Fix P179D85 Mercedes Fault Code Tip_You Don’t Need a New ISM

Experiencing the P179d85 Mercedes Fault Code? This code indicates a problem within the transmission system, specifically concerning the Intelligent Servo Module (ISM) or Direct Select Module (DSM). At AutoExplain, we provide comprehensive troubleshooting and repair solutions, including remote technical support, to resolve this issue efficiently and get you back on the road.

1. Understanding the P179D85 Mercedes Fault Code

The P179D85 fault code in Mercedes-Benz vehicles signals a malfunction within the transmission’s control system. This code often points to issues with the Intelligent Servo Module (ISM), also sometimes referred to as the Direct Select Module (DSM) in newer models. The ISM/DSM is responsible for the electronic gear selection process, essentially acting as the interface between the gear selector and the transmission itself. A failure in this module can lead to various drivability problems, including the inability to shift gears.

P179D85 – Definition:

“The redundancy system for DIRECT SELECT was activated. There is a signal above the permissible limit value.”

Affected Vehicles

Fault code P179D85 can appear in a wide range of Mercedes-Benz vehicles, particularly those using ISM and the DIRECT SELECT system. Common models include:

• Mercedes-Benz E-Class (W212, W213)
• S-Class (W221, W222, W223)
• CLS-Class (W218, C257)
• ML/GLE-Class (W166, V167)
• GL/GLS-Class (X166, X167)
• SL-Class (R231)
• C-Class (W205 with electronic shifter)
• GLC-Class (X253 with ISM-equipped variants)

1.1. What is the Intelligent Servo Module (ISM)?

The Intelligent Servo Module (ISM), designated as A80, is a critical component in Mercedes-Benz vehicles equipped with 7G-Tronic (722.9) and 9G-Tronic transmissions. This module eliminates the mechanical linkage between the gear selector lever and the transmission. Instead, gear selection is done electronically via the ISM, improving the shift quality and overall control.

Mercedes Ism Module

1.2. Symptoms Associated with the P179D85 Fault Code

When the P179D85 code appears, several noticeable symptoms might manifest:

• “Drive to Workshop Without Changing Gear” message: This is the most common warning displayed on the instrument cluster.
• Inability to shift gears: The car may be stuck in Park (P) or another gear, preventing any gear changes.
• Vehicle won’t start: A defective ISM might prevent the engine from starting if it can’t confirm that the transmission is in Park or Neutral.
• Unresponsive shifter: The gear selector lever might not respond when attempting to change gears.
• Gear position not indicated: The gear position indicator on the instrument cluster might not light up or show the current gear.

1.3. Common Causes of the P179D85 Fault Code

The P179D85 code can be triggered by several underlying causes:

1. Defective ISM Module (A80)
   • The most frequent cause. Internal electronic faults within the ISM can lead to invalid gear position signals or failed internal redundancy checks. Components such as position sensors or control ICs may degrade over time.
2. Corroded or Loose Electrical Connector to ISM
   • ISM connectors are susceptible to moisture, vibration, and wear. Even a minor corrosion layer can cause resistance, affecting signal voltage integrity and triggering fault detection.
3. Battery Voltage Instability or Low Battery
   • The ISM relies on stable 12V supply and proper ground reference. If the battery is weak, the system might see voltage spikes or drops during ignition cycles or shifting operations, leading to erratic sensor readings.
4. Damaged or Shorted Wiring Harness
   • A short circuit, broken wire, or high resistance in the ISM wiring (especially CAN or power/ground lines) can interfere with communication or power delivery. Pinched wires during service or rodent damage are common root causes.
5. Faulty Gear Position Sensors (Internal to ISM)
   • The ISM contains Hall effect or magnetic sensors that detect gear lever positions. If these sensors fail or drift out of calibration, the module may misinterpret gear inputs and activate the safety logic.
6. Incorrect Installation or Missing Adaptation After Replacement
   • After ISM replacement, the module must undergo a teach-in process using Xentry. Failure to complete this procedure can result in P179D85, as the system interprets position data as invalid.
7. CAN Communication Errors with Other Control Modules
   • The ISM communicates with the Transmission Control Unit (EGS), Engine Control Unit (ME), and other modules over CAN. Faults in the CAN bus, including latency, interference, or loss of signal, can cause the redundancy mode to trigger.
8. Water Ingress or Environmental Contamination
   • Water entering through poorly sealed connectors, or from the vehicle undercarriage or firewall, can short circuit or corrode the ISM or its connector block, resulting in unpredictable behavior.
9. Software Glitches or Outdated Firmware
   • Like most modern modules, the ISM is software-controlled. Outdated or corrupted firmware may cause internal logic errors. A software update or reprogramming via SCN coding may be required.
10. Incorrect or Incompatible Replacement Part
    • If a used or aftermarket ISM was installed without proper compatibility checks or SCN coding, it may produce invalid signals or fail to communicate with the vehicle network correctly.

