AC Low Pressure Switch

Replace the AC low pressure switch on the 2007-2013 Mercedes-Benz S65 AMG to restore proper air conditioning system pressure monitoring and compressor protection.

Warnings

Tools required

Parts

• AC low pressure switch × 1 — A0025456624 or equivalent
• O-ring for low pressure port × 2 — Use OEM specification
• R134a refrigerant × 1 — Approximately 1400g total system capacity
• PAG 46 compressor oil × 1 — 30ml for service loss
• UV dye (if not present) × 1 — Use OEM specification

Preparation

1. Park vehicle on level ground and engage parking brake
2. Allow engine to cool completely if recently operated
3. Ensure AC system has been diagnosed to confirm low pressure switch failure (common faults: compressor not engaging, fault code B1448)
4. Connect AC recovery machine to high and low side service ports
5. Record current refrigerant weight removed for accurate recharge
6. Open hood and locate low pressure switch on suction line near firewall passenger side (may require removal of engine cover panels)
7. Take photographs of electrical connector orientation for reference

Procedure

1. 1

   Recover refrigerant from AC system
   Connect AC recovery machine to both high and low side service ports. Follow machine manufacturer procedures to completely recover all R134a refrigerant from the system. Record the amount recovered for recharge reference. Wait until machine indicates recovery is complete and system pressure is at vacuum.
2. 2

   Remove engine cover components if needed
   Depending on access, remove upper engine covers to improve visibility and reach to the AC suction line. The V12 configuration makes access tight. Remove plastic covers secured by quick-release fasteners. Set covers aside in a clean area.
   Torque spec

   Cover Screws15 Nm (11 lb-ft)
3. 3

   Disconnect low pressure switch electrical connector
   Locate the electrical connector on the low pressure switch (mounted on the larger diameter suction line near the firewall). Press the locking tab and carefully disconnect the connector. Inspect connector pins for corrosion or damage. Move wiring harness aside to provide clearance.
4. 4

   Remove low pressure switch from suction line
   Using a 13mm wrench, carefully unthread the low pressure switch from the AC suction line fitting. Turn counterclockwise to remove. Some residual refrigerant or oil may escape - have a clean shop towel ready. Remove the old O-ring from the port and discard. Inspect the port threads for damage.
5. 5

   Prepare new low pressure switch
   Install a new O-ring onto the new low pressure switch. Lightly coat the O-ring with PAG 46 compressor oil (approximately 2-3ml) to ensure proper sealing and prevent pinching during installation. Verify the new switch part number matches the original and has the correct pressure range specification.
6. 6

   Install new low pressure switch
   Thread the new low pressure switch into the suction line port by hand to start. Ensure it threads smoothly without cross-threading. Once hand-tight, use the 13mm wrench to tighten to the specified torque. Do not over-tighten as this can damage the O-ring seal or port threads.
   Torque spec

   AC Line Fittings20 Nm (15 lb-ft)
7. 7

   Reconnect electrical connector
   Reconnect the electrical connector to the new low pressure switch. Ensure the locking tab clicks into place securely. Gently tug on the connector to verify it is fully seated and locked.
8. 8

   Evacuate AC system
   Connect AC vacuum pump to both service ports using manifold gauge set. Evacuate the system for a minimum of 45 minutes to remove all air and moisture. Monitor vacuum gauge - it should reach and hold 29+ inches Hg. After evacuation, close manifold valves and observe for 10 minutes to check for vacuum loss indicating a leak.
9. 9

   Recharge AC system with refrigerant
   Using a refrigerant scale, recharge the system with the exact amount of R134a recovered (typically 1400g total system capacity for S65 AMG). Add refrigerant slowly through the low side port with engine off initially, then start engine and set AC to maximum cold to draw in remaining charge. Add PAG 46 oil (30ml) to compensate for service loss if not pre-mixed with refrigerant.
10. 10

    Test low pressure switch operation
    Start the engine and activate the AC system. Verify the compressor engages and disengages properly. Using a diagnostic scanner if available, monitor AC low pressure switch status to confirm it is reading correctly (typically closes circuit at pressures above 1.8-2.0 bar). Check for proper cooling at dashboard vents.

Reassembly

1. Reinstall all engine covers and trim panels removed during access, torquing Cover Screws to specification
2. Double-check all electrical connectors are fully seated and locked
3. Verify no tools or parts are left in engine bay
4. Close hood and clean any PAG oil residue from exterior surfaces

Verification

• Start engine and set AC to maximum cold, maximum fan speed
• Verify compressor clutch engages within 5 seconds and cooling air is felt at dashboard vents within 30 seconds
• Check AC system pressures with manifold gauges: low side should be 20-30 PSI at idle with ambient temperature 70-80°F, high side 200-250 PSI
• Use UV light to inspect new switch installation point for refrigerant leaks (look for fluorescent oil traces)
• If diagnostic scanner available, verify no AC system fault codes are present (particularly B1448 for low pressure switch circuit)
• Test AC operation through full range of temperature settings and fan speeds
• Verify automatic climate control system responds correctly to temperature adjustments
• Perform a final visual inspection of the suction line connection point for any signs of oil leakage after 24 hours of operation

Related trouble codes on this vehicle

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