A2251 2020 13" MacBook Pro

=== IMPORTANT: All CD3217 USB C controllers (U3100_X, U3100_T, U3100_W, U3100_R) are uniquely paired to the board via firmware. If replacement is required, you MUST source them from an 820-01949 board, and you must replace them in the exact order that they came off the donor board. === This isn’t actually true I have successfully replaced the cd3217 with a generic one bought from aliexpress.

I used the u301 to reprogram the rom problem solved. (Daniel: Can you please explain how you did that.)

Daniel: Board 820-01949 starts without battery. {| class="wikitable" ! style="text-align:left;" | Problem ! Solution •Always check if all ports are reading the same. One port reading 5 V ~0.12–0.25 A while the other reads 5 V ≈5 V 0.02 A is more indicative of a CD3217 issue, while all ports reading 5 V ~0.00–0.02 A is more indicative of a PPBUS_G3H issue or T2 corruption.
 * No Power, 5V / 0.00-0.05A as measured by a USB C Amp meter
 * •The most likely cause for a 820-01949 to be stuck on 5 V and low current draw (~0.00–0.02 A) on the USB-C current meter is a short to ground on PPBUS_G3H.
 * •The most likely cause for a 820-01949 to be stuck on 5 V and low current draw (~0.00–0.02 A) on the USB-C current meter is a short to ground on PPBUS_G3H.

Differential Diagnosis for 5 V and low current draw as indicated on the USB-C current meter

•Absent PPBUS_G3H voltage due to a short or creation problem.

•Device stuck in DFU mode due to corrupt T2/BridgeOs firmware.

•Absent or shorted PP3v3_G3H_RTC voltage.

• Failed CD3217 USB C controller (U3100_T, U3100_X, U3100_W, U3100_R)

•Short to ground on any of the power rails generated by U7800 that power the T2 chip, such as PP1v8_SLPS2R or PP1v1_SLPS2R.

• Missing U7800 input power

• Missing or shorted SLPS2R line. (Most commonly PP1v8_SLPS2R due to a short to ground.

• Non-specific areas of corrosion causing T2 or CD3217 communication issues.

➤Diagnostic Steps

1. Check if the device is stuck in DFU or Recovery mode.

- Plug the MacBook into another Mac or MacBook via its master port (Left side, bottom port, closest to the trackpad) and open Apple Configurator 2 to verify that it is or isn't in DFU mode. If the device is in DFU mode, Proceed to "Device stuck in DFU mode due to corrupt T2 firmware" in repair steps below.

2. Check voltage on PPBUS_G3H.

To check voltage on PPBUS_G3H, place your multimeter in DC voltage mode, and place the black probe on ground, and the red probe on PPBUS_G3H. A good spot to measure from on the 820-01949 is C6584 or similar.

~12.60–12.65 V = Normal

~12.28–12.35 V = T2 communication issue with the ISL9240. The likely cause for this is the device is stuck in DFU mode due to corrupt T2 firmware or a failed CD3217. Proceed to repair steps below for corrupt T2 firmware.

~0 V = Short to ground or absent voltage due to a creation problem. Proceed to repair steps below for shorted or absent PPBUS_G3H voltage.

3. Check PP3v3_G3H_RTC voltage.

- Ensure your multimeter is in DC voltage mode. ( Straight line with 3 - - - under the line. ) Place the black probe on ground and the red probe on PP3v3_G3H_RTC. It is recommended to measure PP3v3_G3H_RTC on a capacitor or resistor near one of the CD3217s to rule out a trace/via issue. C3100_W is a good place to measure this line on the 820-01949 logic board.

~3.3 V (3.290–3.425 V) = Normal.

4. Check PP3v3_G3H_PMU_VDDMAIN voltage.

- Ensure your multimeter is in DC voltage mode. ( Straight line with 3 - - - under the line. ) Place the black probe on ground and the red probe on PP3v3_G3H_PMU_VDDMAIN. It is recommended to measure PP3v3_G3H__PMU_VDDMAIN on a capacitor near U7800 to rule out a trace/via issue. C78B0 is a good place to measure this line on the 820-01949 logic board.

~3.3 V (3.290–3.425 V) = Normal.

Values below the above spec should be considered abnormal. Proceed "PP3v3_G3H_PMU_VDDMAIN low or absent" in the repair steps below.

➤Repair Steps

Device stuck in DFU mode due to corrupt T2 firmware:

- Revive T2 firmware via Apple Configurator 2.

NOTE: You must be running the latest version of MacOs for this to work consistently. Check for MacOs updates prior to reviving/restoring T2 firmware.

