1. Field of the Invention
The invention relates to a device for detecting an abnormality in the functioning of a manifold absolute pressure (MAP) sensor on an automobile, and more particularly to a method of this kind which can perform such abnormality determinations by emulating a MAP sensor of the type connected to a microprocessor-based engine control system.
2. Description of the Prior Art
Modern automobiles and similar vehicles include a plurality of engine-mountable sensors which provide signals to an on-board computer having informational value representing engine operating conditions. One such sensor is a manifold absolute pressure (MAP) sensor functioning to provide information relative to the absolute pressure in the intake manifold. As is well known, the amount of intake manifold pressure when coupled with other information, such as engine speed, allows the engine computer to control, among other things, engine fuel requirements.
A microprocessor associated with the vehicle on-board computer reads the pressure signal output from the MAP sensor to control engine operation. For this reason, when the MAP sensor functions improperly, engine performance is adversely affected.
On one end of the MAP sensor is a pneumatic connection between the vacuum hose from the intake manifold and the MAP sensor. On the other end is an electrical connection between the MAP sensor and the vehicle on-board computer.
One obvious way to verify whether a faulty MAP sensor is causing the engine to run rough, involves replacing the vehicle's MAP sensor with an identical off-the-shelf MAP sensor. If engine performance improves with the substituted sensor, the vehicle's present MAP sensor is probably bad. If there is no improvement, the problem lies elsewhere. This test-by-substitution MAP sensor diagnosis is impractical. MAP sensors come in different sizes and shapes, and respond differently to differing levels of input vacuum pressure. Hence, MAP sensors are not interchangeable.
The electrical signal output of a common variety of MAP sensors is a signal whose frequency varies proportional to the vacuum pressure level, measured in inches of Hg (in-Hg), detected at the MAP sensor input. Alternatively, another variety of MAP sensors is voltage varying and generates an output signal, the voltage level of which varies in proportion to a change in the detected vacuum pressure level. Generally, voltage varying MAP sensors are further distinguished between increasing-voltage-variable sensors, which generate an increasing voltage signal proportionate to increases in vacuum pressure, and decreasing-voltage-variable sensors which generate a decreasing voltage signal in response to the increase in vacuum pressure. Typically, both increasing and decreasing voltage-variable type sensors generate an electrical output signal whose voltage levels are positive and typically in the range between +5 and 0 volts. However, it is conceivable that voltage-variable sensors of the type generating negative voltage levels, i.e., 0 to -5 volts may also become common in the future. It would therefore be impractical and inefficient for every automobile technician to have on-hand a wide range of MAP sensors for the purpose of diagnosing a possible single faulty MAP sensor in the vehicle under test. Hence, new diagnostic equipment should include means for testing all types and varieties of MAP sensors.
U.S. Pat. No. 5,214,582 to Gray discloses a general purpose diagnostic work station for use with a vehicle including a network of sensors coupled to an on-board computer. The particular diagnostic work station is described as capable of simulating the operation of a variety of different engine sensors such as, for example, a MAP sensor, independent of the actual operation of such sensor. This is accomplished by way of a network of bypass switches connected between two plug connectors disposed between the on-board computer and the engine sensors. The bypass switches, in turn, are connected to the work station. Depending upon the condition of the bypass switch network, the work station can be connected in parallel with the sensor connectors to monitor the sensor signals, or one or more selected sensors can be disconnected from the on-board computer, and the work station connected in its place. In this latter mode, the work station's internal computer can generate signals to simulate sensor outputs based on operator-entered data representative of the type of sensor under test. That data is then compared to data in a memory-stored look-up table, from which is derived the waveform characteristics of the signal to be simulated. In this arrangement however, because a simulated signal is generated based on predetermined normal sensor outputs, i.e., on a simulated engine input, rather than an actual engine input to the sensor, MAP sensor operation cannot be diagnosed accurately or adequately.