Most oscilloscopes used within the motor vehicle field are handheld allowing portable use.
A modern digital oscilloscope will have a number of functions such as:
● data capture – snapshot, trend, glitch, recording waveforms and set-up functions;
● freeze frame and zoom functions;
● data storage and retrieval, automatic report function;
● auto scope measurement – presents the waveform in the most appropriate format for the less
experienced user.
Waveforms can generally be captured, recorded, reviewed and then stored for later analysis.
Often they can then be added to a report or compared to another signal and printed out.
Oscilloscopes are easy to connect and often have auto functions to aid less experienced users
to sample waveforms.The sampling rate of a good specification digital oscilloscope is very quick
enabling intermittent faults (glitches) to be captured and recorded. Sampling a signal allows
you to check the signal pattern providing additional diagnostics.
figure 3.104
3.105
Information is available illustrating waveforms for a range of different sensors and actuators
for comparative purposes.Waveforms generally have the following characteristics:
● Amplitude – voltage level.
● Frequency – the number of cycles of the waveform per second, dependent on the circuit or
sensor’s operating speed.
● Pulse width – the time current flows through the component (measured in ms).
● Duty cycle – a measurement of the on time compared to the off time of a cycle,measured in
percentage.Duty cycle is often used to control the amount of current flowing in a circuit.
● Signal shape – a flat slow changing analogue signal, sawtooth signal, square-wave signal.Shape
depends on the type and construction of the sensor or actuator.
Waveforms
DC voltage signal analogue waveform (amplitude manipulated)
An analogue DC waveform only tends to change in amplitude. It has no defined signal shape
apart from being a flat line if there is no change in the measured variable, e.g. no change in
temperature.Often the change is slow meaning a good method of recording such a variation is
by recording the waveform using trend. Trend is used to record slow changing signals using
slow sampling rates. For example, a sample rate of 5ms for 6 hours can be used which means
the oscilloscope will record the signal and plot a point on a graph every 5ms for a total period
of 6 hours.The sample can vary from sampling every 2 minutes for 48 hours.This may be used
for data recording of a specific fault associated with the gradual loss of a signal or level of
power in a circuit.
A DC analogue signal can also be measured using the Min and Max selection of a scope.This
enables the measurement of the total variation in voltage to be known which can be compared
to a known value for analysis.
Waveform
The plot in Figure 3.106 shows a reduced volt drop across an NTC sensor.As the temperature
applied to the sensor by the A/C system increases the resistance reduces with a corresponding
reduction in volt drop.
Signal checks:
1. Monitoring of the temperature of the air flowing past the sensor to carry out a comparison.
2. The waveform should correspond to an NTC or PTC graph.
3. The peak voltage should be referenced to the specification of the sensor.
4. Voltage transitions should be steady and reflect a change in temperature.
DC/AC
This is displayed as an alternating DC voltage.This is a DC voltage which alternates between
two voltage points at and above the zero voltage line. For example, the voltage may alternate
between 1.5 and 3.5 volts.The signal will not go into a negative voltage like an AC signal.
Step and pulse voltage
A step and pulse voltage are single occurrences which change from one state to another when
the circuit is triggered.A step voltage occurs when a voltage steps from one level to another.
figure 3.106
A typical example is a compressor clutch (Fig. 3.107).Diodes are connected to these circuits to
prevent voltage spikes which are produced from the collapse of the magnetic field. The spikes
could damage the driver circuit inside the control module.A clean transition from one state to
another should be seen.
A pulsed voltage occurs when the voltage changes from one state to another and then back
to its original state.This is a single pulse, characterised by an instant rise and fall time, and indi-
cates where the voltage spike is being filtered.This is after the clutch is de-energised.
Pulse train
A pulse train can be categorised into a number of subgroups. For example:
● Frequency modulated (change in the frequency, cycles per second).
● Duty cycle (change in the percentage of the on to off time in one cycle).
● Pulse width modulated signal (the time in milliseconds the current is flowing through the
component).
● Pulse shape, a change in the shape of the pulse over time.
● Amplitude, change in the voltage level of the signal.
A voltage signal can include a number of these traits.