How to revive T2 firmware: Plug the device you are working on to another Mac or MacBook via its master port. The master port on the A2251 is the bottom left side USB-C port (closest to the trackpad.) If you are confused, please see the diagram listed on Apple's how to page — https://support.apple.com/guide/apple-configurator-mac/revive-or-restore-an-intel-based-mac-apdebea5be51/mac

Once plugged in, open Apple Configurator 2. You should see a big square icon pop up that says "DFU" or rarely, "RECOVERY". Click the icon, Navigate to the top menu bar click "Actions" then "Advanced". Select Revive device. You will see a progress bar appear. This process can take anywhere from 2 minutes to over 30 minutes in some cases.

SELECTING RESTORE WILL WIPE ALL USER DATA!

PPBUS_G3H Short to ground:

Inject around 1 V with a 5 amp limit into PPBUS_G3H with a DC power supply. A larger tantalum capacitor is preferred to inject voltage to, as you will often need a lot of current to be pushed into the line to cause the shorted component to heat up. 1 V is usually sufficient. It is not recommended to inject over 1 V as if one of the CPU VRMs is shorted, you will end up killing the CPU. A good place to inject voltage to on the 820-01949 is F7000.

With voltage being injected, perform thermal imaging of the board. If thermal imaging is not available, feel around the board to see where it is getting warm. Once the area is localized, add a small amount of isopropyl alcohol to the area to localize the shorted component.

Once the shorted component is localized, replace the shorted component.

For more info on finding short circuits, please see - [2]

PPBUS_G3H Absent or low with no short:

PPBUS_G3H is created by the Intersil/Renesas ISL9240 (U7000) which is a buck or boost converter depending on its input voltage. If USB-C voltage is 5 V, the ISL9240 will boost the 5 V input to ≈12 V. If USB-C voltage is 20 V, the ISL9240 will buck (lower) the 20 V input to ≈12 V.

•When PPBUS_G3H is absent with no short to ground, we need to first make sure that PPDCIN_G3H is making it to the ISL9240 (U7000). C7024, C7028, C702A or C7026 is a good place to measure from on the 820-01949. Measure PPDCIN_G3H with your multimeter in voltage mode. You should get the same voltage that you are getting on the USB-C amp meter (5 V in this case, because that is what we are troubleshooting.) If you get 0v on PPDCIN_G3H, check for a short, and if no short is found, move down to "CD3217 troubleshooting" in the repair steps below.

•If you are getting 5 V on PPDCIN_G3H, the ISL9240 is likely the cause of the problem, however we still need to check a few things before replacing the chip. The ISL9240 relies on a series of current sensing resistors to measure the amount of power being used by its output rail, PPBUS_G3H. If one of these resistors is blown, the chip will think that something on the output is pulling too much power, and it will disable itself as a protection measure.

''To check the current sensing resistors, put your multimeter on resistance mode. Check each resistor listed below by placing your probes on each side of the resistors. The values should be within 5–10% of the reference listed below. Values slightly outside of the reference should be considered insignificant and ignored. If you find a resistor out of spec, replace the resistor AND the ISL9240, as the resistor likely blew as a result of the ISL9240 internally shorting.''

R7021 — 1.00 Ω

R7022 — 1.00 Ω

R7061 — 1.00 Ω

R7062 — 1.00 Ω

If the current sensing values are normal, replace the ISL9240.

PP3v3_G3H_RTC low or absent:

PP3v3_G3H_RTC powers the "brain" functionality of the CD3217 USB-C controllers which allows negotiation with the USB-C charger to allow 20 V. When PP3v3_G3H_RTC is shorted, low, or absent, this communication cannot take place, and the device will be stuck at 5 V. On the 820-01949, U6903 generates PP3v3_G3H_RTC. U6903 is a buck converter which lowers the voltage from PPBUS_G3H (≈12 V) to 3.3 V. Since PPBUS_G3H is the VIN (Voltage Input), we must get PPBUS_G3H before we get PP3v3_G3H_RTC.

•The most common reason for PP3v3_G3H_RTC to be missing or low, is a short to ground, usually on a capacitor around one of the CD3217s, or from a CD3217 itself.

•If PP3v3_G3H_RTC is not shorted, check for its enable signal, GHGR_EN_MVR. CHGR_EN_MVR is produced by the ISL9240 (U7000). The ISL9240 commonly fails in a way that prevents CHGR_EN_MVR from being produced. If the enable signal above is missing, replace the ISL9240 (U7000.) Due to the capacitance of the circuit, PP3v3_G3H_RTC may falsely read as short, measuring around 100 ohms. This should be considered a false positive. Remeasure after the board has sat for a few minutes with power disconnected.

