means has not been connected to a correct one of a plurality of electrical output and/or input connections of the oxygen sensor, said correct connection being the connection which carries the varying voltage signal to be detected.

An advantage of this aspect of the invention is that the correct connection to the oxygen sensor can be easily identified, thus avoiding an erroneous diagnosis of a faulty oxygen sensor due to a voltage on the wrong connection to the sensor being input to the testing device.

Usually, the plurality of electrical inputs/outputs from the oxygen sensor will include an electrical earth (for the oxygen sensor) or a power supply line from the ECU (this depends on which type of oxygen sensor is being used), an input which is connected to a heater for rapid heating of the oxygen sensor to a necessary operational temperature (and for subsequently maintaining the heater at this temperature), an electrical earth for the heater, and the sensor signal output which carries the varying voltage signal to be detected. Preferably, the warning indicator'means of the testing device comprises separate warning lights for each of the earth output or EM power supply line, the heater output and the heater earth, which lights are each formed and arranged for lighting up, in use of the testing device, if the respective one of these inputs/outputs is connected to the signal input means of the testing device. The warning lights are preferably LEDs.

The testing device may conveniently further include a signal input lead having one end adapted for connection to the signal input means of the testing device, and another end adapted for connection to an output lead of the oxygen sensor to be tested. Conveniently, the warning indicator means may include a further warning light adapted to light up if an open circuit condition is experienced i.e. if the signal input lead of the testing device is not properly connected to any one of the inputs/outputs of the oxygen sensor.

The testing apparatus may further include simulator means formed and arranged for generating signals which may be input to an ECU connected to the oxygen sensor being tested, whereby function testing may be carried out on the ECU using the testing device. For example, the simulator means may be formed and arranged for generating simulated "rich" and "lean" oxygen sensor output signals. By sending one or more such simulated 8signals to the EM the response of the ECU, and the resultant effect on the fuel/air mixture supplied to the vehicle engine, may be monitored e.g. by listening to any changing r.p.m. in the engine, or by monitoring the composition of vehicle exhaust gases using other known devices for this purpose which may be located in, or near, the vehicle exhaust.

The testing device may include one or more electrical batteries for powering one or more components of any one or more of: the voltage detection means, the control means, and the first or second indicator means of the testing device. Advantageously, the testing device further includes battery checking means formed and arranged for detecting a fall in the supplied battery power below a predetermined minimum level. A further warning light may be provided which is adapted to light up upon detection of such a fall in battery power by the battery checking means.

It will be appreciated that the signal input means, voltage detection means, control means, and indicator means (including first and second indicator means, where provided) may conveniently be provided together in an electronic circuit arrangement on a printed circuit board (PCB) supported by, preferably substantially within, the housing of the testing device.

According to an improved version of the invention, the oxygen sensor testing device may further include additional control means for controlling the voltage level detection in accordance with a minimum voltage of the varying voltage -9signal input from the oxygen sensor, whereby the testing device is capable of distinguishing between, and testing, at least two different types of oxygen sensor which produce different minimum output signal voltages.

This latter embodiment is advantageous in that the testing device automatically adjusts itself upon connection to the signal outputs from oxygen sensor testers having different minimum output signals, so as to be capable of testing at least two different such sensors.

Said additional control means may comprise circuitry for deriving a signal representing said minimum voltage from the maximum voltage, and the mean voltage, of the varying output signal, and using said derived signal as the minimum voltage in the nominated voltage range within which the indicator means operates.

According to a third aspect of the invention we provide a method of testing oxygen sensors incorporated in vehicle engine management systems, the method comprising the steps of: (1) providing an oxygen sensor testing device according to the first or second aspect of the invention as above described; (2) inputting a varying voltage signal, from an oxygen sensor to be tested, to the testing device; and (3) monitoring the response of the indicator means of the testing device so as to determine whether the oxygen sensor is functioning.

Preferably, step (3) of the method includes monitoring an indicator scale provided in the testing device to detect a -10fluctuating voltage signal indicated by the indicator scale which may, f or example, comprise a series of LEDs.

