This section discusses a wide range of test equipment and approaches available when diagnos-
ing faults on A/C systems. Live data from meters, scopes and serial data obtained from scan-
ners are important elements of this section.The differences between when and how to obtain
such data is crucial to obtaining information reliably and quickly.
Tests for fault diagnosis on A/C modules
Multimeter
Volt drop tests This test involves placing the voltmeter leads across
the input from the power supply to the module and the battery
positive terminal and checking for acceptable volt drops.
Ground- to- ground test involves placing the meter leads from the
ground terminal of the module and battery ground.
The maximum volt drop should not exceed 0.5 V.
A current test can be carried out to check for excessive current due to
an internal or external short circuit which is diverting all the power
(see Fig. 3.98).
A resistance test is not applicable
Continuity test can be carried out with the power isolated and the
A/C module disconnected from its harness connector. This test can
be used to check for any open circuits in wiring and connectors.
A diode test is not applicable
Power probe (with built-in LED display)
The use of a power probe is ideal to power an A/C module if used
cautiously.
The power probe can be used to power the A/C module
directly. This allows the bypassing of any potentially faulty power
supplies. It is also possible to supply a ground signal.
The LED display is ideal when checking power and earth switched pins
on the A/C module. The earth path of the module will power the
green LED and the power to the module will illuminate the red LED.
Serial test
A very important test to read any fault codes, check data lists and any
possible actuator functions. If the A/C module is faulty then this may
prevent you from communicating with this particular system via a
serial tester. You should still be able to communicate with other
modules in this instance. If you cannot then a problem with power
distribution or diagnostic communication is evident.
Break-out box testing
Very useful for pinpointing power supply problems. Checked by using
either an oscilloscope or multimeter connected to pins on the
break-out box which are powered by relays to supply current to
modules.
Oscilloscope
Very useful for measuring any background interference or electrical
noise within the circuit and checking any volt drop going to and
across the relay (under load).
Power- to- power test – this is when the leads of the meter are placed
one on the battery terminal and one to the supply of the relay. This
is to check that the relay supply voltage is acceptable. The maximum
volt drop should not exceed 0.5 V. Earth-to-earth test – this is when
the leads are placed on the earth of the relay and the earth of the
battery. This checks that the earth path is good. The maximum volt
drop should not exceed 0.5 V.
Other tests
Use of any self-tests which may be available within the A/C module.
Using a Multimeter
A high impedance meter with a minimum internal resistance of 50 megaohms/volt is required
for measuring modern sensors and actuators on automotive systems.A meter with a range of
up to at least 50 VDC and as low as 200mV.VAC should be at least 200 VAC and have resist-
ance ranges between 0–2 megaohms. If the multimeter has a frequency, tachometer, PWM,
Duty cycle and a connector for a temperature probe then other data can be obtained. Current
measurement is extremely important. Cheap meters only go up to 10 or 20 amps. If high cur-
rent measurement is required then purchase a quality meter which can measure high currents
with a shunt or a compatible amp clamp. The ideal approach is to purchase a digital oscillo-
scope (Fluke is highly recommended) and amp clamp.
Example specification:
VDC 0–50
VAC 0–200
Resistance 0–2M
Current 0–500 amps (shunt or amp clamp)
Tachometer 0–10 000 rpm (not essential if you have frequency measurement)
Frequency 0–500Hz
Dwell 0–90°
Duty Cycle 0–100%
PWM 0–20ms
Note – when using a multimeter you can only assess the readings obtained by the meter and
not the control signal itself. This means you cannot measure any background interference or
electrical noise.AC readings will be based on the rms (root mean square) voltage (Urms)
which is approximately 70% of the peak value.This means that these meters are not really
adequate for measuring AC type vehicle sensors. Oscilloscopes should be used in these
circumstances.An oscilloscope will measure the peak (amplitude) and the peak-to-peak
value of the waveform including the signal shape, polarity, frequency and any interference.
