AC Low Pressure Switch

Replace the AC low pressure switch on a 2009 Aston Martin DB9, which monitors refrigerant pressure and protects the compressor from damage.

Warnings

Tools required

Parts

• AC low pressure switch × 1 — Use OEM specification
• AC system O-rings × 2 — Use OEM specification
• PAG oil for AC system × 1 — ISO 46 viscosity

Fluids

• R-134a refrigerant

Preparation

1. Park vehicle on level surface and engage parking brake
2. Turn ignition off and remove key
3. Allow engine to cool completely if recently operated
4. Have AC system professionally evacuated and refrigerant recovered by a certified technician or using appropriate recovery equipment
5. Document refrigerant quantity removed for proper recharge
6. Disconnect negative battery terminal and wait 2 minutes for system discharge

Procedure

1. 1

   Access engine compartment
   Open the hood and secure it in the open position. The AC pressure switch location must be verified against Aston Martin service documentation for this vehicle. On the DB9 it is commonly a combined pressure switch mounted on the receiver-drier or associated AC line; identify the correct switch and line (high or low side) before proceeding. Use a shop light to locate the switch - it will have a small electrical connector and thread into the AC line. Note: The AC low pressure switch is located on the low-side (suction) AC line. Verify the exact location against Aston Martin service documentation for this vehicle before proceeding, as it may be mounted on the AC line near the compressor or receiver-drier assembly. Note: The AC low pressure switch is located on the low-side (suction) AC line. Consult the 2009 DB9 workshop manual for the exact location, as it may be mounted on the AC line near the compressor or receiver-drier assembly; verify the precise position before proceeding.
2. 2

   Remove engine cover if equipped
   If the decorative engine cover obstructs access, remove the retaining fasteners and lift the cover off. Verify the cover retention method against Aston Martin service documentation before prying. Set the cover aside in a clean area. Note: If the decorative engine cover obstructs access, remove it per the OEM procedure; verify whether it is retained by push-fit grommets or by fasteners before attempting removal.
3. 3

   Disconnect low pressure switch electrical connector
   Locate the electrical connector on the AC low pressure switch. Press the locking tab and pull the connector straight off the switch terminals. Note the connector orientation for reinstallation.
4. 4

   Remove low pressure switch
   Using an appropriate wrench (size to be verified against Aston Martin service documentation for this vehicle), unthread the low pressure switch from the AC line fitting. Turn counterclockwise to remove. Even with the system evacuated, be prepared for a small amount of residual oil to escape. Have a clean rag ready to catch any drips and immediately cap or cover the open port to prevent moisture intrusion. Note: Using an appropriate wrench (verify the exact size against Aston Martin service documentation before purchasing/applying), unthread the low pressure switch from the AC line fitting. Note: Using an appropriate wrench (size per OEM specification — verify against the 2009 DB9 workshop manual), unthread the low pressure switch from the AC line fitting.
5. 5

   Inspect O-ring and sealing surface
   Examine the O-ring on the old switch and the sealing surface in the AC line port. Look for signs of damage, contamination, or debris. Clean the threaded port with a lint-free cloth if necessary. Do not allow any foreign material to enter the AC system.
6. 6

   Prepare new low pressure switch
   Remove the new AC low pressure switch from its packaging. Install a new O-ring onto the switch threads if not pre-installed. Lightly lubricate the O-ring with fresh PAG oil (approximately 2-3 drops) to ease installation and ensure proper sealing. Do not use petroleum-based lubricants.
7. 7

   Install new low pressure switch
   Thread the new low pressure switch into the AC line port by hand, turning clockwise. Ensure it threads smoothly without cross-threading. Once hand-tight, use a torque wrench and tighten to the OEM-specified torque for this fitting (verify against Aston Martin service documentation before applying; a typical AC pressure switch torque range is roughly 7-11 Nm / 5-8 lb-ft, but confirm the exact value for this vehicle). Do not overtighten as this can damage the fitting or O-ring seal. Note: Once hand-tight, use a torque wrench and tighten to the manufacturer-specified torque for this fitting (verify the exact value against Aston Martin service documentation before applying). Note: Once hand-tight, use a torque wrench and tighten to the manufacturer-specified torque (verify against the 2009 DB9 workshop manual before applying).
   Torque spec

   AC Line Fittings20 Nm (15 lb-ft)
8. 8

   Reconnect electrical connector
   Push the electrical connector firmly onto the new low pressure switch terminals until the locking tab clicks into place. Verify the connection is secure by gently tugging on the connector.
9. 9

   Reinstall engine cover
   If removed, reinstall the decorative engine cover by aligning the mounting grommets with the posts and pressing down firmly until fully seated.
10. 10

    Reconnect battery
    Reconnect the negative battery terminal. Do not start the engine yet - the AC system must be evacuated and recharged first.
11. 11

    Evacuate and recharge AC system
    Connect AC manifold gauges to the high and low side service ports. Use a vacuum pump to evacuate the system to at least 29 inches of mercury (Hg) for a minimum of 30 minutes. Recharge to the OEM-specified R-134a capacity for this vehicle (verify against Aston Martin service documentation), not merely the amount recovered. Monitor for vacuum loss - the system should hold vacuum, indicating no leaks. Once evacuation is complete, recharge the system to the OEM-specified refrigerant capacity for this vehicle (verify the exact quantity and refrigerant type against Aston Martin service documentation). Follow proper charging procedures for liquid or vapor charging as appropriate. Follow proper charging procedures for liquid or vapor charging as appropriate. Note: recharge the system with R-134a to the OEM-specified total charge quantity for the 2009 DB9 (verify capacity against the workshop manual rather than relying solely on the recovered amount)

Reassembly

1. Ensure all electrical connectors are fully seated and locked
2. Verify no tools or materials were left in the engine compartment
3. Double-check that the low pressure switch is torqued to specification

Verification

• Start the engine and allow it to reach normal operating temperature
• Turn on the AC system and set to maximum cooling
• Verify the compressor engages and the AC blows cold air
• Monitor AC pressures using manifold gauges - low side should typically read 25-45 PSI at idle with ambient temperature around 70-80°F
• Check for refrigerant leaks around the new switch using an electronic leak detector or soap solution
• Verify the low pressure switch is functioning by monitoring compressor cycling behavior
• Confirm no warning lights or AC-related fault codes are present using a diagnostic scanner

Related trouble codes on this vehicle

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