Preferred embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which:

Fig.1 is a schematic diagram illustrating an engine management system incorporating an oxygen sensor; Fig.2 is an example graph illustrating typical voltage output signals from a Zirconia type Lambda sensor obtained over a range of excess air factor ( X) values; Fig.3 is a schematic top view of an oxygen sensor testing device according to a preferred embodiment of the invention; Fig.4 is a schematic diagram of the electronic circuit incorporated in the testing device of Fig.3; Fig.5 illustrates a printed circuit board(PCB) with the components of the circuit of Fig.4 implemented thereon; and Fig.6 is a schematic diagram of a modification to the circuit of Fig.4 for implementation on an auxiliary PCB used in an alternative embodiment of the invention.

Fig. 1 illustrates a typical engine management system 1 for controlling the air/fuel mixture injected to the engine 2 of a vehicle (not shown) in which the system 1 is incorporated. The system 1 comprises an electronic control unit 3 (ECU) which is connected to an air intake sensor (not shown), the ECU thus receiving a signal Sa from the air intake sensor. The signal Sa gives a measure of the amount of air being taken -11into the engine (via the engine throttle 5) at any one time. The EM controls the amount of fuel F injected into the engine 2 by fuel injectors 4. The exhaust output 6 of the engine 2 outputs exhaust gases to an oxygen sensor 7, most commonly referred to as a Lambda sensor, which has a signal output 8 feeding back into the EM 3, to form a closed loop C from the EM to the fuel injectors 4, engine 2 and oxygen sensor 7 and back to the EM 3. From the oxygen sensor 7 the exhaust gases E pass to a catalytic converter 10 after which they exit the vehicle from the vehicle exhaust pipe 11.

The oxygen sensor 7 is a catalytic device designed to monitor the air/fuel ratio in the exhaust gases. Fig. 2 illustrates graphically typical output voltage signals from an oxygen sensor 7 of the Zirconia type (see below) across an example range of air/fuel mixture ratios, known as the "excess air factor" or 11 k 11 of the air/fuel mixture taken into the engine 2. As shown in Fig.2, the sensor 7 is a device with essentially two output states, "rich" and "lean", and changes between these states about the stoichimetric point P at which the air/fuel mixture in the exhaust gases is such that pollutants (such as nitrogen dioxide, carbon monoxide and hydrocarbons) in the exhaust gases can largely be absorbed and/or converted to "less harmful" gases by the catalytic converter 10. The excess air factor 11 X 11 at the stoichimetric point is given the nominal value one (1.0) as shown in Fig. 2. The engine management system is designed to constantly maintain the excess air factor at this value of one, and hence -12to maintain air/fuel ratio in the exhaust gases at a desired level (in practice, within a preferred range or "window").

As the engine management system is a closed loop system, a "rich" output from the oxygen sensor will tell the ECU to make the air/fuel mixture (input to the engine 2) leaner (i.e. higher in fuel content), and a "lean" output from the sensor will tell the ECU to make the mixture richer. consequently, the normal operating condition is that the system will "hunt" about the stoichimetric point P, with the sensor output switching between rich and lean with a 50% duty-cycle, at a frequency of a few Hertz Hz.

There are at least two known types of oxygen sensor. The most common type is the Zirconia sensor which produces an output of a few tens of millivolts in the lean condition, and 800 to 100OmV in the rich condition, as illustrated in Fig.2. This type of sensor is connected with one input to ground and the other input to the input of the electronic control unit (ECU).

Another type is the Titania sensor, which is a variable resistance device that has a high impedance in the lean condition and a low impedance in the rich condition. This type of sensor is connected between the +5V supply of the ECU and an input to the ECU, with a pull-down resistor to ground, and with this connection produces a voltage of a few tens of millivolts in the lean condition and about 4.5V in the rich condition.

Oxygen sensors operate at a temperature of 600 to 800 degrees Celsius. They may be unheated, in which case they are mounted -13very close to the engine exhaust manifold in order to attain working temperature quickly, or they may be heated by an electric element, in which case they can be placed further downstream in the exhaust gases where their life will be 5 longer (as in Fig. 1) - Failure of the oxygen sensor will result in failure of the closed-loop control of air-fuel mixture. As sensors tend to fail in a lean output condition, the mixture will become rich.