Wiggle test
A wiggle test is carried out by attaching a meter to a circuit and obtaining a reading.The tech-
nician wiggles the cables and connectors to observe variations in the meter reading in the pur-
suit of verifying a fault.This can also be done using a serial tester and oscilloscope (trend plot).
A serial tester/code reader will produce a fault code if a fault is sensed in a circuit during a
wiggle test. This allows the technician to use the control module as the diagnostic tester. This
test should be a part of a sequence and not relied upon as a singular result.
Load variations
When testing sensors, actuators, modules and cabling a variation in loading on the component
will be required. Similar to a wiggle test,where a load is applied by using a force on a component,
its cabling etc.Temperature sensors need to be subjected to a real temperature variation. Speed,
load, temperature, position, vibration can be used to try to simulate the real environment which
the component is normally subjected to.While the sensor or actuator is being subjected to these
variations a quality tester can be used to view or preferably record the data for analysis.
Figure 3.94
Resistance (ohm )
Resistance measurement is carried out in parallel, across the component.Ohmmeters use their
own power source which means that all power in the circuit being tested must be isolated.
If this is not done and the meter is placed in the circuit then the meter can become damaged.A
resistance measurement is not a dynamic test.This means that the sensor will not be under load.
Resistance measurement with digital multimeter:
Fault symptom:
Lamp H1 does not illuminate.
Pre-condition:
Battery voltage OK, fuse OK, bulb OK. Set digital multimeter to (resistance), fuse F2 pulled
(during the resistance measurement, current must not flow through the lead being measured).
Continuity
A continuity check is based on checking for a complete unbroken electrical path in a circuit,
part of a circuit or through a component.A continuity test checks if a circuit is ‘continuous’
(continuity) and not interrupted (open circuit).A value is not required from the circuit just an
indication that the circuit is open or closed.
Ohmmeter
When testing continuity with an ohmmeter the circuit or component being tested must not be
a part of a live circuit.All power to the circuit or component must be isolated. The meter is
positioned in parallel, across the component. If no continuity exists in the circuit then an infin-
ite sign will appear on the meter.
Voltmeter and ammeter
A voltmeter and ammeter will test the continuity in a live circuit.The voltmeter and ammeter
are positioned in series and generally any reading above ‘000’ indicates continuity.
Voltage – potential difference (V)
Voltmeters should have high internal resistances and only draw a small amount of current.A volt
drop is measured by placing the meter in parallel (across the component) at two points, while
the circuit is under load.The voltage at a point in a system can be measured by ensuring that
one of the points of the meter has a zero reference (fitted to the battery earth).Meters which
have an automatic range will provide the reading as long as it is within the capabilities of the
meter.Analogue meters should not really be used on a vehicle system. If they are used to meas-
ure voltage then the meter should have an internal resistance of no less than 50 kohms/volt.
Permissible volt drops in a circuit
The volt drop in a circuit depends on the cross-sectional area of a cable, its length and the cur-
rent flowing through it. The current flowing through the circuit has the greatest effect. For
example, when the engine is cranked the battery voltage can reduce to as low as 10.5V.This is
a volt drop of 2 volts due to the high current flow. In some circuits this may be acceptable. In
low current circuits up to approximately 2–3 amps a volt drop within the circuit of approxi-
mately 0.5V should not be exceeded.
A power-to-power test is when the leads of the meter are placed one on the vehicle battery
positive terminal and one to the supply of a control module or other consumer powered by
battery voltage. This is to check that the supply voltage is acceptable. The maximum volt drop
between the two points should not exceed 0.5V.Low current systems should be as low as 0.25V
(sensor circuit, i.e. temperature sensor). If the voltage is exceeded then a resistance between
the two points exists, for example faulty relay contacts, poor battery terminal connection,
faulty driver circuit inside the module.
The power to a sensor supplied with 5V from a control unit can also be tested. Place the
ground lead of the voltmeter on the module 5V output pin on the module connector and the
positive lead on the power at the sensor.A volt drop of no more than 0.25V should be expected.
figure 3.95
An earth-to-earth test is when the meter leads are placed on the earth of a component like
a sensor and the earth of the battery. This checks that the earth path is good. The maximum
volt drop should not exceed 0.5V. Low current circuits should be as low as 0.25V (sensor cir-
cuit, i.e. temperature sensor). If the voltage is exceeded then a resistance between the two
points exists, for example a bad earth or trapped wire.
