Air conditioning fault diagnosis table
9.14.2 Air conditioning fault diagnosis table
Possible fault Figure 9.55 Screen heating circuit
Symptom
After stopping the
Air in the system or if no bubbles
compressor, pressure
are seen in the sight glass as the
falls quickly to about
condenser is cooled with water then
195 kPa and then
excessive refrigerant may be the fault
falls gradually Discharge pressure
Fault with the compressor or, if
low
bubbles are seen, low refrigerant Discharge temperature Frozen evaporator
is lower than normal Suction pressure
High pressure valve fault, excessive
too high
refrigerant or expansion valve open too long
Suction and discharge
Excessive refrigerant in the system
pressure too high
or condenser not working due to fan fault or clogged fins
Suction and discharge
Clogged or kinked pipes
pressure too low Refrigerant loss
Oily marks (from the lubricant in the refrigerant) near joints or seals indicate leaks
Figure 9.56 Circuit voltage testing
238 Advanced automotive fault diagnosis
9.14.3 Heating and ventilation
No hot air
Matrix blocked
fault diagnosis table Blend flap stuck
No cold air
Blend flap stuck Blocked intake
Symptom Possible fault
Reduced
Cooling system thermostat stuck open
Heater matrix partially blocked Booster fan not
temperature
Control flap not moving correctly operating at
Open circuit fuse/supply/earth
when set to ‘hot’
Motor inoperative/seized any speed
Dropping resistor(s) open circuit Switch open circuit Electronic speed controller not working
9.14.4 Air conditioning receiver
Booster fan only Dropping resistor(s) open circuit
drier sight glass
works on full Switch open circuit speed
Electronic speed controller not working
A very useful guide to diagnostics is the receiver
drier sight glass. Figure 9.57 shows four possible
Control flap(s) Check vacuum connections (many work
will not move by vacuum operated actuators)
symptoms and suggestions as to the possible
Inspect cables
fault.
HVAC systems diagnostic chart Start
Hand and eye checks (loose wires, loose switches and other obvious faults) – all connections clean and tight
Check battery – must be 70% charged
Check and top up coolant as required
Check system pressures and recharge if necessary
Check operation of
No
Blower motor
control switches
runs correctly?
Check discharge temperature Check supply and earth
Yes
at the motor
Inspect receiver drier If an ECU is used check
sight glass supplies from switches and
Ensure engine is up to
outputs to motor as well as
temperature
the ECU main supply and earth
Inspect for signs of refrigerant leaks (oily deposits)
Air temperature only blows cold
Yes
or only hot?
No
Check sensors if fitted
Check water supply
Is AC
No
working (if
fitted)?
Check operation of compressor clutch
Check flap operation
Yes
End
Electrical systems 239
Speed
ECU Speed
sensor
controller
Figure 9.58 Cruise control – closed control loop
at the touch of a button. Most systems also remember the last set speed and will resume this again at the touch of a button.
To summarise and to add further refinements, the following is the list of functional require- ments for a good cruise control system:
● hold the vehicle speed at the selected value; ● hold the speed with minimum surging; ● allow the vehicle to change speed; ● relinquish control immediately the brakes are
applied; ● store the last set speed;
● contain built in safety features.