Alternator waveform

4.5.1 Alternator waveform

will be lost. A fault within one of the three phases Checking the ripple voltage produced by an alter-

will show a similar picture to the one illustrated nator is a very good way of assessing its condi-

but is three or four times the height, with the base tion (Figure 4.41).

to peak voltage in excess of 1 V. The example waveform shown in Figure 4.42

The voltage scale at the side of the oscillo- illustrates the rectified output from the alternator.

scope is not representative of the charging volt- The output shown is correct and that there is no

age, but is used to show the upper and lower fault within the phase windings or the diodes (rec-

limits of the ripple. The ‘amplitude’ (voltage/ tifier pack).

height) of the waveform will vary under different The three phases from the alternator have

conditions. A fully charged battery will show a been rectified to dc from its original ac and the

‘flatter’ picture, while a discharged battery will waveform shows that the three phases are all

show an exaggerated amplitude until the battery functioning.

is charged. Variations in the average voltage of

Figure 4.41 Alternator

Figure 4.42 Alternator ripple voltage

Oscilloscope diagnostics 59

the waveform are due to the action of the voltage battery is charged, and the starter and associated regulator.

circuit are in good condition. The current for a typical 4 cylinder petrol/gaso- line engine is in the region of 100 to 200 A.

4.5.2 Relative compression

In the waveform shown, the initial peak of cur-

petrol waveform

rent (approximately 400 A) is the current required Measuring the current drawn by the starter motor

to overcome the initial friction and inertia to (Figure 4.43) is useful to determine starter condi-

rotate the engine. Once the engine is rotating, the tion but it is also useful as an indicator of engine

current will drop. It is also worth mentioning the condition.

small step before the initial peak, which is being The purpose of this particular waveform

caused by the switching of the starter solenoid. (Figure 4.44) is therefore to measure the current

The compressions can be compared against required to crank the engine and to evaluate the

each other by monitoring the current required to relative compressions.

push each cylinder up on its compression stroke. The amperage required to crank the engine

The better the compression the higher the current depends on many factors, such as: the capacity of

demand and vice versa. It is therefore important the engine, the number of cylinders, the viscosity

that the current draw on each cylinder is equal. of the oil, the condition of the starter motor, the

condition of the starter’s wiring circuit and

4.5.3 CAN-H and CAN-L

the compressions in the cylinders. To evaluate

waveform

the compressions therefore, it is essential that the Controller area network (CAN) is a protocol

used to send information around a vehicle on data bus. It is made up of voltage pulses that rep- resent ones and zeros, in other words, binary sig- nals. The data is applied to two wires known as CAN-high and CAN-low (Figure 4.45).

In this display, it is possible to verify that data is being continuously exchanged along the CAN bus. It is also possible to check that the peak to peak voltage levels are correct and that a signal is present on both CAN lines. CAN uses a differen- tial signal, and the signal on one line should be a coincident mirror image (the signals should line up) of the data on the other line (Figure 4.46).

The usual reason for examining the CAN sig-

Figure 4.43 Starter and ring gear

nals is where a CAN fault has been indicated by

Figure 4.44 Spark ignition engine cranking amps

60 Advanced automotive fault diagnosis OBD, or to check the CAN connection to a sus-

be viewed. This enables the mirror image nature pected faulty CAN node. The vehicle manufac-

of the signals, and the coincidence of the edges turers’ manual should be referred to for precise

to be verified.

waveform parameters. The signal shown is captured on a fast time- base and allows the individual state changes to

4.6 Summary

‘Scope’ diagnostics is now an essential skill for the technician to develop. As with all diagnostic techniques that use test equipment, it is necessary for the user to know how:

● the vehicle system operates; ● to connect the equipment; ● readings should be interpreted.

Remember that an oscilloscope is really just a voltmeter or ammeter but that it draws a picture of the readings over a set period of time. Learn what good waveforms look like, and then you will

Figure 4.45 OBD socket – pin 6 is CAN-high and pin 14 is

be able to make good judgements about what is

CAN-low

wrong when they are not so good!

Figure 4.46 CAN-high and low signals on a dual trace scope

Knowledge check questions To use these questions, you should first try to answer them without help but if necessary, refer back to

the content of the chapter. Use notes, lists and sketches to answer them. It is not necessary to write pages and pages of text!

1. Explain the terms ‘timebase’, ‘amplitude’ and ‘voltage scale’.

2. Make a sketch of ignition primary and secondary waveforms. Label each part and state which aspects indicate that no faults are present.

3. Describe how to connect an oscilloscope to examine the signal supplied to a single point (throttle body) injector.