[Macbook pro A1708] [Boot loop] [Trackpad ?]

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  • No Lan
    Member
    • Apr 2025
    • 15
    • FRANCE

    #1

    [Macbook pro A1708] [Boot loop] [Trackpad ?]

    Hey everyone,

    Usually, I'm not the type to ask for help, but this time, I'm calling on your wisdom and expertise I've seen what this community can do, so I'm pretty sure you guys can help me out.

    Here's the deal: I noticed my MacBook got stuck in a boot loop. After opening it up and doing some initial troubleshooting, I found that the boot loop stops only when the trackpad is unplugged.

    Now, just so you know, the trackpad actually works — I tested it with another motherboard and it's all good. Same for the cable, no issues there. The problem is only with this motherboard (820-00840).

    Another weird thing I noticed: when the trackpad is plugged in, the fan doesn't spin. But when it's unplugged, the fan runs just fine.

    I checked with L4800 and C4800, no shorts. Also got 5V on PP5V_S4_TPAD_CONN.

    Honestly, I'm a bit stuck on what to do next to find the root cause.

    Oh, and important to mention: I did something I probably shouldn't have, but I wanted to test it out — I waited for the Mac to boot without the trackpad, then plugged the trackpad in hot.

    It worked for about 20 seconds before it rebooted again.

    So yeah, that's the situation! Looking forward to your bright ideas! Thanks a ton
  • mon2
    Badcaps Legend
    • Dec 2019
    • 13830
    • Canada

    #2
    Review the voltage to ground of the I2C lines on the trackpad connector. These are SCL / SDA.

    Comment

    • No Lan
      Member
      • Apr 2025
      • 15
      • FRANCE

      #3
      Thanks you for your reply

      I found these 2 lines on the boardview for J4801 TRACKPAD

      SMBUS_SMC_3_S0_SCL --> 3.3V
      SMBUS_SMC_3_S0_SDA --> 3.3V

      Do i need to make furher test on it ?

      Click image for larger version  Name:	image.png Views:	0 Size:	127.8 KB ID:	3644891

      Comment

      • mon2
        Badcaps Legend
        • Dec 2019
        • 13830
        • Canada

        #4
        Also, pins 8 and 12.

        Post the voltage measurements to ground of each TPAD_*** signal.

        Comment

        • No Lan
          Member
          • Apr 2025
          • 15
          • FRANCE

          #5
          Okey,

          just to be sure, i'm taking measurement without Keyboard and trackpad connect. Is that good ?

          KBD_I2C_SDA --> just after connect USB-C it slowly going from 0 to 1,50v-1,84v --> not stable
          KBD_I2C_SCL --> just after connect USB-C it slowly going from 0 to 1,50v-1,84v --> not stable

          TPAD_SPI_INT_L --> 3,29v
          TPAD_SPI_MOSI --> 0v
          TPAD_SPI_CS_CONN_L --> 3,33v
          TPAD_SPI_MISO --> 0v
          TPAD_SPI_IF_EN --> 3,33v
          TPAD_SPI_CLK --> 0v
          TPAD_VIBE_L --> 0v

          i also noticed something strange. On PP5V_S4 (L4800) i measured 8-9v and sometimes strictly 5v

          Sometimes the Mac boot with trackpad sometimes bootloop..

          Comment

          • No Lan
            Member
            • Apr 2025
            • 15
            • FRANCE

            #6
            Just because I measure 9V on PP5V_S4 one times, should I be concerned that U7650 might be malfunctioning sometimes? Or is it not handling the voltage as expected?

            Comment

            • mon2
              Badcaps Legend
              • Dec 2019
              • 13830
              • Canada

              #7
              What is the current draw for this logic board? Are you at a stable 20v? If not stable 20v, review the article on the internet for the 'CD3215 and 20v'.

              Comment

              • No Lan
                Member
                • Apr 2025
                • 15
                • FRANCE

                #8
                I'm fully stable 20v and between 0,4 and 0,8A without batterie connected.

