Repairing a transmission fluid leak on a 1999 Mercedes-Benz S420 involves several steps, including identifying the source of the leak, replacing any faulty components, and potentially refilling the transmission fluid. click here for more details on the download manual…..

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Here’s a detailed guide to help you through the process:

### Tools and Materials Needed:

1. **Tools:**
– Socket and ratchet set (including metric sizes)
– Torque wrench
– Screwdrivers (flathead and Phillips)
– Pliers
– Transmission fluid pump
– Oil catch pan
– Funnel
– Rags and cleaning cloths
– Jack and jack stands (or a hydraulic lift)

2. **Materials:**
– New transmission fluid (check the owner’s manual for specifications)
– Transmission pan gasket (if applicable)
– Replacement seals or O-rings (for components like the transmission cooler lines)
– RTV sealant (if necessary)
– Brake cleaner or degreaser

### Step-by-Step Guide:

#### 1. Safety First
– **Park the Vehicle:** Ensure the car is on level ground and turn off the engine.
– **Lift the Vehicle:** Use a jack to raise the car and secure it with jack stands. Never rely solely on the jack.

#### 2. Identify the Source of the Leak
– **Visual Inspection:** Look under the vehicle for signs of transmission fluid (usually red or brown). Check around the transmission pan, cooler lines, and any areas where seals are present.
– **Clean the Area:** Use brake cleaner to clean any residue or fluid around suspected areas. This will make it easier to spot new leaks after the repair.

#### 3. Inspect the Transmission Pan
– **Check for Damage:** Examine the transmission pan for dents or cracks.
– **Remove the Transmission Pan:**
– Place an oil catch pan under the transmission pan to catch fluid.
– Loosen the bolts securing the pan in a crisscross pattern to prevent warping.
– Carefully remove the pan and set it aside.

#### 4. Replace the Transmission Pan Gasket
– **Inspect the Gasket:** If the gasket is worn or damaged, it should be replaced. Clean the gasket surface on both the pan and the transmission.
– **Install New Gasket:** Place the new gasket on the pan and reattach it, tightening the bolts in a crisscross pattern to the manufacturer’s specified torque.

#### 5. Check Transmission Cooler Lines
– **Inspect for Leaks:** Check the transmission cooler lines for leaks, especially around the fittings.
– **Replace Seals:** If you find that the O-rings or seals are worn, replace them. It may require disconnecting the lines:
– Use pliers to loosen the clamps and remove the lines.
– Replace the O-rings and reattach the lines.

#### 6. Inspect Other Seals
– **Check for Input and Output Shaft Seals:** These seals can also leak fluid. If they appear to be the source of the leak, they will need to be replaced.
– **Remove drive Shaft (if necessary):** If you need to access the output shaft seal, you may need to remove the drive shaft, which involves:
– Marking the position for reinstallation.
– Unbolting the driveshaft from the transmission and rear differential.
– **Replace Seals:** Remove the old seals using a seal puller or a flathead screwdriver. Install new seals carefully.

#### 7. Reassemble and Refill
– **Reattach Components:** Make sure all bolts and clamps are tightened to specifications.
– **Refill Transmission Fluid:**
– Locate the transmission fluid dipstick tube or fill port.
– Use a funnel and a transmission fluid pump to fill the transmission with the correct type of fluid.
– Use the dipstick to check the level, adding fluid as necessary.

#### 8. Test for Leaks
– **Start the Engine:** Allow it to idle and check for leaks.
– **Shift Through Gears:** Cycle through the gears while checking for fluid leaks under the vehicle.

#### 9. Final Inspection
– **Check Fluid Level Again:** After running the Engine and cycling through the gears, check the transmission fluid level again and top off if necessary.
– **Dispose of Old Fluid:** Make sure to dispose of any old transmission fluid and used parts according to local regulations.

### Conclusion
Repairing a transmission fluid leak on a 1999 Mercedes S420 requires careful attention to detail and the proper tools. Always refer to the vehicle’s service manual for specific torque specifications and procedures. If you’re unsure or uncomfortable with any part of the process, it’s recommended to consult a professional mechanic.
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