Fig.3 shows a top schematic view of an oxygen sensor tester 20 according to a preferred embodiment of the invention. The tester 20 is a hand-held, portable device comprising a substantially hollow housing 22 inside which is contained a printed circuit board(PCB) 40 (as shown in Fig.5) having implemented thereon the electronic circuit 30 illustrated schematically in Fig. 4. An LED display comprising a series of seven LEDs 26 (labelled LD6-LM2 in Fig. 4) are visible through a window 24 in the housing 22. A positive signal input terminal IN has an electrical lead 27 connected thereto, the lead 27 having a piercing clip 29 for connecting it to a signal wire of an oxygen sensor to be tested. An earth terminal GND is connected to electrical earth/ground in use of the testing apparatus.

Fig.4 illustrates schematically the electronic circuit 30 which controls the operation of the testing device. Fig.5 shows the layout of the components of the circuit 30 of Fig.4, on a PCB which is housed within the housing 22 of the testing device.

The operating principle of the tester is to display the switching action which indicates that the oxygen sensor is good. This is achieved by feeding the sensor signal from the oxygen sensor to a voltmeter consisting of a chain of comparators driving LEDs, via a lowpass filter that slows down the rising and falling edges of the waveform so that the LED display can follow it. Because there are difference types of oxygen sensor with different output voltages and tolerances, the tester does not attempt to measure the absolute output voltages of the sensor. Instead, the full-scale range of the voltmeter is set to nominally 80OmV, which is the minimum full-scale range expected from a Zirconia sensor. However, if the output voltage of the sensor is higher than this (as it will be for rich condition outputs from a Titania sensor), the full-scale range of the voltmeter adjusts itself to the output voltage of the sensor. Since a faulty sensor will almost always fail to a lean output condition (low output voltage), with little or no switching action, a good sensor can be identified by watching the LED display run up and down through its range.

circuit Description The input signal to the tester is connected to the input via protection circuitry comprising resistors R17, R20, Capacitor C6 and diode W2. The input will withstand a continuous over voltage of +/15V. The input signal is then fed to two op amps, A1 and A2. The input voltage to A1 is connected via a 3:1 attenuator R10/11, which ensures that the input voltage will never exceed the common-mode range of A1 during normal -15operation, as this can cause signal inversion in the LM324 integrated circuit (IC3 in Fig. 5) in which the two op-amps Al, A2 are provided. R10 and Ril also form a first-order lowpass filter with capacitor C2. This slows down the rising and falling edges of the input waveform so that the LED voltmeter can follow them, otherwise the display would simply switch between maximum and minimum without any intermediate LEDs lighting.

The input to A2 is also connected via a 3: 1 attenuator R12/R14, but is unf iltered. A2 is connected as a peak detector. When it receives a positive input voltage the output will swing positive until D2 is forwardbiased and the voltage on capacitors C3, C4 equals the input voltage. When the input voltage falls below that on C3, C4, the output voltage of A2 will fall, but C3, C4 cannot discharge into the A21s output because of diode D2. Once C3, C4 are charged, they will retain this voltage for several seconds, until they discharge into the reference divider chain (see below) of the LED voltmeter.

The output voltage of Al is connected to the input of the LED voltmeter, which comprises op-ramps A7 to A12 connected as comparators, and the LEDs, LD6 to LD12. The operation of this is very simple. The inverting inputs of the comparators are commoned and connected to the output of Al. The non-inverting inputs derive reference voltages from a resistor chain R30 to R38. The nominal reference voltage across this divider chain is derived from voltage reference diode ZD1, and is set to give a nominal voltage of 80OmV at the junction of R31 and -16R32. This means that LD6 will light if the input voltage falls below about 20OmV. If the peak input voltage exceeds 80OmV, the output of the peak detector will pull the reference voltage higher.

Whenever the input voltage exceeds the reference voltage on a comparator, the output of that comparator (and all those below it) will be low. As the output of the next comparator (and all those above it) will be high, only the LED between the two comparators will be lit.

At the ends of the chain LEDs LD6 and LD12 are connected to only one comparator and to Vcc or ground respectively. once LD6 is lit it will remain lit even if the input voltage rises further. Once LD12 is lit it will remain lit even if the input voltage falls further. This ensures that the display is never unlit.

Lead Identification An oxygen sensor may have up to four of the following connections:

Output signal (signal output 8 in Fig. 1) 2. Ground (or +5V from ECU in the case of Titania Sensors) 3. +12V heater supply (for heated sensors) 4. Heater ground (The ground 9a, heater supply 9b, and heater ground 9c electrical leads connected to oxygen sensor 7 are indicated in Fig. 1).

