Stepper motor waveform

4.3.6 Stepper motor waveform

The stepper or stepper motor is a small electro- mechanical device that allows either an air by- pass circuit or a throttle opening to alter in position depending on the amounts that the stepper is indexed (moved in known steps) (Figure 4.33).

Stepper motors are used to control the idle speed when an idle speed control valve is not employed. The stepper may control an ‘air bypass’ circuit by having four or five connections back to the ECU. The earths enable the control unit to move the

motor in a series of ‘steps’ as the contacts are Figure 4.33 Stepper motor and throttle potentiometer on a

throttle body

Figure 4.32 Signal supplied to a rotary idle control valve

Oscilloscope diagnostics 55

Figure 4.34 Stepper motor signals

amplifier stops increasing the primary current and it is maintained until the earth is removed from the coil. This is the precise moment of ignition.

The vertical line at the centre of the trace is in excess of 200 V, this is called the ‘induced volt- age’. The induced voltage is produced by magnetic inductance. At the point of ignition, the coil’s earth circuit is removed and the magnetic flux collapses across the coil’s windings. This induces a voltage between 150 and 350 V. The coil’s high tension output will be proportional to this induced volt- age. The height of the induced voltage is some- times referred to as the primary peak volts.

Figure 4.35 Direct ignition coils in position

From the example current waveform (in Figure 4.37), the limiting circuit can be seen in operation. The current switches on as the dwell period starts

produce slightly different traces but the funda- and rises until the required value is achieved (usu- mental parts of the trace and principles are the

ally about 8A). At this point the current is main- same (Figure 4.35).

tained until it is released at the point of ignition. In the waveform shown in Figure 4.36, the hor-

The dwell will expand as the engine revs are izontal voltage line at the centre of the oscilloscope

increased to maintain a constant coil saturation is at fairly constant voltage of approximately 40 V,

time. This gives rise to the term ‘constant energy’. which then drops sharply into what is referred to

The coil saturation time can be measured and this as the coil oscillation. The length of the horizontal

will remain the same regardless of engine speed. voltage line is the ‘spark duration’ or ‘burn time’,

Figure 4.37 shows a charge time of about 3.5 ms. which in this particular case is about 1 ms. The

coil oscillation period should display a minimum of four to five peaks (both upper and lower). A loss

4.4.2 Ignition secondary

of peaks would indicate a coil problem.

waveform

There is no current in the coil’s primary circuit until the dwell period. This starts when the coil is

The ignition secondary waveform is a measure- earthed and the voltage drops to zero. The dwell

ment of the HT output voltage from the ignition period is controlled by the ignition amplifier or

coil. Some coils can produce over 50,000 V. Dif- ECU and the length of the dwell is determined by

ferent types of ignition coils produce slightly dif- the time it takes to build up to about 8A. When

ferent traces but the fundamental parts of the this predetermined current has been reached, the

trace and principles are the same (Figure 4.38).

56 Advanced automotive fault diagnosis

Figure 4.36 Primary ignition voltage trace

Figure 4.37 Primary ignition current trace

The ignition secondary picture shown in Figure

4.39 waveform is from an engine fitted with elec- tronic ignition. In this case, the waveform has been taken from the main coil lead (king lead). Suitable connection methods mean that similar traces can

be seen for other types of ignition system. The secondary waveform shows the length of time that the HT is flowing across the spark plug electrode after its initial voltage, which is required to initially jump the plug gap. This time is referred to as either the ‘burn time’ or the ‘spark duration’. In the trace shown it can be seen that the hori- zontal voltage line in the centre of the oscilloscope is at fairly constant voltage of approximately 3 or

Figure 4.38 Spark plugs (Source: Bosch Press)

4 kV, which then drops sharply into the ‘coil oscil- lation’ period.

be faulty. The period between the coil oscillation The coil oscillation period should display a

and the next ‘drop down’ is when the coil is at rest minimum of four or five peaks (both upper and

and there is no voltage in the secondary circuit. lower). A loss of peaks indicates that the coil may

The ‘drop down’ is referred to as the ‘polarity

Oscilloscope diagnostics 57

peak’, and produces a small oscillation in the fluctuating and the display will be seen to move opposite direction to the plug firing voltage. This

up and down. The maximum voltage at the spark is due to the initial switching on of the coil’s pri-

plug, can be seen as the ‘Ch A: Maximum (kV)’ mary current.

reading at the bottom of the screen. The plug firing voltage is the voltage required

It is a useful test to snap the throttle and observe to jump and bridge the gap at the plug’s electrode,

the voltage requirements when the engine is under commonly known as the ‘plug kV’. In this exam-

load. This is the only time that the plugs are placed ple the plug firing voltage is about 12 or 13 kV.

under any strain and is a fair assessment of how When the plug kVs are recorded on a DIS or

they will perform on the road. coil per cylinder ignition system, the voltage seen

The second part of the waveform after the ver- on the waveform (Figure 4.40) should be in the

tical line is known as the spark line voltage. This ‘upright position’. If the trace is inverted it would

second voltage is the voltage required to keep the suggest that either the wrong polarity has been

plug running after its initial spark to jump the gap. selected from the menu or in the case of DIS, the

This voltage will be proportional to the resistance inappropriate lead has been chosen. The plug volt-

within the secondary circuit. The length of the line age, while the engine is running, is continuously

can be seen to run for approximately 2 ms.

Figure 4.39 Ignition secondary trace

Figure 4.40 Distributorless ignition

58 Advanced automotive fault diagnosis

4.5 Other components

If the alternator is suffering from a diode fault, long downward ‘tails’ appear from the trace at reg- ular intervals and 33% of the total current output