Chosen Solution
I’ve got an iPhone 8 that worked fine, it had no issues with booting, running or charging. The screen was slightly cracked and the glass on the back case. It was dropped. But like I said everything ran fine for a long time. I ordered a new screen original and the back housing. (The back housing I ordered was with all small parts and components but they later messaged me it was out of stock so I said cool just send me the housing and I’ll transfer all the small parts over). So the back housing came and I transferred everything over. When I went to boot up the phone to turn it on it went into boot loop. Turned it off, re checked everything and found the very small flex plug which I believe is the Antenna flex to be broken. (I have attached a photo of it so you can see which one I’m talking about). After trying to turn the phone on again it did not respond or turn on. I found it strange that one second its in boot loop and next second its unresponsive. So I measured the voltage in the battery, was fine 4.13 v. I tried plugging power into the charging port and neither did the charging logo appear on the screen. Nothing at all. So I’m thinking: 1- Screen cant be dead cause it just worked. 2- Tried plugging it into itunes encase screen was dead but phone was on (that was not the case itunes did not detect it so phone was off). 3- Battery is fine as tested with multi-meter. I have tested all the pins by known good readings for the J6400. Most where fine apart for these which I found a bit high. Pin 6 - 0.802 Pin 8 - 0.802 Pin 20 - 0.805 Pin 22 - 0.802 Pin 24 - 0.802 Pin 25 - 0.721 Pin 26 - 0.907 Pin 27 - 0.721 Pin 39 - 0.612 Pin 41 - 0.612 (All diode readings) And maybe 50, 53, 54 and 56 (all at 2.32) is a bit high what do you think? From your experience by the readings I provided do you think there is an issue with the U2700 power IC? Or could it be an issue with the original charging port when it was removed and put back into new housing?? What I have done is ordered a new housing with all small elements already installed. If that does not resolve the issue will try replacing the power IC. I know that it regulates power to the phone and also charging, so if the phone is unresponsive it could be due to this. Please let me know your opinion on all this. I appreciate it. Update (09/27/2020)
Here is the picture I forgot to post
Here is J6400. I don’t know where you got J4101 from. If you are trying to sort out a charging problem you are looking in the wrong spot.
Housing replacements are notoriously difficult. The ones that come pre-populated can have low quality, if not outright defective flexes and doing it yourself exposes you to a ton of variable if things dont work. As for your diode readings, the ones you have to worry about are low numbers or shorts (except for ground ;-). Did you put the red probe on ground and probe with the black lead? Your numbers seem high. I would start by testing a bare-bones setup. Start by disconnecting the battery first (always!). Then disconnect the digitizer, LCD, front camera, rear camera and home button extension flex. Now re-connect the battery; you should only have the battery and Lightning Dock flex connected. Connect a known-good, preferably Apple-original Lightning cable to an iTunes enabled computer. Does iTunes recognize your phone? If so, then you should re-connect one flex at a time until the phone stops working. I would start with the Home button flex first, then add the rear camera, LCD & Digitizer then from camera flex. If the bare-bones setup does not get recognized by iTunes, then you most likely have a logic board issue. You could send it (or find a local) to a repair shop that specializes in micro-soldering repair.