Frequency modulated signal with fixed duty ratio, shape and amplitude
The frequency is determined by the number of pulses (oscillations per second).Accordingly, the
frequency increases/decreases proportionally to the number of pulses per second.The frequency
(formula symbol ‘f’) is measured in hertz (Hz).A frequency modulated signal will change in the
number of ‘cycles’ that occur every second with a change in the sensor input of the control of the
actuator. Hall effect speed sensors display this type of signal (Fig. 3.108).As the vehicle speed
increases the signal is switched on and off quicker so the number of cycles increases.The shape
and duty cycle of the signal is fixed by the very nature of the sensor construction.The signal should
be clean with no under- or overshoot providing a constant amplitude throughout the pattern.
figure 3.107
3.108
Signal checks:
1. Monitor a steady change in frequency directly proportional to a change in speed.
2. The peak voltage should be equal to the reference voltage (200–400mV difference is accept-
able). If different then carry out a power-to-power test.
3. Voltage transitions should be straight and vertical (unless switching an inductive component).
4. The lower horizontal lines should almost reach zero.A small volt drop is allowed approxi-
mately 200mV, it should not exceed 400mV. If high carry out earth-to-earth check.
AC voltage signal – amplitude, shape, frequency and pulse width are variable
An AC signal voltage alternates from zero to a maximum positive voltage and then back to zero
and then to a maximum negative voltage (one cycle).These signals are generally produced by an
inductive sensor (variable reluctance sensors) using the theory of movement and magnetism to
produce an AC voltage.The sensor has no power supply only two shielded wires and a coil with
a magnet inside. The trigger wheel is constructed from a permeable material, a low magnetic
reluctance steel.As the trigger wheel rotates, small signal voltages are induced into the coil which
can be measured by a module for the rate and change of speed of the rotor. If a tooth is missing
on the rotor then this will cause one of the cycles to be missing due to no induced voltage.
The signal in Figure 3.109 shows the voltage constantly changing and repeating itself. The
cycle is described as frequency.The number of cycles per second is presented in hertz.The sig-
nal should be smooth and progressive and the peak voltages should be identical with no
change in speed.
figure 3.109
As a change in speed occurs then a change in the shape, frequency, and amplitude will occur
(Figure 3.110).The signal often displays a missing cycle.This is due to a missing tooth on the sen-
sor’s trigger wheel.
AC signals have the potential to induce electrical interference. This is why the cables are
generally heavily shielded.This interference can be viewed on a scope.
Pulse width modulated signal (millisecond measurement)
PWM (Pulse Width Modulation) signals are square-wave or pulse train signals with a constant
frequency, but variable on time.The pulse width is the duration of the active signal.This signal dis-
plays a number of characteristics but the technician is only interested in one. The time current
flows in the circuit measured in time, often milliseconds. A solenoid can be viewed using this
method.Figure 3.111 shows a saturated voltage, a solenoid (injector) with a full voltage applied to
operate it.The solenoid is required to be open for a fixed time.This can be measured using PWM.
figure 3.110
3.111
3.112
3.113
3.114
Duty cycle ratio with fixed frequency
The duty cycle is the ratio between the on and off times of a PWM signal. The duty cycle is
expressed as a percentage. Accordingly, a duty cycle of 25% means that the signal is active
25% of the time; over 1 second of pulse width modulation, for example, the signal is active for
250ms and inactive for 750ms. PWM signals can serve as output (e.g. for controlling a solenoid
valve or blower motor) as well as input (digital sensor) (Fig. 3.112).
PWM signals can serve as outputs for controlling solenoid valves as well as inputs from sen-
sors or control modules. Figure 3.114 provides a duty cycle ratio signal which is sent from the
A/C module to the blower control module to indicate blower speed demand. The duty ratio
variation indicates the required speed.
The duty cycle can be measured with the help of an oscilloscope or a serial tester.
figure 3.115
3.116
Measuring pressure with transducers
Pressure transducers can be linked to scope meters for the direct measurement of pressure
and vacuum.A highly accurate pressure transducer can measure from 0.5 to 350 psi (3.447 to
2413 kPa) and 0 to 29.9 inHg (0 to 76 cmHg).Most are compatible with any automotive fluid,
such as R12, R134a, and possibly CO2.
