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iMac A1418 - Fan spins high when turned off, normal when running.

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  • iMac A1418 - Fan spins high when turned off, normal when running.

    Hi Guys,

    Never worked much on iMacs before. Customer just dropped this one in where the fan runs high when it's turned off.

    He said the HDD was upgraded so maybe thermal sensor, except that the fan runs at a normal speed when the machine is running.

    No idea where to start; I don't even have files for this Mac yet.

  • #2
    Fan runs off 12V_S0 rails usually which of course should never be on when machine is off. Also these suck to open. If HDD has been messed with and they did not use original tape to close it back up you are the one that will crack the LCD. I always inform the customer that if the machine was opened before I will NOT take responsibly for cracking the screen if generic tape or Chinese fake tape kit was used.

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    • #3
      The screen was already removed for me, which is super convenient.

      Originally it (just the logic board) was given to me with a blown L9001 (from memory, backlight filter anyway) maybe this is all related.

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      • #4
        Aren't the fans PWM?

        So that means dumping 12 volts to the rail the fan is on wouldn't necessarily make it spin right?
        Something needs to be telling it to run.

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        • #5
          How can the fan run if the machine is off and powered from S0 rail? PWM is not an issue then.

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          • #6
            Had the same issue a while ago. It was the Q7870. I did not have a FET in same package so i used one from another board and mounted it upside down with wire and conformal coating due to the customers impatience. Customer never came back. If you use an alternative FET then remember it must have a high drain-to-source voltage. I used one that had a maximum rating of 30V.
            Attached Files

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            • #7
              Thanks Sykulski.

              I will investigate this quickly!

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              • #8
                Can someone check my plan please;

                I have the part http://www.mouser.com/ds/2/149/FDMS9600S-348289.pdf

                I have connected

                G1 to FET_EN_P12V_S5_R
                S1 to PP12v_S5
                D1 to PP12v_G3H

                I would have thought S1 and D1 would be the other way around but reading datasheet for the OEM part it seems this way is right.

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                • #9
                  Seems fine to me. But these FETs need heat sinking to the board, just running some wires to them like this it will probably get pretty hot and die. This should only be done for a quick test, then get a FET that fits correctly on the pads.

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                  • #10
                    Thanks for the extra info Duke. I hadn't realised that.

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                    • #11
                      The fan still spins high when the iMac is turned off. :'(

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                      • #12
                        Try using the Q2-part of the FET as it has shotkky diode.

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                        • #13
                          Originally posted by Sykulski View Post
                          Try using the Q2-part of the FET as it has shotkky diode.
                          Can you link me to some info, I'm not following what you're saying.


                          Edit: Do you mean just use the other side (2) of the FET?
                          Last edited by gcain; 05-05-2017, 03:21 AM.

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                          • #14
                            Compare the drawings of your FET to this one http://www.infineon.com/dgdl/irfh370...53561a25871e76

                            The original drain-to-source diode in the schematics has a shotkky diode (drawn with an "S"-formed line next to the arrow instead of a normal straight line which means regular diode). Shotkky diodes switches faster at high frequencies.

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                            • #15
                              "The fan still spins high when the iMac is turned off. :'("

                              This is why you measure before just throwing parts at it. I see no reason why an S5 FET would cause the fan to stay on anyway. Fan runs off 12V_S0, so that bus is staying on. So Q7950 is a more likely candidate but measure first the enable and gate. Use brains, no plz Bro solutions.....

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