Fig. 5.22

Details of the strain gauge type of MAP sensor

When the sensing diaphragm flexes, the physical sizes of the resistors R1 to R4 change and this affects their electrical resistance. The bridge circuit shown in Fig. 5.22(c) represents the electrical circuit in the sensor. Under the influence of manifold pressure the electrical resistance of R1 and R3 increases and that of R2 and R4 decreases by a similar amount. This alters the voltage levels at the two inputs to the differential transformer and leads to a sensor output voltage that is proportional to manifold absolute pressure. Figure 5.22(d) shows an idealized representation of sensor voltage against manifold absolute pressure and it shows that a MAP sensor can be checked for accuracy of performance by conventional electrical test methods.

5.7.2 OTHER MAP SENSORS Variable-capacitance type

Figure 5.23 gives an indication of the principle of operation of the variable-capacitance type of MAP sensor.

Fig. 5.23 Principle of a capacitive-type MAP sensor

Capacitance C = eo A/d, where eo = permittivity in a vacuum, A = area of the metallized plates and d = the distance between the plates. The metallized plates of the capacitor are placed on each side of an evacuated capsule. This capsule is placed in a chamber which is connected to manifold pressure and, as the manifold pressure changes, the distance d between the capacitor plates changes. This change in distance between the capacitor plates causes the value of the capacitance C to change. The capacitor is connected into an electronic circuit that converts changes in capacitance into an electrical signal.

Variable-inductance type

The variable-inductance type of MAP sensor relies on the principle that the inductance of a coil is altered by varying the position of an iron cylinder placed in the center of the coil. Figure 5.24 illustrates the principle involved.

Connec1'"" rr- ^mtrAA absolute

In this simplified version, the iron cylinder moves in or out of the coil under the influence of the diaphragm and spring. Variations in manifold absolute pressure increase or decrease the 'suction' force acting on the diaphragm and the resultant changes in inductance are related to the manifold absolute pressure. The coil (inductance) forms part of an electronic circuit and this circuit is designed so that the changes in frequency of the square-wave output are accurate representations of manifold absolute pressure.

Figure 5.25 shows the approximate form of the variable frequency output of sensors of this type.

As with all vehicle systems, it is vitally important to have access to the precise figures and data that relate to the system and vehicle being worked on. The information given here is intended as a guide to general principles. In many cases the suppliers and makers of diagnostic equipment will provide a back-up service that will provide test data for a wide range of vehicles and systems. This is a factor that must be considered when making decisions about the purchase of equipment.

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