Testing multiplex signal communication between nodes (modules)
The CAN (Control Area Network) standard supports half duplex communication with only two
wires to send and receive data forming the bus.The nodes have a CAN transceiver and a CAN
controller for bus access. At both ends, the bus must be terminated with a resistor, typically
120 .The CAN transmits signals on the CAN bus which consists of a CAN-high and CAN-low.
These two wires carry anti-phase signals in opposite directions to minimise noise interruption
that simultaneously interferes with the bus.The CAN bus line can have one of two logical states:
‘recessive’ and ‘dominant’.Typically, the voltage level corresponding to recessive (logical ‘1’) is
figure 3.117
2.5V and the levels corresponding to dominant (logical ‘0’) are 3.5V for CAN-high and 1.5V
for CAN-low.The voltage level on the CAN bus is recessive when the bus is idle.
When analysing the CAN bus signals, it is of interest to measure the peak-to-peak voltages
and to verify that the CAN signals are disturbance free. Each input can be viewed on a dual
scope (two measurements to be made simultaneously – Figure 3.93).
Multiplex wiring diagram example
Figure 3.117 shows a mid-speed CAN network. On the MS CAN bus, the transmission rate is
125 kbit/s. Cabling is between two nodes, the touch screen A363 module pins 15 and 5 and the
A/C module A205 (EATC) pins 18 and 19. CAN-high signal is between pins 5 and 18 and
CAN-low signal between pins 15 and 19.
Motor speed control
Blower motor example
Speed control is required for the interior fan of the ventilation system.Generally this is achieved
by controlling the current within the circuit.
The current is controlled through a series of resistors which reduce the current flow.Figure 3.118
shows the motor M3 receives a switched feed to its positive terminal.The ground of the motor
is then directed by switch N73 allowing current to flow through the resistor pack to ground or
directly to ground. If the current flows to the resistor pack then it will reduce in value allowing
the fan to operate at a reduced speed (some of the electrical energy will convert to heat and be
wasted).
If the current goes straight to earth then the fan will operate at its maximum speed due to
maximum current flow.
Figure 3.119 is a simplified circuit of the blower motor. The switches represent the multi
switch N73 which has three speed positions. In position one (left switch) the motor has the
maximum amount of current flowing and will operate at maximum speed.This can be seen on
figure 3.118
3.119
the waveform in Figure 3.120. For the first 2 seconds the maximum amount of current is flow-
ing to the motor so it will be rotating at its maximum speed.To reduce the speed of the motor
we reduce the current by including more resistors in the circuit.
Switches 2, 3 and 4 direct the current through more resistors which cause the motor to reduce
in speed.
figuure 3.120
The waveform shows the reduction in current over time which caused a reduction in rota-
tional speed of the motor. The switching points can be seen on the graph by the sudden drop
in current. In reality the blower circuit uses about 15 amps at full load. The above graph with
its current values is used just as a simulation.
Electronic control of brushless blower motor
The above waveform is analogue and is a typical way of controlling the speed of a motor. It is
very inefficient due to the wasted energy dissipated by the additional resistors. The resistor
pack R21 has a large heatsink enabling it to do this job. Not all motors have to be controlled
in this way.The ultimate aim is to reduce either the voltage or the current to the motor which
can be achieved through switching the current on and off very quickly, often as quick as 500
times per second (500Hz).
Pulse width modulation and duty cycle switching
By switching the motor on for one millisecond and then off for one millisecond and then on and
so on, the motor accelerates during its ‘on’ phase and freewheels during its ‘off’ phase. The
switching is very quick and the acceleration and deceleration are not noticeable until very low
switching rates are achieved; like 1 cycle per second (1Hz).When the motor is switched on it
will be connected to a power supply of 12 volts (V). If the signal is on for 50% of the time and
off for the other 50% of the time then the average voltage will be only 6V.This waveform is said
to have a duty cycle of 50%. If the on time is extended and the off correspondingly reduced, for
example 75% on and 25% off, then the duty cycle will be 75% with average voltage being 9V.
A number of motors and solenoids are controlled in this manner on a motor vehicle. By redu-
cing the voltage there is no waste heat created and a method of fine adjustment is achieved.
There is neither any current being drawn during the off time except for the electronics to drive
the circuit.This method of control is called Pulse Width Modulation (PWM).
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