[Repair Case] 2016 Mercedes-Benz GLA200 remote key cannot be unlocked

Model: 2016 Mercedes-Benz GLA200 (156.943), equipped with 270.910 engine and 724.001 transmission.

Driving Mileage: 12,430 miles.

Issue: The vehicle cannot be locked/unlocked using the remote-control key.

Diagnosis: The customer reported that using the lock/unlock button on the remote-control key doesn’t lock/unlock the vehicle, and the same is true when using a different remote control key. However, the unlocking and locking switches in the car can be unlocked and locked normally. In this way, the hardware problem of the lock block can be quickly eliminated. But this car does have abnormal locking failures, so use a Mercedes-Benz special diagnostic instrument to read the failure code. It can be known that there is a fault code reported in the keyless start control unit N69/5, and the fault code refers to a functional failure of the antenna module. 

To quickly solve this problem, we must first understand the functional principle of the central locking system and find the network block diagram, as shown in Figure 1.

Figure 1

This car is set up in the B configuration. Vehicles equipped with KEYLESS-GO/code 889 or KEYLESS-GO start function code 893: When the unlock button or lock button on the remote control key is activated, it will pass the radio signal through the rear window antenna and frequency modulation (FM), amplitude modulation (AM)), the central locking system (CL) antenna amplifier is sent to the KEYLESS-GO control unit. Then, the KEYLESS-GO control unit sends this radio signal to the electronic ignition switch control unit via the in-car controller area network (CAN). In both cases, the electronic ignition switch (EIS) control unit must check the entry authorization code of the remote-control key.

If the entry authorization is valid, the electronic ignition switch control unit will send the "lock" request to the following control units via the in-car controller area network (CAN):

 ◆◆Signal acquisition and actuation control module (SAM) control unit

◆◆Left front door control unit

◆◆Right front door control unit

 The left front door control unit locks/unlocks the left front door lock through the left front door central locking system motor. The front right door control unit locks/unlocks the front right door lock through the center door lock motor of the right front door. The front signal acquisition and actuation control module (SAM) control unit locks/unlocks the left rear door lock through the left rear door central locking system motor, and locks/unlocks the right rear door lock through the right rear door central locking system motor. The signal acquisition and actuation control module (SAM) control unit enables or disables the operation of opening the trunk lid or the opening tailgate through the external operation switch of the trunk lid/opening tailgate. The status of the rotary lock pin switch of the open tailgate is directly read by the signal acquisition and actuation control module (SAM) control unit.

 The lock function is simplified to understand that when the unlock or lock button on the remote-control key is pressed, the key sends the wireless signal to the rear window amplifier A2/32 antenna and the radio frequency modulation, the central locking system antenna A2/18 antenna amplifier. The keyless control unit N69/5 and then the keyless control unit N69/5 send radio signals to the electronic ignition switch N73 via the in-car CAN. N73 detects the authorization code and assumes it is valid. The electronic ignition switch will send the lock or unlock signal to the corresponding door module and the front signal actuation control module SAM through the in-car CAN, to perform each corresponding motor work to lock or unlock. See the functional schematic diagram for the specific working logic, as shown in Figure 2.

 

Now focus on detecting the problem between the remote-control key and the ignition switch. Because the fault persists after changing a remote-control key, eliminate the key problem. In addition, the keyless control unit N69/5 reported that the antenna module had a functional failure, so the antenna should be checked.

 After filtering according to this model and configuration, the A2/32 rear antenna amplifier is connected to the central locking system antenna and the radio signal antenna A2/18 through a through line, and the A2/18 antenna is then connected to the keyless through a through line Start on the control unit N68/5. The A2/18 antenna is also responsible for the FM and AM signals of the radio. At this time, we tested the radio broadcast and found that it was not working properly.

 Next, focus on checking A2/18. The inspection found that the internal corrosion of A2/18 was serious, which was obviously caused by water ingress. Because it is installed at the rear section of the rear windshield and the large roof, the sunroof is not draining smoothly, causing water to enter, so it malfunctions. Finally, the A2/18 antenna was replaced, and the water leakage was handled, and the problem was solved.

 Troubleshooting: Replace A2/18 antenna and deal with water leakage.

 Summary: Although this fault is a simple fault that the central control door lock cannot be locked normally, after the initial troubleshooting of all door lock hardware faults through the in-vehicle switch, the focus of the problem is to check the access permission transmission path. The transmission path can be quickly obtained by checking the functional schematic diagram and network block diagram.