-17The positive test lead 27 of the tester should obviously be connected to the signal output 8 of the sensor. In order to avoid mis-connection to one of the other leads, op-amps A3 to A6 sense the DC voltage at the input, and it is corresponds to one of the following fault conditions, a warning LED will be lit. Because the sensor voltage may swing down almost to ground, or to greater that 4. 5V in the case of a Titania sensor, the input to A3-A6 is fed through a first-order lowpass filter consisting of resistor R15 and capacitor C5, so that these comparators will respond only to the average DC input voltage.

1. Ground If the test lead is connected to the sensor ground, the input 15 voltage will be close to zero, depending on any voltage drop If it is less than 10OmV. the outDut in the ground lead. voltage of A6 will be high, and LDS will be lit Note: In the case of some heated oxygen sensors with a three-wire connection (ground common to sensor and heater) the heater current may cause a voltage drop greater than 10OmV along the ground wire, in which case this error indication will not work, and a connection to ground may be indicated as an open circuit connection. (See below) 2. open circuit If the piercing clip is not making contact, the input voltage will be pulled up to about 150mV by R9. The output voltage of A6 will be low, whilst that of A5 will be high. LD4 will be -18lit. If the input voltage exceeds 20OmV, the output of A5 will go low and LD4 will be extinguished.

3. Connection to EM +5V Supply If the input voltage exceeds 4.5 volts, the output of A4 will be low, and LD3 will be lit, indicating a connection to the EM +5V supply.

4. Connection to +12V heater Supply If the input voltage exceeds 1OV, the output of A3 will be low and LD2 will be lit, indicating a connection to the heater +12V supply of other + 12V point.

Battery Check The supply voltage of the battery BY1 powering the circuit 30 is constantly compared against the reference voltage in a comparator comprising transistors Tr3 and Tr4. If the battery voltage falls below 7V, Tr4 will turn on, turning on Tr3 and lighting LD1.

Power on/off Power on/off switching is performed electronically under the control of two push-button switches S1 and S2. When S1 is pressed, base current is supplied via R1 to transistor Trl, which turns on, supplying power to circuit. This also supplies base current to transistor Tr2 via R2, turning it on, so that the supply remains latched on after S1 is released. To switch the supply off, S2 is pressed, which shorts out the base-emitter junction Tr2, turning it off, which in turn switches off Trl.

Simulator Rich and lean conditions can be simulated by overriding the oxygen sensor output with voltage pulses from the tester. IC4 is a dual 556 timer connected as two monostable multivibrators. When S3 is pressed, the output 1 of IC4 goes high for about 4 seconds, turning on Tr6, which pulls the output low to simulate a lean condition. When S4 is pressed, output 2 of IC4 goes high for about four seconds, turning on Tr5, which pulls the output high to simulate a lean condition. Diode D4 clamps the output from IC3 to 0.6V above Vref. The base-emitter voltage of Tr5 and the forward voltage drop of diode D3 ensure the output voltage does not exceed about 4.5V, which is less than the ECU supply. By simulating rich and lean outputs, an engineer can use the tester to check if a fault is due to a bad oxygen sensor, or due to the ECU not responding to the oxygen sensor output. This may be done, f or example, by listening for changing r.p.m. of the engine in response to simulated pulses from the tester and/or by monitoring pollution levels in exhaust gases from the vehicle using a suitable pollution level tester located in the exhaust.

A modified version of the oxygen sensor tester of the invention will now be described with reference to Fig.6. It is known that one carmanufacturer in particular in fact uses a Zirconia type oxygen sensor which has an output range which is different to the afore- described conventional output ranges of both the Zirconia and Titania type sensors. In this known sensor the output voltage range is offset (as compared to the -20conventional Zirconia sensor output range) such that it ranges from about 40OmV-70OmV in the lean state up to about 1.4-1.7V in the rich state. To accommodate this type of sensor it is necessary not only to recognise and adjust the LED scale for different full scale (maximum voltage) outputs, but also to recognise and adjust the scale for different minimum voltage outputs. It is not particularly easy to make a negative (minimum) peak detector unless there is a negative supply for the op-amp, which we do not have in the circuit of Fig.4.