Voltage measurement with digital multimeter:
Example fault symptom:
Lamp H1 does not illuminate.
Pre-condition:
Battery voltage OK, fuse OK, bulb OK. Set digital multimeter to VDC (direct voltage).
Different meter configuration giving different readings
figure 3.96
The simulation in Figure 3.96 shows a simple motor circuit in three different configurations.
The configurations only change with the position of the voltmeter positions.The simulation is
designed to show the different readings obtained by placing the meter in different configur-
ations. In all the configurations the meter is reading the potential difference between the posi-
tive and the negative terminal. The difference in the meter readings are due to whether the
negative connector is attached to a zero reference or across the component. The circuits in
Figure 3.96 are not operational due to the switch being open. This means that the motor is
ground switched. By placing the meter across the motor you get a potential difference (pd) of
zero due to 9V being measured both sides of the meter. The two other meters show voltage
due to the ground connector being ground referenced (attached to battery earth). If the switch
is integral to a control module then voltage will be measured all the way to the pin on the module
figure 3.97
but no current will flow until the switch inside the module is closed.This can often lead to con-
fusion when trying to understand when current is flowing and when it is not.
The closed circuits have produced two different meter readings and one meter which has
not changed. If a component is ground switched like the example in Figure 3.97 then placing
the meter across the component will provide a zero voltage when the component is off and
supply voltage (minus volt drop caused by current flow) when on. If the meter is placed on the
supply side of the component which is ground switched then no change in voltage will be
measured during its operation. If the meter is placed on the ground side of the component then
during its operation it will change from supply voltage to between zero and 500mV (volt drop
depends on amount of current flowing) also called a ground-to-ground test.
Measuring the pd across the component is always recommended. If just the supply to the
component is required then place the multimeter cables on the supply of the component and
a zero ground reference (battery ground).A power-to-power check should also be made by
placing the cable of the meter onto the supply of the component and the other meter cable on
the battery supply (or other voltage supply, i.e. 5V from ECU).This will provide a reading on
the difference of the two voltages.This difference should not be more than about 500mV.
Measuring AC voltage
Alternating voltage is only produced on a motor vehicle by the alternator which is then recti-
fied into DC before charging the battery. The only other means of measuring AC on a motor
vehicle is measuring the output from the sensor.An inductive sensor produces an AC signal
which can be used to measure the speed, position and rate of change (velocity) of a variable
(e.g. the vehicle speed). The signal will be transmitted to a control module. The best way to
measure an AC signal is using an oscilloscope. Inductive signals change in amplitude, frequency,
and voltage as well as shape.Obtaining peak-to-peak voltages, frequency, shape and amplitude
from a multimeter is beyond its ability.
Measuring current
Measuring current through a circuit is the most important test available to the technician. For
example, if the current was measured flowing through the power supply of an A/C module the
test would give the technician a great deal of information.To understand how much information
which could be obtained with such a test an appreciation of the current path is required.The cur-
rent flows from the vehicle battery and terminals through cabling, fuses, relays,more cabling and
eventually the module connector.Once inside the module the current will supply internal driver
and memory circuits, other sensors which the module powers, and eventually go to earth. The
earth path includes earth connection to the chassis, then through the chassis and back to the bat-
tery.During the test the circuit is under load.Compare this to measuring the potential difference
of two points. Current is the preferred measurement but the problem of finding data on current
values is not always easy. It is always good practice to take measurements from vehicles that are
operating correctly and record data for comparison purposes.The value of potential difference
measurement is important but current should always be applied whenever possible.
