Transmission Speed Sensor - Output

Replace the output speed sensor on the 722.9 7-speed automatic transmission to restore proper speedometer and transmission shift operation.

Warnings

Tools required

Parts

• Output speed sensor (Y3/6n2) × 1 — A0005429218 or equivalent
• Sensor O-ring × 1 — Use OEM specification
• Transmission drain plug crush washer × 1 — Use OEM specification

Fluids

• Mercedes-Benz ATF 134 — 2 qt

Preparation

1. Park vehicle on level surface and apply parking brake
2. Allow transmission to cool if recently driven (fluid will drain during procedure)
3. Raise vehicle on lift or securely support on jack stands at all four corners
4. Remove undertray panels using T30 Torx fasteners to access transmission underside
5. Locate output speed sensor on driver's side of transmission case, near output shaft

Procedure

1. 1

   Disconnect electrical connector
   Press the locking tab on the output speed sensor electrical connector and carefully pull connector straight off sensor. The connector is located above the sensor body. Use a pick tool if necessary to release the lock tab.
2. 2

   Position drain pan
   Place a drain pan with at least 2-quart capacity directly below the output speed sensor. Approximately 1-1.5 quarts of ATF will drain when the sensor is removed.
3. 3

   Remove output speed sensor mounting bolt
   Using a 10mm socket, remove the single mounting bolt securing the output speed sensor to the transmission case. Keep the bolt for reinstallation.
4. 4

   Extract output speed sensor
   Carefully pull the output speed sensor straight out from the transmission case. Transmission fluid will begin draining immediately. Allow fluid to drain completely into the pan. Inspect the sensor tip for metal shavings or debris which may indicate internal transmission damage.
5. 5

   Clean sensor bore
   Use a clean shop towel to wipe the sensor bore in the transmission case. Remove any debris or old O-ring material. Do not allow contaminants to enter the transmission case.
   ⚠Do not use compressed air to clean the sensor bore as this can force debris into the transmission.
6. 6

   Prepare new sensor
   Install the new O-ring onto the new output speed sensor. Lightly coat the O-ring with clean ATF 134 fluid to ease installation and prevent damage. Ensure the O-ring is properly seated in the sensor groove.
7. 7

   Install new output speed sensor
   Carefully insert the new output speed sensor straight into the transmission case bore until it seats fully against the case. Ensure the sensor is fully seated and the mounting hole aligns with the case hole.
8. 8

   Secure sensor mounting bolt
   Install the sensor mounting bolt and torque to specification using a torque wrench. Ensure the sensor remains fully seated while tightening.
   Torque spec

   Sensor Bolts15 Nm (11 lb-ft)
9. 9

   Reconnect electrical connector
   Push the electrical connector firmly onto the output speed sensor until the locking tab clicks into place. Verify the connection is secure by gently pulling on the connector.
10. 10

    Refill transmission fluid
    Locate the transmission fill plug on the passenger side of the transmission pan. Remove the fill plug and add approximately 2 quarts of Mercedes-Benz ATF 134 using a fluid pump or transmission funnel. The transmission must be filled to the proper level.
11. 11

    Verify fluid level
    Start the engine and bring transmission fluid to operating temperature (80-100°C) by driving or allowing to idle in gear for 10-15 minutes. With engine running and transmission in Park on level ground, fluid should drip slowly from fill hole. Add or drain as needed to achieve proper level, then reinstall fill plug.
    ⚠Transmission fluid level must be checked at operating temperature with engine running. Incorrect level can cause transmission damage.
12. 12

    Reinstall undertray panels
    Reinstall all undertray panels and secure with T30 Torx fasteners. Ensure all panels are properly aligned and secured.

Reassembly

1. Verify all electrical connections are secure and properly locked
2. Double-check transmission fluid level is correct at operating temperature
3. Ensure all undertray panels are reinstalled and fasteners are tight

Verification

• Clear any stored fault codes using a Mercedes-Benz compatible scan tool
• Test drive the vehicle through all gears and verify smooth shifting operation
• Verify speedometer displays correct speed and transmission shifts normally
• Check for transmission fluid leaks around the output speed sensor after test drive
• Re-scan for fault codes to ensure P0720, P0722, and P0723 codes do not return

Related trouble codes on this vehicle

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