1.4. Vehicles Affected by ISM/DSM Issues

The ISM and DSM modules are commonly found in Mercedes-Benz models manufactured from 2006 onwards, particularly those equipped with 7G-Tronic (722.9) and 9G-Tronic transmissions and featuring a steering column-mounted gear selector. Here are some of the affected models:

MODEL	YEARS	TRANSMISSION TYPE
C-Class	2007-2014	7G-Tronic
E-Class	2009-2016	7G-Tronic, 9G-Tronic
S-Class	2006-2013	7G-Tronic
CLS-Class	2006-2017	7G-Tronic, 9G-Tronic
GLK-Class	2009-2015	7G-Tronic
ML-Class (GLE)	2006-2019	7G-Tronic, 9G-Tronic
GL-Class (GLS)	2007-2019	7G-Tronic, 9G-Tronic
SLK (SLC)	2011-2020	7G-Tronic
SL	2006-2020	7G-Tronic, 9G-Tronic

2. Diagnosing the P179D85 Fault Code

Diagnosing the P179D85 fault code requires a systematic approach.

Tools Needed

• Mercedes-Benz Xentry Diagnostic System (official software)
• Multimeter for voltage and continuity checks
• Wiring diagram (WIS or Alldata)
• Battery tester or jump pack
• Torque wrench for any module replacement

=> Install Xentry Diagnosis Software here!

=> Learn How to Use Xentry Diagnostics Software

Here’s a step-by-step guide to help you pinpoint the root cause of the problem:

2.1. Initial Inspection

• Check the battery voltage: Ensure the battery voltage is within the normal range (typically 12.4-12.7 volts with the engine off). Low voltage can cause electronic modules to malfunction.
• Inspect wiring and connectors: Thoroughly inspect the wiring and connectors associated with the ISM/DSM. Look for signs of damage, corrosion, or loose connections. Pay particular attention to the connector at the ISM itself.

2.2. Scan for Fault Codes

Use a professional-grade scan tool capable of reading Mercedes-Benz specific fault codes. This will not only confirm the presence of the P179D85 code but may also reveal other related codes that can provide valuable diagnostic clues.

2.3. ISM Module Specific Diagnostics

Using the scan tool, access the ISM module and perform the following tests:

• Read fault codes: Record all fault codes present in the ISM module.
• Clear fault codes: Clear the fault codes and attempt to restart the vehicle and reproduce the problem. See if the P179D85 code returns.
• Check live data: Monitor live data parameters related to the ISM, such as gear selector position, output shaft speed, and ISM temperature. Look for any anomalies or inconsistencies in the data.
• Perform actuation tests: Use the scan tool to perform actuation tests on the ISM. This will allow you to test the functionality of the internal components.

2.4. Additional Diagnostic Steps

• Check the transmission fluid level: Low transmission fluid can sometimes cause shifting problems.
• Inspect the transmission for mechanical issues: Listen for unusual noises or vibrations coming from the transmission.

2.5. Common Fault Codes Associated with ISM Issues

Besides P179D85, other fault codes often accompany ISM malfunctions:

• Mercedes 1994 Fault Code: Component A80 (Direct Select) Defective / Cannot reach the specific gear target position
• Mercedes 5399 Fault Code: ESP CAN Signal Gear Selected from N15/3 implausible
• Mercedes C103 Fault Code: ETC No CAN Message Received from A80 (Intelligent Servo Module) for direct selection.
• Mercedes 1974 Fault Code: Selector lever position P was engaged due to a fault occurring in component a80 (intelligent servo module for DIRECT SELECT) or the vehicle communications
• Mercedes 1922 Fault Code: System Shut down due to an internal Fault
• Mercedes 1904 Fault Code: The internal mechanism in component A80 for engaging select or lever position P is not in the correct position.
• Mercedes P179D00 Fault Code: The redundancy system for DIRECT SELECT was activated

3. How to Fix The Mercedes Fault Code P179D85 Without Replacing ISM

Step 1: Confirm the Fault

1. Connect Xentry to the vehicle.
2. Navigate to:
   • Control Units > Drive > A80 (ISM)
3. Read out fault codes and confirm P179D85 is present.
4. Check if any related codes are stored (e.g., CAN errors, voltage faults).

