[Repair Case] 2014 BMW 520Li central control door lock can only be locked but not opened.

Fault phenomenon: The owner of this car reported that the central control door lock did not work properly. The specific phenomenon is that when the door is locked with the remote control, the 4 doors can be locked normally. When the door is opened with the remote control, the 4 doors cannot be unlocked. Only the left front door can be opened with the mechanical key to enter the car, and the vehicle starts normally.

Troubleshooting: After picking up the car, we first confirm the fault. It is indeed as described by the car owner that the door can only be locked with the remote control. When the unlock button on the remote control is pressed, none of the 4 doors respond, but the turn signals can be seen flashing from outside the car. According to this phenomenon, we analyzed that the remote control sent out the door opening signal, and the remote-control signal receiving part of the vehicle also received this command, but it did not execute normally.

To confirm the diagnosis, we used special equipment to detect the remote control, and found that when the unlock button is pressed, there was an electromagnetic wave signal output of 433MHz signal, which basically confirmed that the remote control worked normally, and the fault point was on the car.

Based on the meaning of the fault codes scanned, it was assumed that the problem may be a blown fuse, or a damaged micro relay inside the junction box electronics (JBE).

Enter the JBE system with the diagnostic instrument and perform the component test. It was found that when the door was locked, the four door locks could work normally, and when the door was opened, the sound of the action of the micro relay inside the module could be heard, but the door lock motor had no response.

We learned that the JBE control box was installed behind the glove box on the co-pilot side. We could see this control module after removing the glove box. Because the right front door cannot be opened, it was very difficult to access when checking the safety piece, so the JBE was first removed, and the two micro relays were checked. The specific methods were as follows:

After taking the JBE case apart, use a hand-grinding wheel carefully to rub off the top covers of the two relays, and then pry off the remaining case. After observation and analysis, it is found that each micro-relay has 8 pins, which are 4 pins for the positive and negative terminals of the two groups of coils, two power input terminals for positive and negative electrodes and two output terminals for the motor. There are two relays encapsulated inside each shell, and then according to the structure of the relays, a multimeter is used to detect the on and off states of the normally open contacts and the normally closed contacts. After inspection, it was found that the relationship between turn-on and turn-off is normal. The resistances of the measuring coils are all about 225Ω, which is within the normal range. The location of the relay on the circuit board is shown in Figure 1.

Figure 1

Additionally, we polished the contacts with fine sandpaper, reinstalled the JBE control box, and tested again, and found that the fault phenomenon had not changed. The only thing left was to rule out whether there was a blown fuse.

The entire fuse box was checked, and it was found that a 15A fuse was blown at the position shown in Figure 2. After replacing it, use the remote control to open the door. The four doors were unlocked normally, and the fault was eliminated.

Figure 2

After trial and error, the failure did not recur. Because the reason for the burning of the fuse was not found, we ordered a new relay, and the vehicle was temporarily delivered.

Fault summary: The fault of this car was because the fuse was blown, so the vehicle can only be locked by the remote control and cannot be opened by the remote control. Thus, it was difficult to measure the safety piece, so we adopted the elimination method and firstly checked the micro relay in the control module.

Why does the fuse blow? We haven't found the definitive answer. We assume that the door lock motor may be frozen, resulting in excessive current; or the relay contacts are abnormally stuck, resulting in excessive current and burning out the fuse.