An ammeter is a low resistance instrument and is very easy to destroy. It is also connected
n series with a circuit and is polarity conscious.This can cause problems due to the inability to
break the circuit to place the meter in series. Fuseholders are often a very good location for fit-
ing an ammeter.Upon removal of the fuse the ammeter set on the correct meter range can be
placed across the pins of the fuse holder to allow all the current of the circuit to flow through
he meter. Short circuits, open circuits and low current flow due to a high resistance in a circuit
can be measured. Always invest in a good ammeter. Vehicle batteries can supply up to and
above 500 amps so if a short circuit to earth exists within a circuit, this will flow through your
meter. Shunts are often used to protect ammeters and are worth investing in. Current meas-
urement of a compressor coil can benefit the diagnosis of a faulty coil or a large air gap.
In the case of a lightbulb fitted to a fuseholder of a circuit which is not operating due to being
open circuit, if the circuit becomes intermittently closed then the bulb will light up.This is often
done while carrying out wiggle tests (wiggling cables and connectors) on electrical cables during
faultfinding.
AMP clamp
A current clamp is a passive tester which has sensing circuitry that calculates accurately how
much current is flowing through a circuit. It is passive because the circuit being measured can
remain complete and not be disconnected like conventional methods of testing currents in
series.The clamp connects via a BNC connector to the multimeter leads of an oscilloscope or
figure 3.98
conventional multimeter.The meter is set to voltage and the clamp senses the effects of current
flowing through the cable and produces a voltage reading that represents a current flow through
conversion, e.g. 100mV/A so 500mV means that 5 amps is flowing in the circuit.Amp clamps
are a must for modern diagnostic testing.
Measurement of PWM using a voltmeter
If we measure PWM by means of a voltmeter (AC), the instrument will show the average volt-
age carried by the cable so that a higher pulse ratio will show a higher average voltage reading.
Using the voltmeter, we can obtain a rough estimate of the pulse ratio. A pulse ratio of 9%
(switched by 13V) will often give a reading of about 1.2V (0.09 X 13V = 1.17V). In the case of
positive-triggered PWM, connect the red test lead to the cable and the black test lead to a good
ground. In the case of negative-triggered PWM, connect the black test lead to the cable and the
red test lead to battery positive. Select ‘Smooth’ on the voltmeter if it has this function.
Power probe
Power probes are useful when a current or a ground signal needs to be applied to a vehicle sys-
tem. The probe is connected to the vehicle battery and can supply voltage at 12–14V (if the
engine is running) and current up to approximately 20 amps. The probe has Light Emitting
Diodes (LEDs) built into the casing which indicate if a wire is live (red LED) or ground (green
LED). Power probes also have trip switches in case of an accidental short circuit. If a voltage is
absent or weak, additional voltage can be applied to the circuit by pressing a switch on the probe.
It must be noted that components can be destroyed through the misuse of a power probe.
Power probes should not be used to apply voltage to a control module. Power probes are very
useful for testing relay circuits (power and earth points) and upon the removal of a relay the
probe can be used to power an A/C compressor clutch providing the fault is electrical and not
with the A/C system, i.e. lack of refrigerant (care must be taken or damage to the A/C system can
occur due to lack of oil or refrigerant in the system). Power probes can also be used to provide
voltage to low current DC motors like recirculation motors.
LED logic probe tester
LED probe testers have two LEDs which are used for a range of tests.One LED is green and
the other is red. The red will illuminate whenever it touches a voltage above 0.7V; the green
LED will illuminate whenever it senses a ground signal.The most useful test is a continuity test.
The sensor will light when a voltage above 0.7V is sensed.This tester is very useful for testing
pulsed circuits.These circuits include stepper motors,Hall sensors or variable reluctance sensors
where the voltage drops to zero and rises above 0.7V.This causes the LED to switch on and off
rapidly (flash) indicating a signal is present.They are also very useful for placing across an A/C
cycling switch to provide an indication of when the switch is cycling.
