A driver trail me his BMW X5 2008 year with 55 thousands mileage, he said: its rear door keyless entry not working.
Firstly connect the car and confirm where is the trouble, after test its right rear door keyless entry function all not working, but the other doors keyless entry function is working.
BMW X5 keyless entry function means when the key is within 2m from the door, as long as the driver press the door handle, and the door transmit the sensor signal to the starting system control unit (CA) and Comfort Access System (CAS) by CAN-BUS, then active the sleepy CAS and CA. CA active FBD antenna in the multiple choice merit-based module, meanwhile the activated CA by CAN-BUS sends the signal to the door requesting the antenna in the hand to emit 125kHz key identification radio signals. After the key out of BMW X5 car receives its radio signal, decode and identify it, CA send 868 kHz high-frequency radio Key identification signal to FBD antenna in the multiple choice merit-based module, FBD antenna send the received key identification signal to the radio signal receiver DIV in the multiple choice merit-based module for identification, and send the results to CAS, after successfully verification, CAS send out unlock authorization and notify junction box control unit to execute unlock instruction, the door unlock.
Then I connect BMW ICOM ISID to test BMW X5, and it has no any fault code, according to the above-mentioned controlling principle, I analyze and find out the reasons are as follows: the antenna in the handle of right rear door; the door handle identification device has troubles; circuit trouble exist between door handle and CA or CAS.
After I change the antenna in the right rear door handle, the trouble still exists; after I change the right rear door handle, the trouble still not troubleshoots. Then I check the related circuit gram, and measure the circuit between the door handle to CA or CAS. When I check the connector of the right rear door handle and prepare to connect adapter to measure the circuit signal, I surprisingly find there are 3 lines coming, but the connector only has 2 ports, then I check the related circuit gram, it says there should be 3 lines. Then I make the 3 line skip the connector and directly connect to the door handle, then to test, the right rear door keyless entry function can work.
Troubleshoot: after the connector in the right rear door handle is changed, the trouble is solved.