AutoFocus: All locked up with no place to go

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With some of today’s cars diagnosing those supposedly ‘simple functions’ such as power door locks, power seats, or power windows can get overly complicated. For example, I took a call recently on a 2001 Mercedes C-class. The driver’s complaint was that the door locks wouldn’t work after the car sat for more than an hour, but once they started to work again, they would be fine as long as you used them.

The technician found that in order to get into the car, you would have to unlock the car manually because the key remote would not work. Normally that would not be a big problem, but in this case, it would set off the car’s alarm every time. To compound the issue, the key would not turn in the ignition switch. So, to stop the alarm activation, the technician had to repeatedly press the lock/unlock button on the remote. Then, suddenly, the key remote would start working, allowing the key and the locks to function, the key would then turn in the ignition switch, and the engine would start.

The first step of the diagnosis was to check for any stored fault codes. Fortunately, the technician had a factory scan tool and was able to test for codes in every control unit. He didn’t find any codes that would provide any clues to the problem. Using the scan tool, a guided test of the antennas, keys and ignition switch verified that the car was working correctly every time the test was performed. Next the technician performed a function test to see if a component could be isolated by the symptom, in order to find out what was causing the locks and the ignition not to work.

The theory of operation for door locks goes a little like this: When the unlock button is pressed on the key, two signals are sent to the car. One is infrared and the other is radio. The infrared signal is used for operation of the locks and for the operation of convenience opening of windows and sunroof. In order for the convenience opening to work, the remote must be held within an inch or two of the infrared sensor on the outside driver’s door handle. The radio signal is read by an antenna amplifier module located at the top of the rear window glass. The information is then sent to the Rear Signal Activation Module (SAM) which then sends the information over the vehicle’s Control Area Network (CAN) bus to the Electronic Ignition Switch (EIS).

Upon receiving the information, the EIS will check for access authorization of the key. This is needed because every time a remote button key is pressed, every Mercedes within range of the remote will read the signal in this manner. If the key is verified as the correct key for the vehicle, the EIS will send the ‘unlock’ message over the CAN bus for all the body control units to see. At that time, all four of the Door Control Modules (DCM) will perform the unlock operation of their respective door lock actuators, and the rear SAM will then command both the left and right front DCM -- along with the front SAM – to flash the parking lights for a visual verification to the customer that everything worked as planned. The infrared signal is similar, but it takes a short cut.

When pressing the unlock button, the information is sent from the sensor in the door handle to the left front DCM, which will then send that information to the EIS. From there, it works the same as the radio operation.

To understand the operation of the EIS when it comes to allowing the key to actually turn to the “on” position … When a key is inserted into an EIS, the EIS will produce its own signal to read the data chip inside the key. If the key is found to be the correct one, the EIS will authorize the release of the steering lock control unit and will activate an internal lock solenoid, allowing the key to turn.

So, after waiting for the car to act up, it was determined that the remote would not work when trying to use the infrared signal, indicating the antenna amplifier and the rear SAM were out of the equation. That idea was reinforced since the locks could be operated using the interior lock/unlock switch. This meant that the CAN bus signal for lock/unlock was sent to the rear SAM and the DCM’s responded by operating the lock actuators. At that time, the only components left were the EIS itself and the key.

The possibility of a key working and then not working was low but still possible. Luckily, a second key was brought in by the customer, and it was quickly found that no matter which key was used, the outcome was still the same. Focusing on a problem EIS, the technician removed enough of the interior to be able to back probe the wiring for testing powers and grounds. All voltage testing checked OK. The EIS is the control unit in charge of the CAN bus and has no outside influence, other than itself, that would prevent the ignition key from turning.

Previous testing with the guided tests of the factory scan tool along with the function tests performed during time of failure proved that it was time to order a new EIS.

After the proper coding of the new EIS with the factory scan tool and using the required programming “orange” key, both of the old keys failed to work, so a new key needed to be ordered as well. This solved the unlocking and starting problems.

In the end, it was theorized that after the EIS went into sleep mode, it would not fully wake itself up, yet it would still wake up enough to communicate with the rest of the components on the CAN bus to allow functions such as lights and alarm activation. It’s amazing how a simple complaint of door locks not working can become so overly complicated.

Service data provided by Identifix Inc. For more information, visit the company’s Web site at

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