Low voltage on 3V42 line

I have a 820-2327 logic board fault which occurred in odd circumstances.

The laptop was working OK and I had powered it off (I thought I had) and was beside me while I was doing something else. I then became aware of the laptop fan running at high speed. The fan could only be stopped by removing the battery. Replacing the battery resulted in the fan either starting immediately or random running on and off.

The fault has progressed to the fan not running when the battery is connected (which is good as it should not be running when switched off). However the laptop will not boot and there is no green light when the charger is connected.

On further investigation the PP3V42_G3H measures less than 1v. Would explain the lack of green light on charger.

The PP3V42_G3H point measures 4.8ohm to ground. On a working board is 65K.

So something is low resistance on the 3.42v line.

Looking around the U6990 area I am hoping that C6999 on the line to ground is the culprit, although not yet had a chance to remove it to check. Also there is so much else on that line that could cause a low resistance.

My question has anyone seen a similar fault (fan running when powered off).

Also anyone have any ideas on the low 3V42 on where the next best area to check is.

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what do you get at adapter sense


Thank you for your interest and reply. I have measured 1v on J6900 pin 5 Adaptor Sense.

I can understand your logic as the 'Adaptor sense' is the second input to the compariter which feeds the voltage divider which the schematics notes read:

VOLTAGE DIVIDER FROM DCIN ENSURES Q6910 Vgs is met when SYS_ONEWIRE is high or low. Q6920 used as bilateral switch to ensure SYS_ONEWIRE doesn’t drive unpowered U6990.

U6990 being the 3v42 "G3Hot" supply.

I have also checked the voltage at R6905/2 which is 17.1 V , a bit low as it is the PP18V5 DCIN and effectively the VIN tof U6990.

My current thinking s that U6990 could be faulty which would cause the only 1v on 3V42 Line. The other thinking is that one of the many items on the 3V42 is low resistance.

Or your thinking on the logic side which prevents the sys_onewire from driving an unpowered U6990. U6990 is powered but a tadge low.


This one wire system is still used many mac laptops and following Louis Rossman on YouTube has provided an interest in fixing this. In my case I have bought another working spare board from ebay and got my laptop working but still feel a repair here is possible and is also some fun in finding out the fault.


@andy1248 do you have the boardview file for that model ?


Yes but cannot attach file to comment.


should be able to access it here.


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