•PP3v3_G3H_RTC can also be pulled low by PP3v3_UPC_W_LDO or PP3v3_UPC_X_LDO, so if you have no measurable short to ground, and your enable/VIN is present, check both the above rails for a short or low resistance to ground. Low resistance to ground/short on the above LDO lines will usually be caused by a bad CD3217.

'''If you found and resolved a short circuit, and still have low voltage on PP3v3_G3H_RTC, check voltage on both sides of R6934/35. These resistors can blow in response to a short circuit.'''

PP3v3_G3H_PMU_VDDMAIN low or absent:

Most commonly, PP3v3_G3H_PMU_VDDMAIN will be absent do to a corroded probe point or resistor. R7900 passes through PP3v3_G3H_RTC to PP3v3_G3H_PMU_VDDMAIN.

• Failed or corroded CD3217 • Corroded TBT ROM or corresponding resistors or traces. • Incompatible CD3217 used as a replacement. • Device stuck in DFU mode. ➤Diagnostic Steps:**IMPORTANT: Was prior work performed on the board? If so, the previous shop likely used a incompatible CD3217. When replacing CD3217s on the 820-01949, you MUST use a chip from another identical 820-01949 and you MUST replace the chip in the same format it came off the donor board. New chips, or chips from other model boards will not work, and swapping the TBT rom from another board also will not work.**
 * 1) Ensure you have power to pin 1 of the resistor. (R7900.) The adjacent probe point may become corroded in the presence of liquid damage. The trace is easily visible and readily traced back to the possibly offending probe point.
 * 2) If power is present to pin one, check voltage on pin 2 of the resistor. You should have 3.3v. If voltage is measured to be low, you may have a blown resistor and/or a short to ground. R7900 is a 0 ohm resistor and is essentially used as a fuse, if resistance is out of spec, check for a short to ground. If no short to ground is identified, replace the resistor and retest. If the resistor blows again, replace U7800.
 * No Power, 5v and ~0.23-0.31A, Cycling or not cycling
 * Differential diagnosis for No Power, 5v and ~0.23-0.31A, Cycling or not cycling on the 820-01949 logic board:
 * Differential diagnosis for No Power, 5v and ~0.23-0.31A, Cycling or not cycling on the 820-01949 logic board:

1. Check if the device is stuck in DFU or Recovery mode.

- Plug the MacBook into another Mac or MacBook via its master port (Left side, bottom port, closest to the trackpad) and open Apple Configurator 2 to verify that it is or isn't in DFU mode. If the device is in DFU mode, Proceed to "Device stuck in DFU mode due to corrupt T2 firmware" in repair steps below.

2. Check all 4 ports to see if one port cycles or has a different amp draw. If one port cycles, or has a different amp draw than the others, the offending CD3217 is likely tied to this port.

- CD3217s rarely fail, even when corroded. If the device has experienced liquid damage, and there is visible corrosion around the CD3217s, reball them. Replacement should only be performed as a last resort due to the firmware issues that replacing CD3217s may cause on this board.

- In the presence of CD3217 issues, you will almost always find one port reading differently; with the most common finding being cycling.

3. Visually inspect UB260 and surrounding resistors and traces. Address as necessary if findings suggest damage.

4. Visually inspect U3060 and surrounding resistors and traces. Address as necessary if findings suggest damage.

➤Repair Steps

LEFT SIDE BOTTOM PORT IS CONTROLLED BY U3100_T

LEFT SIDE TOP PORT IS CONTROLLED BY U3100_X

RIGHT SIDE BOTTOM PORT IS CONTROLLED BY U3100_W

RIGHT SIDE TOP PORT IS CONTROLLED BY U3100_R


 * 1) IF LIQUID DAMAGED - Check for a short to ground on PP1v5_UPC_LDO_CORE and PP3v3_UPC around the CD3217 corresponding to the port that reads differently than the rest. If a short is found, remove and replace the shorted component, which may be a capacitor, or the CD3217 itself. If findings suggest a failed CD3217, reball the chip tied to the offending port. (See above.) Some visual delamination is typical on CD3217s when exposed to heat, this usually will not affect the functionality or longevity of the IC. Inspect all surrounding resistors and replace corroded resistors as necessary. If a resistor appears to be corroded, remove and replace it. Pad damage is common, especially on lines fed by 3.3v. Pad damage can be easily missed and can mimic a CD3217 issue.
 * 2) IF NOT LIQUID DAMAGED - Check for a short to ground on PP1v5_UPC_LDO_CORE and PP3v3_UPC around the CD3217 corresponding to the port that reads differently than the rest. If a short to ground is identified, remove and replace the shorted component ; otherwise, replace the CD3217 tied to the offending port.

