Bought a Macbook Pro A1502 off eBay as 'parts only'. It was a 2014 model with a 820-3476-A board. All worked except the SSD wasn't being detected. Thought I'd post this as I haven't seen this fault really addressed anywhere else.
Checking the SSD connector, all rails and enable were present. I did find however that SSD_RESET_CONN_L was stuck low, so the SSD never got out of reset. This is fed by U3740, but SSD_RESET_L was stuck low at around 0.2V, sometimes getting as high as 1V. Looked like the PCH was at fault, so I tried a donor BIOS, but no luck there. This data line was definitely damaged in the PCH, so I tried altering the pullup to a lower value. But only thing that worked was pulling up pin 3 of U3740 to PP3V3_S0SW_SSD. My thought was since this is switched, it's not a hard pullup, so this reset line will still be low initially, but will turn off when this switched supply comes on. (Note: Reset is active low). I haven't found a datasheet for the SLG4AP016V IC, but the schematic seems to indicate that it holds the SSD in reset internally after a delay. The SSD_RESET_L is a master reset.
I then noted that pin 4 was only around 1.6V, even without a SSD fitted. Swapping out U3740 fixed that. So my assumption here is this chip was damaged (from ESD maybe during SSD insertion), and potentially damaged that line in the PCH as well. There original SSD had been removed so no idea if that was faulty.
Anyway, this hack appears to work and the SSD is now detected ok. Booted and reinstalled MAC OS just fine. As the other MAC's like Air etc use an identical design, this kind of fault isn't just restricted to this board.
Checking the SSD connector, all rails and enable were present. I did find however that SSD_RESET_CONN_L was stuck low, so the SSD never got out of reset. This is fed by U3740, but SSD_RESET_L was stuck low at around 0.2V, sometimes getting as high as 1V. Looked like the PCH was at fault, so I tried a donor BIOS, but no luck there. This data line was definitely damaged in the PCH, so I tried altering the pullup to a lower value. But only thing that worked was pulling up pin 3 of U3740 to PP3V3_S0SW_SSD. My thought was since this is switched, it's not a hard pullup, so this reset line will still be low initially, but will turn off when this switched supply comes on. (Note: Reset is active low). I haven't found a datasheet for the SLG4AP016V IC, but the schematic seems to indicate that it holds the SSD in reset internally after a delay. The SSD_RESET_L is a master reset.
I then noted that pin 4 was only around 1.6V, even without a SSD fitted. Swapping out U3740 fixed that. So my assumption here is this chip was damaged (from ESD maybe during SSD insertion), and potentially damaged that line in the PCH as well. There original SSD had been removed so no idea if that was faulty.
Anyway, this hack appears to work and the SSD is now detected ok. Booted and reinstalled MAC OS just fine. As the other MAC's like Air etc use an identical design, this kind of fault isn't just restricted to this board.
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