Serial testing (OBD 16- pin connector)
Serial testers are diagnostic testers used to communicate with vehicle systems via the multi-
plex communication network. This means that the serial tests must be able to ‘communicate’
via the protocols used throughout all the different manufactured vehicles. It is important to
note that the information a serial tester provides is simulated to aid diagnostics and inform the
technician on how the system is functioning.The data is not live and is often not ‘seen’ in the
same format as it is presented by the diagnostic tester.An example to aid the explanation is an
air distribution door stepper motor.The serial data may be presented as a percentage of open-
ing: 0% closed, 50% half open and 100% fully open. The signal the A/C module sends to the
motor is a pulse train used to rotate the door.This means that the serial information will be in
a different format than the actual signal which is live and would be sampled using a different
process.
Serial testers play a very important role in modern diagnostics.The most common use of a
serial tester is for fault code analysis.This allows the technician to see if any faults have been
recorded by the modules within the system and logged using a unique fault code which is
assigned to every sensor and actuator on the vehicle. Serial testers often provide the technician
with the ability to see more simulated information at a glance than say using a dual oscillo-
scope when only two signals can be monitored at once. The opportunity exists to carry out
actuator tests where actuators are energised to witness operation. Serial testers also have
important programming functions so that adjustments to a module’s memory (data stored in
EPROM) can be made allowing the vehicle system to be reprogrammed ‘in service’. Repro-
gramming changes the operational parameters the system puts in place.Advanced serial testers
which are dedicated to manufacturers’ use, often have comprehensive guided diagnostic cap-
abilities.As vehicles advance the units can be updated by being programmed via intranet or
CD ROM and interface with the other knowledge-based systems which the manufacturer uses
(technical data systems and data collection).
Serial test – fault code check
While the parameters, or readings, required by OBD II regulations are uniform, auto manu-
facturers had some latitude in the communications protocol they used to transmit those read-
ings to scanners. Expensive scanner consoles costing thousands of pounds often include
decoding software and firmware for all protocols in their units, making them universal. Less
expensive units are usually customised for a specific communication protocol. Be sure the
scanner you are using suits the protocol of your customer base.
For the retrieval of such codes the diagnostic tester is plugged into the Diagnostic Link
Connector (DLC).The diagnostic tester communicates with the system (if networked) or indi-
vidual module to match the protocols the system is using. If a match cannot be found then a
communication error will be displayed. If the communication is successful then information
about the vehicle or system will be displayed.A simple menu structure is used on the tester
providing a range of facilities for diagnostics. DTC will allow the user access to any stored
trouble codes (referred to as historical codes) which should be recorded by the technician and
then removed. DLC monitoring is continuous so the vehicle can be taken for a test drive and
monitored for any codes that may present themselves.A DLC often only provides an indica-
tion that a fault exists within a particular system or associated with a component. It will not tell
you exactly what is wrong.Additional testing is often required.
Note – always check fault code history. If in doubt of system performance then record and
clear the fault code history and take the vehicle for a dealer drive cycle test.
Data collection
On some serial testers data collected during a test session may be saved in the instrument for
further analysis and also transferred to a PC for future reference. The serial tester is able to
perform efficient fault diagnostics on different types of control system, including engine man-
agement,ABS, airbag and of course air-conditioning. The serial tester is portable, has a built-
in battery supply and may be used for on-road testing.
figure 3.99
3.100
Data list
A serial tester reads current data from the vehicle’s control unit.The values are updated con-
tinuously (small delay).
Data list information can be used to test sensors’ wiring.A typical example is using a jumper
wire across terminals to check extreme sensor variations.A jumper wire may be placed across
the input and output of a sensor to test the wiring of the sensor.
Connect fused jumper wire to:B135 sensor/air quality wiring harness connector (wiring harness
side) terminal 3 and terminal 2.
Diagnostic tester data list parameter air quality sensor will read 0%.
Actuators
In most cases, actuators can simply be switched on and off.
Snapshots
All screens that display data received from the vehicle can be saved as ‘snapshots’.The snap-
shots can be downloaded to a PC to create test reports.The saved information is displayed in
the same way as when the information was saved.
