Transmission Speed Sensor - Input

This procedure covers removal and replacement of the transmission input speed sensor on a 2019 Mercedes-Benz G-Class with the 4.0L turbo V8 engine and 9G-TRONIC automatic transmission.

Warnings

Tools required

Parts

• Transmission input speed sensor × 1 — A0005429118 or equivalent OEM
• Sensor O-ring seal × 1 — Use OEM specification
• Transmission pan gasket × 1 — Use OEM specification (if pan removal required)

Fluids

• MB 236.17 ATF — 3 qt

Preparation

1. Park vehicle on level surface and engage parking brake.
2. If engine has been running, allow transmission to cool for minimum 2 hours.
3. Disconnect negative battery terminal and wait 3 minutes for system capacitor discharge.
4. Raise vehicle on lift or support securely on jack stands at all four corners.
5. Remove engine undertray panels by removing Torx fasteners around perimeter (typically T25 or T30 fasteners).
6. Locate transmission input speed sensor on driver's side of transmission bell housing, near torque converter area.
7. Clean area around sensor and transmission case with brake cleaner to prevent contamination during removal.

Procedure

1. 1

   Disconnect Sensor Electrical Connector
   Locate the input speed sensor electrical connector on the driver's side of the transmission. Press the locking tab and pull connector straight away from sensor body. Do not pull on wiring harness. If connector is secured with a wire clip or bracket, remove the securing fastener first (typically one Torx T20 or T25 screw).
2. 2

   Assess Sensor Access
   Inspect the area around the input speed sensor mounting location. On the 2019 G-Class with 9G-TRONIC (725.0) transmission, the input speed sensor is internal to the transmission and integrated with the electronic control module/valve body. Transmission pan removal (Step 3) is required to access it. Verify the exact removal procedure in Mercedes-Benz WIS before proceeding.
3. 3

   Lower Transmission Pan (If Required)
   If sensor access requires pan lowering: Position fluid catch pan under transmission pan. Loosen all pan bolts in a crosswise pattern, leaving front bolts partially engaged. Remove the transmission pan completely to access the internal speed sensor on the mechatronic unit. Position fluid catch pan, remove pan bolts in a crosswise pattern, and drain fluid. Verify total fluid drain quantity against Mercedes-Benz specification for the 725.0 transmission. This provides clearance to access the sensor mounting bolt without complete pan removal.
4. 4

   Remove Input Speed Sensor Mounting Bolt
   The input speed sensor mounting/retention method must be confirmed against Mercedes-Benz WIS for the 725.0 transmission. Note fastener type, size, length, and any sealing washer before removal for correct reinstallation.
5. 5

   Extract Speed Sensor
   Grasp the sensor body firmly and pull straight outward from the transmission case. The sensor may require moderate force to overcome O-ring seal friction. Twist gently while pulling if sensor is stuck. Once sensor begins to move, continue pulling straight out to avoid damaging the sensor bore. Note the sensor probe depth and orientation as removed for correct reinstallation. Confirm sensor design and installation depth against Mercedes-Benz service documentation for the 725.0 transmission.
6. 6

   Inspect and Clean Sensor Bore
   Inspect the sensor mounting bore in the transmission case for debris, metal particles, or damaged threads. Clean the bore opening with brake cleaner and a clean lint-free cloth. Use a wire brush to clean the mounting surface around the bore. Check for and remove the old O-ring from the sensor bore if it remained in the transmission (it may have come out with the old sensor). Inspect the tone wheel or reluctor ring visible inside the bore for damage or excessive buildup.
7. 7

   Prepare New Speed Sensor
   Remove new speed sensor from packaging. Install the new O-ring seal onto the sensor body groove, ensuring it is seated evenly around the circumference. Lightly lubricate the O-ring with clean MB 236.17 ATF to ease installation. Verify the sensor electrical connector pins are clean and undamaged.
8. 8

   Install New Speed Sensor
   Align the new sensor with the mounting bore, ensuring the sensor probe is oriented correctly (tab or flat on sensor body aligns with corresponding feature in bore). Push sensor straight into bore until the mounting flange contacts the transmission case. Do not rotate sensor during insertion to avoid damaging O-ring. Sensor should insert smoothly with light hand pressure.
9. 9

   Install and Torque Sensor Mounting Bolt
   Install the sensor mounting bolt with any sealing washer if equipped. Hand-tighten the bolt, then torque to specification using a torque wrench. Ensure sensor body remains fully seated against transmission case during tightening.
10. 10

    Reinstall Transmission Pan (If Lowered)
    If pan was lowered in step 3: Clean the pan mating surface and transmission case surface with brake cleaner. Inspect the pan gasket for damage; replace if gasket seal is compromised or if reusable gasket shows tears or compression damage. Raise pan back into position and install all pan bolts finger-tight. Torque pan bolts in a crosswise pattern, working from center outward, to specification.
11. 11

    Reconnect Sensor Electrical Connector
    Push the electrical connector onto the sensor until the locking tab clicks into place. Tug gently on connector to verify it is locked. Reinstall any wire clips or securing brackets removed in step 1.
12. 12

    Refill Transmission Fluid
    Locate the transmission fluid fill port (on passenger side of transmission case, accessed from underneath). Remove the fill plug. Using a transmission fluid pump, add MB 236.17 ATF through the fill port. Refill with the quantity of MB 236.17 ATF specified by Mercedes-Benz for the 725.0 transmission (verify exact volume in WIS). Perform the fill/level check using the overflow-tube method at the specified fluid temperature per the Mercedes-Benz procedure, not simply to the fill port edge at room temperature. Set the fluid level using the overflow method at the Mercedes-Benz specified fluid temperature (per WIS for the 725.0 transmission) with the vehicle level, allowing excess fluid to drain from the overflow tube/plug. Reinstall fill plug and torque to specification.
13. 13

    Reinstall Undertray Panels
    Reinstall all engine undertray panels removed during preparation. Install all Torx fasteners and tighten securely, working from one end to the other to ensure even panel alignment.
14. 14

    Reconnect Battery and Clear Fault Codes
    Reconnect negative battery terminal. Use Mercedes-Benz XENTRY/DAS (or equivalent) to read and clear stored transmission fault codes and to perform any required control-module adaptation or coding after sensor replacement. Confirm applicable Mercedes-Benz fault codes rather than assuming generic OBD-II codes. The transmission control module may have logged multiple codes during sensor failure.

Reassembly

1. Ensure all electrical connectors are fully seated and locked.
2. Verify transmission pan bolts (if loosened) are torqued in proper crosswise sequence.
3. Double-check transmission fluid level with vehicle level and fluid at operating temperature (approximately 80°C) per Mercedes-Benz service procedure.
4. Confirm no fluid leaks are present around sensor mounting location or pan gasket before lowering vehicle.

Verification

• Start engine and verify no warning lights remain illuminated on instrument cluster.
• With engine running and brake applied, shift transmission through all gear positions (P-R-N-D-S). Ensure all shifts occur smoothly without delay or harsh engagement.
• Use diagnostic scan tool to verify transmission input speed sensor is reporting data. Monitor sensor output while slowly driving vehicle forward; sensor should show increasing speed signal proportional to vehicle speed.
• Road test vehicle at various speeds (city and highway) to verify proper transmission shifting patterns and absence of limp mode.
• After road test, recheck fluid level with transmission at operating temperature and add fluid if necessary to reach proper level at fill port.
• Recheck area around sensor and pan (if disturbed) for any fluid seepage after road test.

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