On W211 models, the ignition switch is referred to as the electronic ignition switch (EIS). The EIS is a plastic housing with a slot for the remote key. The key has a sequential code to prevent vehicle theft. The EIS reads the code from the key and allows or prevents the vehicle from being turned on and started, depending on if the key code is accepted or not. This is done in conjunction with the DME, which stores a rolling code.
The EIS is installed in Mercedes-Benz models from C-Class to ML SUV vehicles, (W208 / W210 / W203 / W209 / W211 / W215 / W219 / W220 / W230). This article will cover replacing the battery in the black and chrome remote key, as well as some common problems and tips on faults and possible causes.
If you have symptoms similar to below, be sure to diagnose the system properly using an advanced Mercedes-Benz scan tool, something similar to an Autologic tool. Multiple modules are involved in starting the vehicle and the fault can happen in any of them. Fault codes, system signals and data stream values can help to pinpoint the cause.
If the EIS fails, the rotate key and start function will be deactivated and the module will be locked for security purposes. Your vehicle will not start. A fault with the EIS can produce:
Remote key will not turn EIS.
Vehicle will not start.
Key turns, but engine will not start.
Intermittent remote key faults
The EIS unit, as all other electronic modules in modern Mercedes-Benz models are sensitive to voltage and current fluctuations in the electrical systems. If you have symptoms of a faulty EIS and recently jump started your vehicle, your technique could have caused damage. If any of the following occurred while starting:
Jump start attempt while inserting remote key.
Voltage or current surge.
Low battery voltage when starting.
High A/C voltage output from battery charging.
Some models with electronic steering lock (ELS) can exhibit issues similar to an EIS failure. If you have any of the following symptoms, be sure to confirm the ELS is not the issue.
Remote key in EIS, steering wheel will not unlock.
Instrument cluster displays "Remove Key From Ignition".
Turning key remote to accessory position: instrument panel lights do not illuminate, radio won't turn on, power windows inop.
Vehicle will not start.
If you have a new remote key and notice the LED is not illuminating when locking and unlocking the vehicle, the remote key needs to be inserted into the vehicle EIS and used to activate LED function.
Remember your car may have been serviced before and parts replaced with different size fasteners used in the replacement. The sizes of the nuts and bolts we give may be different from what you have so be prepared with different size sockets and wrenches.
Protect your eyes, hands and body from fluids, dust and debris while working on your vehicle. If working with the electrical system, disconnect the battery before beginning. Always catch fluids in appropriate containers and properly dispose of any fluid waste. Recycle parts, packaging and fluids when possible. Never work on your vehicle if you feel the task is beyond your ability.
Our vehicle may vary slightly from yours as models do change and evolve as they grow older. If something seems different, let us know and share your info to help other users. Questions or want to add to the article? Leave a comment below. When leaving a comment, please leave your vehicle information.
If your remote key is not working, the first step is to replace the battery. Next try to program it. If the key will not program and your spare key works, the key will have to be replaced. A new remote key can be purchased from MB using your registration information and proof of ownership.
If you have the message in your instrument cluster, "Replace Key Battery", use the following steps to replace the battery. To replace the battery, start by sliding the lock away from the metal key slot and removing the metal key from the remote.
Next, flip the remote key over so the buttons are facing down. Use a small thin flathead screwdriver or a small ruler (red arrow) to release the rear cover lock. Slightly twist the tool until the chrome top moves away from the plastic cover, note the small gap in the photo.
Lift the plastic cover up and remove it from the remote key.
The battery will fall out as you flip the remote key over. Clean the battery contact points with electronics cleaner, be sure to clean carefully and not wet remote internals. Replace battery with new, positive side facing out, as shown. When installing the cover, insert the tabs on cover into back of remote key first (red arrows). Then press the top in until it snaps into place, then push the chrome top back down until seated. Once the battery is replaced, you may have to program the key;
Red Arrow: Lock: Pressing 1 x (global locking), Pressing and holding 1 x, at the same time access to infrared receiver (convenience closing).