461 Vacuum Pumps And Gauges

A vacuum pump and gauge of the type shown in Fig. 4.17 can be used for a range of purposes, such as checking a MAP sensor performance, EGR valves and other devices that are vacuum operated.

Fig. 4.15 Part of the procedure for use of the Bosch KTS 500 instrument

When used to check the performance of a manifold absolute pressure sensor, or an exhaust gas recirculation valve, the vacuum pump is used to replace the manifold vacuum, as shown in Fig. 4.18.

When the vacuum gauge and pump are used to test a MAP sensor, the vacuum pipe from the inlet manifold is removed and the pump and gauge are connected to


Fig. 4.19 A manifold absolute pressure sensor test


Fig. 4.19 A manifold absolute pressure sensor test the vacuum connection of the sensor. The meter is then connected to the signal wire of the sensor with electrical energy supplied but the engine not running. Figure 4.19 gives an impression of the set-up.

Starting with zero vacuum, i.e., at atmospheric pressure, the frequency reading should be written down. The vacuum should then be increased in steps of 25 mm of mercury (vacuum gauge reading) and the frequency output at each step noted until the maximum vacuum of approximately 750 mm of mercury is reached. The figures can then be compared with those that relate to the sensor under test. The frequency normally lies in a range of approximately 70-150 Hz, where the lower reading applies to maximum vacuum.

4.7 Calibrating test instruments

One should be reasonably certain that a new instrument will be accurate. However, during the course of normal use, it is not uncommon for instruments to become less accurate and it is good practice to check instrument readings periodically against 'known good' ones. This process which is known as 'calibration' should be performed on all measuring instruments at regular intervals. Figure 4.20 shows the procedure for calibrating the test leads of the Bosch PMS 100 oscilloscope.

The supplier of any test instrument will normally provide a calibration service and often this service is included in the equipment service engineer's routine.

4.8 Location charts and wiring diagrams

A location chart of the type shown in Fig. 4.21 shows the position on a vehicle where the individual sensors and actuators etc. are situated. Such charts are useful when making visual inspections and also when locating the device to check connections or to back probe for test purposes.

Circuit diagrams, or wiring diagrams as they are commonly known, are an essential aid to checking circuits. Figure 4.22 shows a typical wiring diagram. Many modern items of diagnostic equipment now include location charts and circuit diagrams in the software, so that 'on screen' displays are readily to hand.

4.9 Sources of diagnostic data

Manufacturers and suppliers of the type of equipment described in this chapter normally provide a service that ensures that equipment users have a ready supply of diagnostic data relating to a range of vehicles. In some cases a subscription ensures that updates are provided as and when required. In addition, most companies provide a 'hotline' service to help with diagnostic problems. A number of publishing companies also produce books of data and fault codes and these are readily available from companies such as those listed in the Appendix.

Fig. 4.20 Calibrating the test leads on the Bosch PMS 100
Fig. 4.21 A typical location chart

4.10 Exhaust gas emissions and emission system testing

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