Main Relay

Replace the main relay (fuel pump relay) in the front Sam unit, which controls fuel pump operation and various electrical systems in the Mercedes-Benz S65 AMG.

Warnings

Tools required

Parts

• Main relay (fuel pump relay) × 1 — A0025427619 or equivalent
• Dielectric grease × 1 — Use OEM specification

Preparation

1. Park vehicle on level surface and ensure transmission is in Park
2. Turn off all electrical accessories and remove key from ignition
3. Allow engine to cool if recently operated
4. Open hood and secure in open position
5. Locate the front Sam unit (Signal Acquisition Module) in the engine compartment on the left side near the battery

Procedure

1. 1

   Disconnect battery negative terminal
   Using a 10mm socket, loosen the negative battery terminal bolt. Remove the negative cable from the battery post and secure it away from the terminal to prevent accidental reconnection. Wait at least 3 minutes for system capacitors to discharge.
   Torque spec

   Battery Terminal Bolts6 Nm (4 lb-ft)
2. 2

   Remove engine cover components if necessary
   Depending on access, you may need to remove the engine cover. Lift the cover straight up to disengage from mounting grommets. Set aside in a clean area. The front Sam unit is located on the left side of the engine compartment near the fuse box.
3. 3

   Access the front Sam unit
   Locate the front Sam unit housing, which is a black plastic enclosure mounted on the left side firewall area. The unit contains multiple relays and control modules. Identify the access cover secured by Torx screws.
4. 4

   Remove Sam unit cover
   Using a Torx T20 or T30 driver (depending on your specific model year), remove the cover screws securing the Sam unit access cover. There are typically 3-4 screws. Carefully lift the cover away to expose the relay bank inside. Keep screws organized for reinstallation.
   Torque spec

   Cover Screws15 Nm (11 lb-ft)
5. 5

   Identify the main relay
   Locate the main relay (fuel pump relay) within the Sam unit. It is typically labeled on the cover or housing diagram. The relay is usually a larger relay with multiple pins, often black or brown in color. Consult the cover diagram or owner's manual to confirm correct relay position before removal.
6. 6

   Remove the main relay
   Firmly grasp the main relay and pull straight up with steady pressure to extract it from the socket. If resistance is encountered, use a relay removal tool or gently rock the relay side-to-side while pulling upward. Do not twist or bend the relay during removal to avoid damaging the socket pins.
   ⚠Inspect socket pins for damage, corrosion, or bent contacts before installing new relay.
7. 7

   Inspect relay socket and clean contacts
   Examine the relay socket for any signs of corrosion, heat damage, or contamination. Check that all socket pins are straight and make proper contact. If corrosion is present, clean contacts with electrical contact cleaner and allow to dry completely. Look for signs of melting or burning which may indicate a larger electrical issue.
   ⚠If socket shows heat damage or melted plastic, the Sam unit may require replacement or professional diagnosis.
8. 8

   Prepare and install new main relay
   Verify the new relay matches the original part number and pin configuration. Apply a small amount of dielectric grease to the relay pins to ensure good electrical contact and prevent corrosion. Align the relay with the socket, ensuring correct orientation (check for alignment tab or notch). Press firmly straight down until the relay is fully seated and flush with surrounding relays.
9. 9

   Reinstall Sam unit cover
   Position the Sam unit cover back onto the housing, ensuring any sealing gasket or weather seal is properly aligned. Install all cover screws and tighten in a cross pattern to the specified torque to ensure even pressure and proper sealing.
   Torque spec

   Cover Screws15 Nm (11 lb-ft)
10. 10

    Reconnect battery
    Clean the battery terminal posts and cable clamp if any corrosion is present. Reconnect the negative battery cable to the negative terminal. Tighten the terminal bolt to specification, being careful not to overtighten and damage the terminal.
    ⚠Do not exceed specified torque on battery terminals as overtightening can crack the battery case or damage terminals.
    Torque spec

    Battery Terminal Bolts6 Nm (4 lb-ft)

Reassembly

1. Reinstall engine cover by aligning with mounting grommets and pressing down until seated
2. Perform window initialization if necessary (close each window fully, hold button for 1 second, open fully, hold button for 1 second)
3. Reset clock and radio presets as needed

Verification

• Turn ignition to position II (on) without starting engine - listen for fuel pump priming (2-3 second hum from rear of vehicle)
• Check instrument cluster for any warning lights related to fuel system or electrical faults
• Start engine and verify normal operation with no rough running or hesitation
• Allow engine to idle for 2-3 minutes and confirm no warning lights appear
• Cycle ignition off and on several times to verify consistent fuel pump priming
• Test operation of electrical accessories to ensure Sam unit is functioning properly
• Use scan tool if available to check for any stored fault codes related to main relay or fuel system

Related trouble codes on this vehicle

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