Why does my Mercedes-Benz E-Class W212 (2009-2016) transmission hesitate when shifting?

Symptoms of Transmission Hesitation

1. Delayed Shifting: Noticeable pause before the transmission shifts to the next gear.
2. Harsh Engagement: A jolt or roughness when the transmission shifts.
3. Slipping: The engine revs without corresponding acceleration.
4. Check Engine Light: Possible illumination of the check engine light indicating potential transmission issues.

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Diagnostic Steps

Step 1: Visual Inspection

• Tools Required: Flashlight
• Instructions:
  1. Inspect the transmission fluid level using the dipstick. Ensure the fluid is at the correct level and is clean (red and clear).
  2. Check for any visible leaks around the transmission pan, seals, and lines.
  3. Look for any signs of damage or wear on the transmission mounts and linkage.

Step 2: Check for Diagnostic Trouble Codes (DTCs)

• Tools Required: OBD-II Scanner
• Instructions:
  1. Connect the OBD-II scanner to the vehicle's diagnostic port.
  2. Retrieve any stored trouble codes related to the transmission.
  3. Note the codes and refer to a service manual for definitions.

Step 3: Fluid Condition Analysis

• Tools Required: Transmission fluid tester (if available)
• Instructions:
  1. Remove the transmission dipstick and check the fluid condition.
  2. If the fluid appears dark, burnt, or has a burnt smell, it may need to be replaced.

Step 4: Test Drive

• Instructions:
  1. Take the vehicle for a test drive under various conditions (city, highway, different RPMs).
  2. Pay attention to when the hesitation occurs (e.g., during acceleration, deceleration, or at specific speeds).

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Possible Causes

1. Low or Contaminated Transmission Fluid: Can cause erratic shifting.
2. Worn Transmission Components: Such as clutches, bands, or solenoids.
3. Faulty Transmission Control Module (TCM): May misinterpret signals.
4. Throttle Position Sensor (TPS) Issues: Affecting shifting.
5. Vacuum Leaks: May impact transmission function.

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Repair Instructions

Repairing Low or Contaminated Fluid

1. Tools Required: Socket set, funnel, new transmission fluid (refer to the owner’s manual for specifications), fluid pump.
2. Instructions:
   • Drain the Transmission:
     1. Raise the vehicle using a jack and secure it with jack stands.
     2. Place a drain pan under the transmission pan.
     3. Remove the transmission pan bolts and carefully lower the pan to drain the fluid.
   • Replace Filter (if applicable):
     1. Remove the old filter and install a new one.
   • Refill Transmission Fluid:
     1. Reattach the transmission pan and tighten bolts to specifications.
     2. Using a funnel, refill the transmission with new fluid through the dipstick tube.
     3. Start the engine and cycle through the gears while checking fluid levels, adding more as necessary.

Repairing Faulty Components

1. Tools Required: Basic hand tools, multimeter (for electrical checks).
2. Instructions:
   • Inspect and Replace Solenoids:
     1. Locate the transmission solenoids (refer to service manual for specific locations).
     2. Disconnect the electrical connectors and remove the solenoid using appropriate tools.
     3. Install the new solenoid and reconnect the electrical connectors.
   • Check TCM and TPS:
     1. Use a multimeter to test the TPS for correct voltage output per manufacturer specifications.
     2. If faulty, replace the TPS and clear any DTCs.

Final Steps

1. Clear Codes: After repairs, use the OBD-II scanner to clear any codes from the system.
2. Test Drive Again: Re-test under various driving conditions to ensure the problem is resolved.

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Conclusion

If the problem persists after following these steps, it is advisable to seek professional assistance or consider further diagnostics, as internal transmission issues may require specialized equipment and expertise.