898

(b)
Fig. 7.37 Diesel injection advance
Fig. 7.38 A test on an ABS wheel speed sensor

The test requires that the road wheel be rotated at sufficient speed to generate a signal. This can normally be achieved by safely jacking up the vehicle and rotating the wheel by hand. [Any other method of rotating the wheel must only be in accordance with the vehicle manufacturer's recommended procedure.] The sensor output should be of the form shown in Fig. 7.39. The voltage will vary with speed, as will the frequency.

Fig. 7.39 Testing an ABS wheel sensor

The main points to look for on the voltage trace are:

• a regular waveform

• any gaps in the waveform that may indicate a missing tooth or displaced reluctor ring

Some ABS and traction control systems make use of Hall-type sensors. The Hall sensor requires a power supply and testing this type of sensor will require special attention to be given to this aspect. The remainder of the test would be similar to that shown above and, once again, special attention must be given to safe jacking up of the vehicle and rotation of the wheel. A Hall-type sensor will produce a pattern, of varying frequency, of the form shown in Fig. 7.40.

Fig. 7.40 The pattern of the signal from a Hall type ABS sensor
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