Each time I plug in the IO Flex Cable into the main logic board, it shorts out PP3V42. The other end can remain unplugged to smaller board and the short persists. I tried a different cable and the same thing occurred. If I remove the flex cable, I read 3.4 volts like I am supposed to.
I am unsure if the 3.42V is routed through the cable to other parts of the board using the connector.
The connector looks fine.
Injecting voltage produces no heat.
I am quite confused.
Thanks for any help offered.
This is confusing me, is the short on the mainboard or the I/O board? (IE: unplug the I/O board and measure on the mainboard)
How have you determined that it’s short to ground on PP3V42? resitance to ground?
I’m guessing your asking if PP3V42 goes to the I/O board? if yes, then yeah I’m pretty sure it does… been a while since I worked on Macbooks but pretty sure it goes to the magsafe for feedback… though it’s been a while. Circuitry will be similar to the older mainboartds which had the I/O stuff on it
IO board has pp3v42 bio direction at pin21 on J9500 ribbon flex connector.
Test with known good IO charge board and flex, and measure pin21 resistance to ground without flex
Hi Severence, yes, the short is on the mainboard but only when the flex cable is plugged in to the mainboard. I can leave the other end of the cable disconnected from the I/O board and the short persists.
I have determined PP3V42 is shorted by inserting the flex cable, placing one probe on ground, and the other probe on L7095 or any of the capacitors in the area. Also, when the flex cable is not plugged in, I get 3.4 volts on L7095. As soon as I plug the cable in, 3.4 volts is gone.
PP3V42 is felt on pin 21 of J9500 but I am unsure if the flex cable routes the voltage back out of J9500 on another pin.
Thanks for the image. I have tested with a known good flex cable with the IO board completely disconnected, the flex cable inserted, and 0 ohms. From ground to pin 21 with flex cable disconnected, ~150k ohms.
Ah I see. I don’t have the schematics or boardviews on my PC but I would be examining them to determine if PP3V42 deviates IE: follow it’s path and see if it changes naming convention by going way of something else (which is typical on apple schemaitcs) in order to determine if it’s another rail/line/IO is dragging it down subsequently. Also, just other rails in general, what other rails derive from PP3V42 which if shorted could in turn pull PP3V42 down?
The only other thing I could think of if you haven’t got the other end of the ribbon attached to the I/O board and still have the short is something physically wrong with the connector mainboard side Probably it’s fine if it looks ok, but if you’ve got donor board with this connector, probably wouldn’t hurt to just swap it out just to rule it out
That’s a known good flex and known good io board?
Check the one wire circuit on schematic. If 150k on main board side. I believe the short comes from io board
You that is a great point. Perhaps, the signal comes in as PP3V42 and then leaves via some other signal. I will have to ponder that. I inspected the port via the microscope and there is nothing crazy going on. No damage or corrosion. I will order another connector and swap it just to rule it out.
A known good flex cable but I don’t have an IO board. I am not saying the IO board is good, but it isn’t causing the short because I completely disconnected it, and the short is still present.
I did spin the cable around and the short went away, but I think the pinout is different between the two ends of the cable.
It is almost like the cable is the thing that routes the signal coming in on P21 to another pin going back to the mainboard. I just can figure out which pin.
I mean, pull the connector off the IO board and put it on the mainboard and then put the mainboard connector on the IO board and see if the short transfers over to IO board when the ribbon is connected - that is if the connectors are the same. Save you buying another connector is all just to verify.
Sorry for the delay, I had to wait 24 hours to post again. After review, the connector on the small IO board won’t work. I did remove the port and inspect it for any type of damage (there was none) and then resoldered it. When I have time, I am going to inject voltage on pin 21 of the connector, and then measure each pin on the connect and see where the 1 V shows up. This way I will know which pin the voltage is leaving the port at.
if the suspicion is it’s joined on the ribbon and is coming out somewhere else under a different name, then just disconnect the ribbon, put one of your probes on the PP3V42 “pad” and in continuity sweep the other pads with your other probe. You can also do the same with it connected board end if you want or board solo on the connector itself, no need to “inject” voltage
There is no circuitry on the ribbon afaik so your just looking for direct continuity as it can’t really be anything else. Now board side is a little different, I’m almost certain PP3V42 will morph into somthing else / change names (at least elsewhere on the board)
OK well I am here to report my findings and it is with great chagrin that I do so. This is the first Macbook I have ever touched, much less tried to fix. When I disassembled it, the flex IO was connected and I assumed that was the proper orientation for the cable. After losing my mind, I decided to reverse the cable and see what happens. When I did so, the green LED illuminated on the charger and the laptop booted.
I thought, finally it is fixed, but alas, it was not. The backlight did not work correctly. I troubleshot that and determined that L7701 was open but upon later inspection, one of the legs was broken. I repaired that so it was making a good connection and now the laptop boots with backlight.
Now all I have to do is figure out what kind of HDD it takes and learn how to install MacOS.
Thanks for all the help, and while I am pretty embarrassed, I truly learned a lot trying to figure this out. Thanks again!
I was scratching my head a little as it could only really be the ribbon itself or the connector mainboard side at fault but if you go back, you figured it out earlier, no short with the ribbon reversed and then you subsequently figured it out, so nice work
Yeah, I kept looking at the flex cable thinking, why would the engineers route this cable across the fan impeding airflow. Looking back, I realize now, they didn’t lol.
Probably it’s fine if it looks ok, but if you’ve got donor board with this connector, probably wouldn’t hurt to just swap it out just to rule it out 