Step 2: Perform Visual Inspection

• Locate the ISM Module (A80):
  • Often mounted near the transmission or integrated with it
• Inspect:
  • Connectors for corrosion, bent pins, loose fit
  • Wiring harness for rubbing, damage, or water ingress
  • Grounding points near ISM and battery

Step 3: Check Battery and Power Supply

1. Measure battery voltage with the ignition off and on:
   • Should be ~12.6V off, and >13.5V with engine running
2. Check ISM power feed and ground continuity
3. Check for voltage drops between battery and ISM power pins

Step 4: Evaluate Actual Values and CAN Communication

• Use Xentry to check actual values in the ISM module:
  • Gear lever position
  • Voltage readings
  • Status of the redundancy system
• Use CAN analyzer or Xentry logs to check for interruptions in communication

Step 5: Perform Teach-In / Learning Procedure

If no physical faults are found:

1. Navigate to:
   • Control Units > A80 ISM > Adaptations or Teach-in Process
2. Select “Teach in gear positions” or “Initial startup of ISM”
3. Follow on-screen steps:
   • Brake pedal applied
   • Do not start engine
   • Cycle through gear positions as instructed

This allows the ISM to relearn and synchronize gear sensor values.

Step 6: Check Software and Coding

• Ensure the ISM has correct software and SCN coding
• If recently replaced, check if the adaptation process was completed
• Perform a software update if recommended by Xentry

Step 7: Clear Faults and Test Drive

• After any repair or teach-in:
  • Clear all fault codes
  • Cycle ignition and check for immediate reappearance
  • Test drive the vehicle and monitor live data

3. How to Repair The P179D85 Fault Code (If Teaching Gear Positions Couldn’t Help)

Once you’ve diagnosed the cause of the P179D85 fault code, you can proceed with the necessary repairs. Here are some common solutions:

3.1. ISM/DSM Replacement

3.1.1. When You Need to Replace ISM 

Replacing the ISM (Intelligent Servo Module) should only be considered after thoroughly diagnosing the system and ruling out all other causes such as wiring issues, power supply problems, and teach-in failures. Here are specific conditions that justify ISM replacement:

1. Irrecoverable Internal Faults
   • If the ISM reports internal hardware failures that persist after clearing codes and performing teach-in procedures (e.g., stuck gear sensor readings or actuator faults), the module is likely defective.
2. Communication Failure
   • If the ISM is completely unresponsive to Xentry diagnostics (no communication), and the power supply and CAN wiring are verified to be functional, the module may be dead and require replacement.
3. Failed Teach-In Process
   • If the ISM cannot successfully complete the “Teach-In of Gear Positions” or “Initial Startup” even after multiple attempts with correct conditions (battery voltage stable, brake applied, ignition on), this suggests internal calibration or sensor issues that are non-repairable.
4. Incorrect or Implausible Actual Values
   • If actual values such as gear position, actuator travel, or signal voltages remain fixed or outside expected ranges during diagnostics, and these readings do not respond to physical changes, the ISM sensors or internal logic may be compromised.
5. Repetitive Activation of Redundancy Mode
   • If the redundancy system is repeatedly activated without any external cause (such as wiring faults or voltage issues), the ISM may be overly sensitive or internally failing.
6. Documented TSB or Known Defect
   • If Mercedes-Benz technical service bulletins (TSBs) or recalls specifically identify certain ISM part numbers or software versions as faulty or incompatible, replacement may be recommended proactively.

3.1.2. How to Remove & Replace ISM

If the ISM is found to be defective, replacement is often the most reliable solution.

• New ISM: Purchase a new ISM module from a Mercedes-Benz dealership or a reputable aftermarket supplier.
• Used ISM: A used ISM can be a more affordable option, but it will require programming to match your vehicle’s VIN.
• Removal and Installation:
  1. Disconnect the negative battery cable.
  2. Raise the vehicle and secure it on jack stands.
  3. Locate the ISM on the side of the transmission housing.
     Location of the ISM module on the side of the Mercedes-Benz transmission.
  4. Disconnect the electrical connector from the ISM.
  5. Remove the bolts securing the ISM to the transmission.
  6. Install the new or refurbished ISM, ensuring proper alignment.
  7. Connect the electrical connector.
  8. Lower the vehicle.
  9. Reconnect the negative battery cable.

After replacement:

1. Use Xentry to perform initial startup of the new ISM
2. Run Teach-in of gear positions
3. Perform SCN coding if needed

Preventive Measures and Tips

• Always disconnect battery before removing ISM
• Avoid aftermarket ISM modules unless validated for compatibility
• Maintain good battery health; voltage dips cause many electronic faults
• Use dielectric grease on connectors during reinstallation
• Keep wiring secured and away from heat or vibration sources

3.2. Wiring and Connector Repair

If the P179D85 code is caused by wiring or connector problems, repair or replacement of the affected components is necessary.