Programming
Advanced serial testers can be used to program control modules or other components like
transponder keys.
figure 3.101
3.102
Wiggle test
A wiggle test detects intermittent faults in harnesses and/or connectors. When the test is
performed the vehicle module is used to detect and record any faults that present themselves
during the test.Hence, it is the intelligence of the module that determines the accuracy of the
test.During the test, wiggle multi-plugs and wiring without using excessive force or detaching
connectors. Some testers provide an audible beep when a fault has been found in the affected
plug or associated wiring.
A/C systems with built-in self-test diagnostics
ATC and SATC systems have the facility to monitor their own inputs and outputs by means of
a self-test. The system recognises continuous or sporadic faults and stores them under an
appropriate fault code. These fault codes can be read out by activating a diagnostic mode.
Diagnostic mode is accessed by sequential operation of a certain key combination on the cli-
mate control selection panel.To start self-diagnosis and read out the fault memory, the ignition
key must be turned to the ‘ON’ position and the battery voltage must be between 9 and 16V.
Generally codes are presented in a two-digit format or a flashing LED (blink code).They are
presented on the climate control graphics interface.
This is in addition to fault codes accessed via a diagnostic tester by connecting the 16-pin
Data Link Connector (DLC) to the vehicle.
Activation of self-diagnosis
Activation can be automatic which means that as soon as a fault is present an LED will flash
or message will be displayed on the graphics interface. More common is activation through
pressing a sequence of buttons.
For example, briefly press the ‘OFF’ and ‘FOOTWELL’ buttons on the A/C controls simul-
taneously for at least 2 seconds, then press ‘AUTO’ within 1.5 seconds.The self-diagnosis which
then starts lasts a few seconds.An animated display appears during this time.Any faults found
are displayed in the form of trouble codes. Example: first of all, ‘93’ flashes for 2 seconds, then
‘42’ flashes for 2 seconds – DTC 9342. If no faults are stored, then all of the segments in the dis-
play are actuated. Diagnosis mode can be stopped at any time by pressing any button on the
A/C controls. If the ‘DEFROST’ button is pressed to end the diagnosis mode, all DTCs in the
fault memory will be deleted!
Read out stored faults
On the A/C controls briefly press the ‘OFF’ and ‘FOOTWELL’ buttons simultaneously for at
least 2 seconds, then press ‘HEADROOM’ within 1.5 seconds.Any stored intermittent faults
are output on the graphics display and should be noted for safety reasons. By pressing the
‘DEFROST’ button, the fault memory is cleared and diagnosis mode is ended.To end diagno-
sis mode without clearing the DTCs, press any other button on the A/C control.
The above is just one example of a self-test activation procedure.Always refer to manufac-
turers’ specifications.
Break-out box testing (parallel communication)
Whenever possible, break-out boxes should be used for testing electrical signals.A break-out box
is a parallel interface which attaches to the existing control module connectors allowing easy con-
nection to wiring without reducing its integrity. In simple terms, remove the connector (female) of
a module which may contain hundreds of pins and plug it into the connector of a break-out box
(male) and then plug the connector (male) of the break-out into the module.All the electrical sig-
nals travelling into and exiting the control module will travel via the break-out box.
As an example, if the module has 104 pins on its connector then a 104 pin break-out box can
be attached allowing the technician quick and easy access to testing all of the electrical signals.
The pin numbers on the module connector usually match the pin numbers on the break-out
box. For example, pin 1 on the module connector should be pin1 on the break-out box.
Some aftermarket suppliers produce generic break-out boxes that fit to a range of manu-
facturers.They often use overlays on the break-out box pins to match the pin numbers of the
module with the numbers on the box.
figure 3.103
Some manufacturers use break-out boxes that can interface with their serial testers which
allow the monitoring of live data instead of simulated data via the 16-pin serial connector.
Other manufacturers attach oscilloscopes to break-out boxes to monitor, record, and plot live
data. Break-out boxes are extremely useful, even small boxes with 2, 3, 4, 5 pins fitted to allow
the connection to individual sensors and actuators.
If a break-out box is not available then suitable adaptor leads must be used to take readings
from the module connector. Wiring information on connector views and colour codes are
required for such a task.
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