DIY Mercedes Benz Diagnostic Testing

by Richard Ristow

A Mercedes Benz has multiple diagnostic systems within the vehicle. Each of these correspond with a warning light on the Mercedes' dashboard. A number of these systems are accessible to a home mechanic, but you need appropriate hardware. In most cases, each system, whether it's the On-Board Diagnostics, Anti-Locking Braking System, or Supplemental Restraint, requires a separate scanner or code reader. Some newer scanners are multi-functional, and with the appropriate software bundles, a laptop can also be a diagnostic scanner.

Open the Mercedes' driver-side door and take a seat behind the steering wheel. Start the vehicle's engine. All the dashboard lights will momentarily come on and then switch themselves off. Look for any warning lights that remain lit. These indicators will tell you what parts of your Mercedes need troubleshooting. For example, an active "service engine" light means the Mercedes' OBD-II system has detected malfunctions. An Anti-Lock Brakes light means there is a problem with the brakes.

Turn the engine and the electrical system off. Leave the key in the Mercedes' ignition.

Look into the leg-space beneath the steering wheel. There, you will find a 16 pin-receiving computer outlet. This Data Link Connector accommodates scanners for all diagnostic systems. However, this outlet's location depends on the year and model of the Mercedes in question. For most vehicles, it will be under the dash and to the left or right of the steering column. In some models, it will be behind an access panel, and in others, it will be out in the open.

Connect your diagnostic hardware to the Mercedes' DLC outlet. Switch the device on. Most scanners come with a diagnostic cable, which connects the scanner to the DLC. Some devices may plug directly into the DLC.

Turn the key in the ignition to the "On" designation. This will bring the electrical system up. Depending on the system and the scanner brand you are working with, you may need to turn on the engine. If your scanner will not become fully operational otherwise, you will need to start the Mercedes.

Enter a code-retrieving command. How to do this depends on the exact device you are using. Configuration, orientation, and programming differs by the brand and system being used. It is a lot like how operating systems are different by brand of cell phone or laptop. Consult your hardware's user's manual for precise instruction. Once the codes have been retrieved from the system, they will appear on the scanner's display screen.

Look up coding definitions. Some device manuals will contain such definitions in the rear, as an appendix. If the manual doesn't contain diagnostic coding information, you can find further information online. OBD-II and ABS codes, for example, are relatively easy to find. Some of the more complex systems, like SRS air bags, will be a little more difficult and may require a technical source book.

Look up definitions that correspond with all the codes you copied from the scanner. These definitions will help you make an informed decision. You can either troubleshoot and repair the problems yourself or drive the Mercedes to a mechanic. If you lack experience with the highly complex systems, like ABS brakes or SRS air bags, seriously consider involving a repair garage. Your code list will save you money in diagnostic fees.

Items you will need

About the Author

Richard Ristow has written for journals, newspapers and websites since 2002. His work has appeared in "2009 Nebula Showcase" and elsewhere. He is a winner of the Science Fiction Poetry Association's Rhysling Award and he edits poetry for Belfire Press. He also holds a Master of Fine Arts from the University of North Carolina at Wilmington and has managed an automotive department at WalMart.

Photo Credits

  • photo_camera dashboard with speed meter image by robert paul van beets from