Instead, in the present case we use the following technique. We first assume that the duty-cycle of the oxygen sensor output signal waveform, averaged over several seconds, is 50%. In this case the mean value of the sensor voltage lies midway between the peak voltage and the minimum voltage of the sensor output. Based on this assumption, we generate the reference voltage Bs for the bottom of the "potential divider" resistor chain R33-R37 (which controls the on/off operation of the LEDs) by using the following algorithm:

BS = MS where Ms is the Minimum sensor output voltage, and Ns = (Ps + Ms)/2 where Ps is the Peak sensor output voltage and Ns is the Mean sensor output voltage, so Ms = 2Ns - Ps This algorithm is performed by the circuit shown in Fig.6 which derives a signal for Ms using Ps and Ns. The circuit in Fig.6 is provided on an auxiliary PCB which is connected to -21the PCB on which the circuit of Fig.4 is implemented, in the manner now described, in order to provide a modified circuit. In the modified circuit, the resistors R13, R32 and R38 are removed from the circuit of Fig.4 and replaced with wire wrapping pins to which the auxiliary PCB carrying the circuit of Fig.6 is connected. The circuit of Fig.6 has two inputs Il,I2 which are connected in place of the top and bottom ends respectively of resistor R32, the function of R32 being effectively taken over by the resistor R11 connected between 11 and 12 in Fig.6. Two further inputs 13,14 are connected in place of the top end and bottom (ground) end respectively of resistor R13, so that the output from op-amp A1 (in Fig.4) is input to a buffer A21 in Fig.6, the function of R13 being taken over by resistor R21 in Fig.6. Another two inputs 15,16 are connected in place of the top end and bottom (ground) end respectively of resistor R38 (at the bottom of the resistor chain R33- R37), the bottom end reference voltage (i.e. Ms as calculated above) being connected into the resistor chain via resistor R91 in Fig.6. (It will be appreciated that references to "top" and ''bottom" ends are made with reference to the orientation of the components shown in Fig.4 with respect to the top and bottom of the page.) The jumper link (LINK) next to R32 in Fig.4 is also removed thus allowing the 9V supply which was carried thereby on the main PCB to be supplied to the auxiliary PCB between the +IN and +OUT end connections shown in the top right-hand corner of Fig.6. (These are connected in place of the afore-mentioned jumper link.).

The functions of the other components of the circuit of Fig.6 are as follows:

-22All buffers the peak voltage from capacitors C3,C4 on the main PCB; A21 buffers the sensor voltage from A1 on the main PCB; R31 and Cl' heavily filter the sensor waveform to extract its 5 mean DC voltage, and A31 buffers that voltage; A41 amplifies the mean voltage by 2 (gain= 1 + R71/R51) and the peak voltage by -1 (gain = -R71/R51); R61 balances the impedances at the two outputs of A4 to reduce the effect of input bias current; and R81 provides a pulldown load for the output of A41 It will be appreciated that various modifications to the described embodiments are possible without departing from the scope of the invention. For example, it will be appreciated that the modified circuit described above with reference to Figs.4 and 6 can be provided in a single PCB if more convenient, rather than as a main PCB and auxiliary PCB which are connected together. The spatial layout of the two circuits of Figs.4 and 6 would of course need to be re-designed in order to do this.

-233 -

Claims (19)

1. An oxygen sensor testing device suitable for use in testing oxygen sensors incorporated in vehicle engine management systems, the testing device comprising a housing supporting: signal input means into which, in use of the testing device, a varying voltage signal from an oxygen sensor to be tested may be input, voltage detecting means for detecting the varying voltage signal input from the oxygen sensor, and indicator means for indicating the detection of the varying voltage signal by the voltage detection means, wherein the testing device further includes control means formed and arranged for controlling the voltage level detection capacity of the voltage detection means in accordance with a maximum voltage of the varying voltage signal input from the oxygen sensor, whereby the testing device is capable of distinguishing between, and testing, at least two different types of oxygen sensor which produce different maximum output signal voltages.

M -1

2. A device according to claim 1, wherein the voltage detection means is formed and arranged for monitoring, over a nominated voltage range, the varying voltage signal input from the oxygen sensor.

3. A device according to claim 2, wherein the indicator means comprises an indicator scale which indicates different relative values of the varying voltage signal, within said nominated voltage range, as the signal varies over time.

