Aston Martin diagnostic trouble code
P0645: A/C Clutch Relay Control Circuit
Use this reference to understand the recorded condition, confirm the available V8 Vantage or DB9 manual coverage, and follow the documented diagnostic checks before replacing parts.
- Code family
- Powertrain
- Vehicle system
- Transmission
- Manual coverage
- V8 Vantage, DB9
- Warning-lamp status
- V8 Vantage: Not stated; DB9: MIL
What Aston Martin code P0645 means
A/C Clutch Relay Control Circuit
DB9 preliminary diagnostic manual · section 2-156
Manufacturer fault-analysis procedure
Manual fault definition
P0645 - A/C Clutch Relay control circuit
Cautions and notes
CAUTION: This code should be generated only in
the Primary Powertrain Control Module (PCM).
Check that you are working on the correct half of
the engine.
the Primary Powertrain Control Module (PCM).
Check that you are working on the correct half of
the engine.
Test sequence and decision path
1. Connect the WDS or scan tool. With the engine
warm, select A/C on. Monitor the level of the AC
Clutch Request signal at primary PCM pin
C0635-25.
If the AC Clutch Request signal remains at 12V,
go to step 2.
If the AC Clutch Request signal switches to 0V, go
to step 3.
2. Check the level of the A/C Request (ACCS) signal:
Monitor the A/C Request signal at primary PCM
pin C0635-15 whilst switching the A/C system on
and off.
The Clutch Request signal should be at 0 volts
with the A/C system On.
If the A/C Clutch Request signal remains at 12
volts, check for a short from this line to VPWR.
If no short circuit exists, consult Aston Martin
Technical Support.
If a short circuit is detected, repair the defect.
Reset the PCM and rerun the KOER Test.
3. Check the Air Con clutch relay:
Remove the A/C clutch relay and check the coil
resistance. Measure between relay pins 85 and
86. The coil resistance should be between 40 and
85Ω. If the coil resistance is incorrect, replace the
relay. Reset the PTEC and rerun the KOER Test.
Check for internal shorts in the relay:
Measure resistance from relay pin 85 to pins 30,
87 and 87A. The resistance should be greater
than 10k Ω. If the resistance is less than 10k Ω,
replace the relay.
If the relay resistance checks are good, refit the
relay and go to step 4.
If the relay is defective and is replaced, reset the
PCM and rerun the KOER Test.
4. Check the A/C clutch relay circuit:
Disconnect the PCM. Key on, check for greater
than 10.5 volts at PCM connector pin C0635-25.
If greater than 10.5 volts is not present, service
the open circuit in the VPWR or the A/C clutch
signal lines.
Key off. Reconnect the PCM. Key on. Switch on
the A/C (Mode switch from ECO to A/C for immediate response). Check for greater than 10.5 volts
at A/C clutch relay pin 30. If the voltage is less
than 10.5 volts, go to step 5.
If the voltage is greater than 10.5 volts, the supply
from the A/C clutch relay is good.
Measure the voltage at A/C clutch relay pin 87. If
less than 10.5 volts with the A/C selected on, the
clutch relay is defective. Replace the relay, reset
the PCM and rerun the KOER Test.
Open the corresponding manual page for wiring diagrams and figures
warm, select A/C on. Monitor the level of the AC
Clutch Request signal at primary PCM pin
C0635-25.
If the AC Clutch Request signal remains at 12V,
go to step 2.
If the AC Clutch Request signal switches to 0V, go
to step 3.
2. Check the level of the A/C Request (ACCS) signal:
Monitor the A/C Request signal at primary PCM
pin C0635-15 whilst switching the A/C system on
and off.
The Clutch Request signal should be at 0 volts
with the A/C system On.
If the A/C Clutch Request signal remains at 12
volts, check for a short from this line to VPWR.
If no short circuit exists, consult Aston Martin
Technical Support.
If a short circuit is detected, repair the defect.
Reset the PCM and rerun the KOER Test.
3. Check the Air Con clutch relay:
Remove the A/C clutch relay and check the coil
resistance. Measure between relay pins 85 and
86. The coil resistance should be between 40 and
85Ω. If the coil resistance is incorrect, replace the
relay. Reset the PTEC and rerun the KOER Test.
Check for internal shorts in the relay:
Measure resistance from relay pin 85 to pins 30,
87 and 87A. The resistance should be greater
than 10k Ω. If the resistance is less than 10k Ω,
replace the relay.
If the relay resistance checks are good, refit the
relay and go to step 4.
If the relay is defective and is replaced, reset the
PCM and rerun the KOER Test.
4. Check the A/C clutch relay circuit:
Disconnect the PCM. Key on, check for greater
than 10.5 volts at PCM connector pin C0635-25.
If greater than 10.5 volts is not present, service
the open circuit in the VPWR or the A/C clutch
signal lines.
Key off. Reconnect the PCM. Key on. Switch on
the A/C (Mode switch from ECO to A/C for immediate response). Check for greater than 10.5 volts
at A/C clutch relay pin 30. If the voltage is less
than 10.5 volts, go to step 5.
If the voltage is greater than 10.5 volts, the supply
from the A/C clutch relay is good.
Measure the voltage at A/C clutch relay pin 87. If
less than 10.5 volts with the A/C selected on, the
clutch relay is defective. Replace the relay, reset
the PCM and rerun the KOER Test.