• Inspect Wiring: Check for damaged, frayed, or corroded wires.
• Repair or Replace: Repair damaged wires using appropriate splicing techniques. Replace corroded or damaged connectors.
• Test Continuity: Use a multimeter to test the continuity of the wiring between the ISM and the TCM.

3.3. Software Updates

Outdated software can sometimes cause issues with the ISM. Check for available software updates for the ISM and TCM.

• Dealer Update: The dealership can perform software updates using the MB Star diagnostic tool.
• Independent Specialist: Some independent shops can also perform software updates.

4. Cost Considerations

The cost of repairing the P179D85 fault code can vary depending on the cause of the problem and the chosen repair method.

• ISM Replacement (New): $1200 – $1900 (including parts and labor)
• ISM Replacement (Used): $600 – $1000 (including parts, programming, and labor)
• Wiring/Connector Repair: $100 – $300
• Software Update: $100 – $200

5. Preventing Future ISM Issues

While not always preventable, some steps can be taken to minimize the risk of future ISM issues:

• Regular Maintenance: Follow the manufacturer’s recommended maintenance schedule for your vehicle.
• Battery Maintenance: Ensure the battery is in good condition and properly charged.
• Protect Wiring: Keep the wiring and connectors clean and protected from moisture and corrosion.
• Prompt Repairs: Address any warning signs or symptoms of transmission problems promptly.

AutoExplain – Your Partner in Mercedes-Benz Repair

At AutoExplain.com, we understand the complexities of modern automotive systems. We offer a range of services to help you diagnose and repair issues like the P179D85 fault code, including:

• Remote Technical Support: Our team of experienced technicians can provide remote assistance via WhatsApp or email. We can help you diagnose the problem, guide you through the repair process, and offer expert advice.
  • WhatsApp: +1(936)2896695
  • Email: [email protected]
• Diagnostic Guides: We provide detailed diagnostic guides for various Mercedes-Benz fault codes, helping you understand the cause of the problem and the steps required to fix it.
• Programming and Coding Services: We can assist with ISM programming and SCN coding, ensuring your replacement module is properly integrated with your vehicle.

We are committed to providing exceptional service and technical expertise to our customers. Contact us today to learn more about how we can help you resolve your Mercedes-Benz issues.

Don’t let the P179D85 fault code keep you off the road. With AutoExplain.com, you can access the knowledge and support you need to diagnose and repair your Mercedes-Benz efficiently and effectively. Our experienced technicians are standing by to assist you with remote technical support, diagnostic guides, and programming services. Contact us today and experience the AutoExplain.com difference!

FAQ: 

Here are some frequently asked questions about the P179D85 Mercedes fault code:

Q1: What does the P179D85 fault code mean on a Mercedes-Benz?

A1: The P179D85 fault code indicates a malfunction within the transmission’s control system, specifically related to the Intelligent Servo Module (ISM) or Direct Select Module (DSM).

Q2: What are the common symptoms of the P179D85 fault code?

A2: Common symptoms include the “Drive to Workshop Without Changing Gear” message, inability to shift gears, vehicle won’t start, unresponsive shifter, and gear position not indicated.

Q3: What are the common causes of the P179D85 fault code?

A3: Common causes include ISM/DSM failure, wiring or connection problems, low battery voltage, software issues, and mechanical issues within the transmission.

Q4: Can I drive my Mercedes-Benz with the P179D85 fault code?

A4: It’s generally not recommended to drive your Mercedes-Benz with the P179D85 fault code, as it can lead to unpredictable shifting behavior or complete immobilization of the vehicle.

Q5: How do I diagnose the P179D85 fault code?

A5: Diagnose the P179D85 fault code by performing an initial inspection, scanning for fault codes, accessing the ISM module with a scan tool, checking live data, and performing actuation tests.

Q6: Can I repair the P179D85 fault code myself?

A6: Depending on your mechanical skills and experience, you may be able to repair the P179D85 fault code yourself. However, some repairs, such as ISM programming, require specialized equipment and knowledge.

Q7: How much does it cost to repair the P179D85 fault code?

A7: The cost of repairing the P179D85 fault code can vary depending on the cause of the problem and the chosen repair method, ranging from $100 to $1900.

Q8: Can a used ISM module be installed in my Mercedes-Benz?

A8: Yes, a used ISM module can be installed, but it requires programming to match your vehicle’s VIN.

Q9: Where is the ISM module located on a Mercedes-Benz?

A9: The ISM module is located on the left side of the transmission housing (driver side for LHD vehicles).
Location Of Mercedes-Benz Ism Module

Q10: Where can I get help with diagnosing and repairing the P179D85 fault code?

A10: You can get help with diagnosing and repairing the P179D85 fault code from AutoExplain.com, a Mercedes-Benz dealership, or a qualified independent mechanic specializing in Mercedes-Benz vehicles.