-244. A device according to claim 3, wherein the indicator scale comprises a linear array of light emitting diodes (LEDs) which are connected to the voltage detection means.

5. A device according to claim 4, wherein each LED is formed and arranged to be illuminated when the monitored voltage signal falls within a particular sub-range of the nominated voltage range.

6. A device according to any of claims 3 to 5, wherein the control means is formed and arranged so as to adjust the nominated voltage range by increasing or decreasing the ratio between the voltage detected and the level indicated by the indicator scale, depending on the maximum voltage of the varying voltage signal input to the testing device from the oxygen sensor.

7. A device according to any of claims 3 to 6, further including signal delay means formed and arranged for slowing down rising and falling edges in the varying voltage signal input from the oxygen sensor to the signal input means of the testing device, in use of the device, so as to allow sufficient response time for the indicator scale to respond to increases and decreases in voltage in the varying voltage signal.

8. A device according to claim 7, wherein the signal delay means is provided in the form of a low-pass filter.

-259. A device according to any preceding claim, further including additional control means for controlling the voltage level detection in accordance with a minimum voltage of the varying voltage signal input from the oxygen sensor, whereby the testing device is capable of distinguishing between, and testing, at least two different types of oxygen sensor which produce different minimum output signal voltages.

10. A device according to claim 9, wherein said additional control means comprises circuitry for deriving a signal representing said minimum voltage from the maximum voltage, and the mean voltage, of the varying output signal, and for using said derived signal as the minimum voltage in the nominated voltage range within which the indicator means operates.

11. A device according to any preceding claim, further including second, warning indicator means for indicating if the signal input means has not been connected to a correct one of a plurality of electrical output and/or input connections of the oxygen sensor, said correct connection being the connection which carries the varying voltage signal to be detected.

12. An oxygen sensor testing device suitable for use in testing oxygen sensors incorporated in vehicle engine management systems comprises a housing supporting: signal input means into which, in use of the sensor, a varying voltage signal from an oxygen sensor to be tested may be input, voltage detection means for detecting the varying -26voltage signal input from the oxygen sensor, and first indicator means for indicating the detection of the varying voltage signal by the voltage detection means, wherein the testing device further includes second, warning indicator means for indicating if the signal input means has not been connected to a correct one of a plurality of electrical output and/or input connections of the oxygen sensor, said correct connection being the connection which carries the varying voltage signal to be detected.

13. A device according to claim 11 or claim 12, wherein said plurality of electrical inputs/outputs from the oxygen sensor includes: at least one of an electrical earth and a power supply line, from an ECU controlling fuel injection to the vehicle engine of a vehicle in which the oxygen sensor is incorporated; an input which is connected to a heater for rapid heating of the oxygen sensor to a necessary operational temperature (and for subsequently maintaining the heater at this temperature); an electrical earth for the heater; and the sensor signal output which carries the varying voltage signal to be detected, and wherein said warning indicator means comprises separate warning lights for each of the earth output and/or ECU power supply line, the heater output and the heater earth, which lights are each formed and arranged for lighting up, in use of the testing device, if the respective one of these inputs/outputs is connected to the signal input means of the testing device.

14. A device according to any of claims 11 to 13, further including a signal input lead having one end adapted for -27connection to the signal input means of the testing device, and another end adapted for connection to an output lead of the oxygen sensor to be tested, and wherein the warning indicator means includes a further warning light adapted to 5 light up if an open circuit condition is experienced.

15. A device according to any of claims 11 to 14, further including simulator means formed and arranged for generating simulated "rich" and "lean" oxygen sensor output signals which may be input to an ECU connected to the oxygen sensor being tested.

16. A device according to any of claims 11 to 15, further including one or more electrical batteries for powering one or more components of any one or more of: the voltage detection means, the control means, and the first or second indicator means, and wherein the device further includes battery checking means formed and arranged for detecting a fall in the supplied battery power below a predetermined minimum level.

17. A device according to claim 16, wherein a further warning light is provided which is adapted to light up upon detection of a said fall in battery power by the battery checking means.

18. An oxygen sensor testing device substantially as described herein and with reference to Fig.4.

19. An oxygen sensor testing device substantially as described herein and with reference to Figs.4 and 6.