{"id":1713,"date":"2020-04-06T16:28:12","date_gmt":"2020-04-06T15:28:12","guid":{"rendered":"http:\/\/www.beady.com\/blog\/?p=1713"},"modified":"2025-03-07T18:20:11","modified_gmt":"2025-03-07T18:20:11","slug":"p38-becm-communications","status":"publish","type":"post","link":"https:\/\/www.beady.com\/blog\/p38-becm-communications\/","title":{"rendered":"P38 BECM communications (Updated Mar 2025)"},"content":{"rendered":"\n

When making my p38 camper I needed to make a new wiring loom but still wanted to keep the look of the P38 internally as I feel it is a very tidy interior, I also wanted to remove the BECM and make a small neater one that did the bits I just needed.<\/p>\n\n\n\n

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The BECM talks to all the other systems (ABS,ENGINE,HVAC) via simple wires going hi and lo to indicate errors or status, <\/p>\n\n\n\n

The outstations in the doors, display and the switch panel use a form of serial comms to talk to each other.<\/p>\n\n\n\n

I only reverse-engineered the display but the comms system is the same on the others it will take someone else to sniff the codes using a simple cheap Arduino.<\/p>\n\n\n\n

There is a working arduino sketch at the end of the post.<\/p>\n\n\n\n

you just need three wires to the display, +12v and ground to make it work with any of the components., if you only want to listen then you can just sniff the data wire, to know which is sending the data is a little harder to do, because you cant detect which end pulled the line low.<\/p>\n\n\n\n

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The system runs on three wires, <\/p>\n\n\n\n

    \n
  1. Clock wire<\/li>\n\n\n\n
  2. Data Direction<\/li>\n\n\n\n
  3. Data wire<\/li>\n\n\n\n
  4. ground is always needed \ud83d\ude42 <\/li>\n<\/ol>\n\n\n\n

    The data wires to the display are easy to get to on the BECM they are on the plug just below the fusebox and can easy be identified because of the dual coloured wires. push a paperclip into the data wire and connect that to the a input pin on the arduino with a 1k resistor in series then connect a earth from the car to your arduino and then make a sketch to relay, use 4800 8N1 bps comms on the BECM wire to the serial monitor on the computer. then press things or make things happen on the car and see what gets sent to\/from the BECM. you should see the mileage being sent from the display to the BECM it is easy to work it out.<\/p>\n\n\n\n

    Bytes : A6 13 83 73 45 <\/p>\n\n\n\n

    A6 is the code to says its mileage then the mileage is in plain text here the mileage is 138373 (45 is the checksum)<\/p>\n\n\n\n

    Here is how the connections are wired into the switch panel for the windows<\/p>\n\n\n\n

    \"Range<\/figure>\n\n\n\n

    three wires OG,LG and R (black = GND)<\/p>\n\n\n\n

    on the display the signals wires are doubled up for reliabilty .<\/p>\n\n\n\n

    \"Range<\/figure>\n\n\n\n

    basically there is a red a green and a orange wire 2 & 12 ,3 & 13 , 5 & 15<\/p>\n\n\n\n

    orange is Data direction<\/p>\n\n\n\n

    red is clock signal<\/p>\n\n\n\n

    green is the actual data<\/p>\n\n\n\n

    the clock wire runs at 4.8 khz<\/p>\n\n\n\n

    the data dir wire is pulled low when sending data, you must wait 5ms before writing to the data line after pulling this low<\/p>\n\n\n\n

    and the data is 4800 8N1 just like any rs232<\/p>\n\n\n\n

    So what does this data ?<\/h4>\n\n\n\n

    The data is in 10 byte blocks <\/p>\n\n\n\n

    which consist of two 5 byte sections
    byte 0 : ID
    byte 1 : data
    byte 2 :data
    byte 3 :data
    byte 4 : Checksum byte : bytes 0,1,2,3 all XORed together
    <\/p>\n\n\n\n

    The display sleeps if it doesn’t get a valid data stream, so if you have power and nothing is on the screen then your data stream is wrong somehow.<\/p>\n\n\n\n

    to keep the screen awake when you dont need to send any data then you must send AA AA AA AA cs 00 which is the keep alive signal<\/p>\n\n\n\n

    <\/p>\n\n\n\n

    Byte 1 is the control\/action code<\/p>\n\n\n\n