System Fault Diagnosis; Fault Finding - Aston Martin DB7 Workshop Manual

Air Conditioning
System Fault Diagnosis
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System Fault Diagnosis
Probable causes of faults can be found by comparing actual system pressures, registered on the manifold gauge set
or recovery-recharge-recycie station, and the pressure to temperature relationship graphs found on the previous
page. The chart below shows the interpretation that may be made by this difference. The 'Normal' condition is that
which is relevant to the prevailing ambient and evaporator temperatures.
Note: If erratic or unusual gauge movements are experienced, check the equipment against a known manifold gauge set.
Low Side
Normal
Gauge
Normal to low
Low
Low
Low
Low
High
High
High Side Gauge
Normal
Normal
Low
Low
Low
Normal to high
Low
High
High
Fault Finding
High
Symptom
Discharge air initially cool
then warms up
As above
Discharge air slightly cool
Discharge air warm
Discharge air slightly cool or
frost build up at expansion
valve
Discharge air slightly cool
Compressor noisy
Discharge air warm and high
side pipes hot
Discharge air warm
Sweating or frost at evaporator
Diagnosis
Moisture in system
As above
HFC 134A charge low
HFC 134A charge very low
Expansion valve stuck closed
Restriction in High side of system
Defective reed valve
HFC 134A charge high or
condenser malfunction
Expansion valve stuck open
Caution: The microprocessor is extremely sensitive and should only be tested using a digital multimeter with no less than a 3.5
digit display and a resistance of no less than 2 M ohms. The use of any other form of multimeter will damage the microprocessor
irreparably.
Note: Always allow time for the servo motors and blower motors to come to a rest before starting a check.
The car should be in a workshop and the ambient temperature should be stable e.g. 24''C (75°F) for at least 30 minutes before
commencing the automatic check.
At 24°C the sensor voltage is 2.972 V ± 2 mV. The rate of change is lOmVper 1 °C.
Unless stated otherwise, all checks are carried out at the ECM test plugs
M o d e Switch: Off Ignition Switch: Aux 2
Signal Pin No.
Battery supply 1
Recirc input 9
Earth-ground 2
Earth-ground 6
Earth-ground 10
Earth-ground 38
Earth-ground 45
From mode switch 44
To mode switch 12
Voltage
n to 14v
0 t o 2 V
0 to 40mV
0 to 40mV
0 to 40mV
0 to 40mV
0 to 40mV
0 to 12V
10 to 13.3V
8-30
May 1996