what the code P179D means and how to troubleshoot it.

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 Intelligent Servo Module (ISM) or Direct Select Module (DSM) in some Mercedes-Benz vehicles. The ISM or DSM is a device that allows the driver to select the gear position electronically without using a mechanical linkage. It is located on the transmission and communicates with the engine control unit (ECU) and the instrument cluster.

The code P179D means that the redundancy system for DIRECT SELECT was activated. The redundancy system is a backup mechanism that ensures the vehicle can still be driven in case of a failure in the ISM or DSM. The redundancy system consists of two switches, one for park and one for neutral, that are activated by a solenoid when the ISM or DSM detects a fault. The code P179D also indicates that there is a signal above the permissible limit value, which could mean a short circuit to positive, a low voltage supply, or a communication error with the Electronic Ignition Switch (EIS/EZS).

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

- Check the battery voltage and make sure it is within the normal range (12-14 volts). If the battery voltage is too low, it could cause the ISM or DSM to malfunction and activate the redundancy system. Charge or replace the battery if necessary.

- Check the wiring and connectors of the ISM or DSM and make sure they are not damaged, corroded, loose, or shorted. Repair or replace any faulty wiring or connectors as needed.

- Check the fuse and relay of the ISM or DSM and make sure they are not blown or defective. Replace them if needed.

- Perform a quick test with the scan tool and check for any other codes related to the ISM or DSM, such as P179A12, P179A16, P179C16, B210A00, or B210D00. These codes could indicate more specific problems with the ISM or DSM, such as a short circuit to positive, a low voltage supply, or a communication error with the EIS/EZS. Follow the diagnostic procedures for each code and clear them after fixing them.

- Perform a learning process with the scan tool and check if the ISM or DSM can recognize and display the gear positions correctly. The learning process is a procedure that calibrates the ISM or DSM to match the transmission shift points. If the learning process fails or does not improve the situation, it could mean that the ISM or DSM is faulty and needs to be replaced.

- If replacing the ISM or DSM does not remedy the problem, please create a PTSS case and attach previous repair quick tests, new repair quick tests, and a control unit log from the affected module. The next steps will be provided through PTSS.

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

It seems like you are having trouble with your **ISM- Intelligent Servo Module** or **DSM- Direct Select Module** in your **Mercedes-Benz S-Class W221 Diesel**¹. This code means that **the redundancy system for DIRECT SELECT was activated** due to a possible **short circuit to positive** or a **low voltage**². This could prevent you from starting the car or shifting gears²³. You may need to replace the ISM or DSM module or check the wiring and power supply¹².