                Comment

                • mon2
                  Badcaps Legend
                  • Dec 2019
                  • 13830
                  • Canada

                  #9
                  Do you ever see the Apple logo or hear the boot chime in any of these attempts?

                  What is the voltage to ground of PPBUS_G3H? ALL_SYS_PWRGD?

                  Comment

                  • No Lan
                    Member
                    • Apr 2025
                    • 15
                    • FRANCE

                    #10
                    Just to clarify

                    if trackpad is disconnected --> Mac boot with fan --> Logo Apple --> Screen for user authentication --> Stable
                    If trackpad is connected --> Mac boot without fan (fan work only for few second) --> Logo Apple --> Boot loop OR Mac boot without fan (fan work only for few second) --> Logo Apple --> Screen for user authentication crash and restart

                    for your question

                    PPBUS_G3H--> 13,2v
                    ALL_SYS_PWRGD --> 3,23v

                    Comment

                    • mon2
                      Badcaps Legend
                      • Dec 2019
                      • 13830
                      • Canada

                      #11
                      What is the motherboard that works with this same trackpad with the same cable? Is it the 820-00840?

                      Comment

                      • No Lan
                        Member
                        • Apr 2025
                        • 15
                        • FRANCE

                        #12
                        Yes it is the exact same motherboard 820-00840

                        Comment

                        • mon2
                          Badcaps Legend
                          • Dec 2019
                          • 13830
                          • Canada

                          #13
                          With the trackpad disconnected, you should have a high fan spin since the (trackpad) thermal sensors are not being detected. Is this correct? You can review the other working board for the expected high fan spin condition.

                          Comment

                          • No Lan
                            Member
                            • Apr 2025
                            • 15
                            • FRANCE

                            #14
                            I agree with you.
                            with the 2 motherboard (the one for this post and the other working well) i have a high fan speed when trackpad disconnected !

                            Comment

                            • mon2
                              Badcaps Legend
                              • Dec 2019
                              • 13830
                              • Canada

                              #15
                              Boot into SAFEMODE with the trackpad connected.

                              To start an A1708 MacBook Pro in safe mode, you need to restart the computer and press and hold the Shift key until the login window appears. Once logged in, you can verify safe mode by checking the System Information app.

                              Does this trackpad with the same cable function with this (non-working) logic board?

                              Review the mating connectors for possible dry solder or lifted pin on the SMD joints.
                              Last edited by mon2; 05-25-2025, 09:54 AM.

                              Comment

                              • No Lan
                                Member
                                • Apr 2025
                                • 15
                                • FRANCE

                                #16
                                I cannot connect as the system keep restarting before or during login windows :/
                                even when starting in safe mode

                                Comment

                                • mon2
                                  Badcaps Legend
                                  • Dec 2019
                                  • 13830
                                  • Canada

                                  #17
                                  Remove all power. Meter in resistance mode. Mate the trackpad to this board.

                                  Measure the resistance to ground of:

                                  PP5V_S0_FAN
                                  FAN_LT_PWM
                                  FAN_LT_TACH

                                  Comment

                                  • No Lan
                                    Member
                                    • Apr 2025
                                    • 15
                                    • FRANCE

                                    #18
                                    Here the result

                                    PP5V_S0_FAN --> 1,8
                                    FAN_LT_PWM --> OL
                                    FAN_LT_TACH -->OL

                                    Comment

                                    • mon2
                                      Badcaps Legend
                                      • Dec 2019
                                      • 13830
                                      • Canada

                                      #19
                                      What are the units of measurement?


                                      PP5V_S0_FAN --> 1,8


                                      1R8 ohms to ground?

                                      Comment

                                      • No Lan
                                        Member
                                        • Apr 2025
                                        • 15
                                        • FRANCE

                                        #20
                                        Sorry i was in diode mode.

                                        Multimeter on 2000K ohms and i observe between 220 and 240 on PP5V_S0_FAN. took on C4870

                                        FAN_LT_PWM --> OL
                                        FAN_LT_TACH -->OL

                                        Comment

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