P179D The redundancy system for DIRECT SELECT was activated

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In this blog post, I will explain what the code P179D means and how to troubleshoot it. P179D is a diagnostic trouble code (DTC) that indicates a problem with the redundancy system for DIRECT SELECT, which is a feature of the Intelligent Servo Module (ISM) or the Direct Shift Module (DSM) in some Mercedes-Benz models. The redundancy system for DIRECT SELECT is designed to ensure that the transmission can be shifted into park (P) position in case of a failure of the ISM or DSM. The code P179D can be triggered by various causes, such as:

- A short circuit to battery (+) in the wiring or connectors of the ISM or DSM.

- A low voltage supply to the ISM or DSM due to a weak battery, a faulty alternator, or a blown fuse.

- A faulty ISM or DSM that cannot communicate with the Electronic Ignition Switch (EIS/EZS) or the Electric Parking Brake (EPB).

- A faulty brake caliper that prevents the EPB from engaging.

To diagnose and fix the code P179D, you will need a compatible scan tool that can access the ISM or DSM module and perform some tests. Here are some steps you can follow:

1. Check the battery voltage and make sure it is above 12 volts. If not, charge or replace the battery and clear the code.

2. Check the fuses and relays related to the ISM or DSM and replace any that are blown.

3. Check the wiring and connectors of the ISM or DSM for any signs of damage, corrosion, or loose connections. Repair or replace as needed and clear the code.

4. Perform a learning process for the gear positions using the scan tool. This will calibrate the ISM or DSM and ensure that it recognizes the correct gear position.

5. Perform an actuation test for the EPB using the scan tool. This will check if the EPB can be applied and released properly.

6. If none of the above steps resolve the code P179D, you may need to replace the ISM or DSM with a new one. Note that this is a theft-related item and requires programming and adaptation with a dealer-level scan tool.

I hope this blog post was helpful and informative. If you have any questions or comments, please feel free to leave them below.

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