Replacing CD3217s on the A2251/820-01949
YOU MUST USE CD3217s FROM AN IDENTICAL 820-01949 DONOR BOARD. (RAM, CPU, and storage configurations do not matter.) NEW CHIPS, OR CHIPS FROM OTHER BOARDS WILL NOT WORK, EVEN IF YOU SWAP THE TBT ROMS. THE CHIPS MUST BE REPLACED IN THE SAME FORMAT THEY ARE REMOVED FROM THE DONOR BOARD. U3100_T MUST BE USED TO REPLACE U3100_T AND SO ON. YOU CANNOT MIX THE CHIPS UP, IT WILL NOT WORK. Replacing or mixing up the chips with the wrong firmware may permanently corrupt the TBT ROMs.

If corrosion or trace damage is noted near UB260 or U3060, replace corroded components and repair traces as necessary using jumper wires. DO NOT REPLACE EITHER CHIP. If chip is corroded, remove and clean pads and reuse the existing chip. Both ROMs have customized firmware. If replacement is required due to a short, or severe pad damage on the chip, it must be sourced from an identical 820-01949. Differential diagnosis for no display backlight on the 820-01949 logic board:
 * No Backlight on display
 * Troubleshooting backlight problems on the 820-01949 is identical in process to all the other USB-C boards from 2016-2020.
 * Troubleshooting backlight problems on the 820-01949 is identical in process to all the other USB-C boards from 2016-2020.
 * Failed display TCON board.


 * Failed display LED strip.


 * Short to ground on backlight output caused by a failed output decoupling capacitor.


 * Failed backlight driver (U8400)


 * Backlight driver circuit fault (resistors, traces, capacitors, MOSFETs.)
 * Low voltage on PP5v_S0SW_LCD


 * SMC_LID_LEFT or SMC_LID_RIGHT voltage too low due to a failed hall effect sensor or a issue within the SMC_LID circuit.

The most common cause for a 820-01949 to have no backlight in the absence of liquid damage, is a short to ground on backlight output due to a failed capacitor.'

➤Diagnostic Steps

1. Check for a short to ground on backlight output (PPVOUT_LCDBKLT_F). C8471 or adjacent capacitors in the series is an appropriate area to measure, since it does not require removal of the system board. If a short to ground is measured, unplug the display connector to rule out a potential short to ground within the TCON board or LED strip of the display. If the short is persistent with the screen unplugged, proceed to the "Backlight output shorted to ground" repair steps below.

2. If no short to ground is measured, and there is no signs or history of infiltrative liquid damage to the system board, test the device with a known working display. If the device has backlight with a known working display, proceed to the repair steps below for "Device has backlight output on a known good display, but not on the originally installed display" below.

3. The following section will focus on determining if U8400 is the cause of the backlight issue.

Failure of U8400 without the presence of liquid damage is uncommon, however if you have no backlight with a known good display, and do not have a short on backlight output, U8400 may be bad or you may have another circuit fault such as a bad resistor or broken trace within the U8400/backlight generation circuit.

If U8400 is corroded, replace it along with any other corroded resistors or capacitors adjacent to it.

It is also common for current sensing pins (pins 9 and 10) of U8400 to be very corroded with corresponding pad/trace damage. If this is the case, run a wire for each pad and connect them to R8400. Pin 9 (ISNS_LCDBKLT_N) can be run to Pin 4 of R8400 and Pin 10 (ISNS_LCDBKLT_P) can be run to pin 3 of R8400.

①. Ensure that the backlight circuit has its input voltage (VIN) by measuring on BOTH SIDES of F8400, which is the backlight fuse. Voltage on both sides of the fuse should be equivalent to PPBUS_G3H. If voltage is normal on both sides of the backlight fuse, proceed to the next step. The backlight fuse can blow in response to a previous short to ground in the backlight circuit, or due to the screen being plugged in without the battery disconnected. Rarely, the backlight fuse can become open (blown) due to development of sulfur deposits on the fuses' filament, or due to micro manufacturing defects on the fuses filament. If the backlight fuse is blown, check for a short on backlight output (PPVOUT_S0_LCDBKLT_F) and on backlight input (PPVIN_S0SW_LCDBKLT_F). If the fuse is blown, proceed to the "Backlight fuse (F8400) blown in the absence of a short to ground" repair steps below.

② Check that the backlight circuit is being enabled. The screen will need to be connected for the enable signal to be sent out. With the board on your bench, with a known good screen connected, measure voltage on pins 1 and 2 of R8442. You should have ~3.3v. Low voltage on pin 2 of R8442 is suggestive of an issue with U8400. If you have 0v on pin 1 of R8442, the backlight circuit is not being told to turn on, which raises the possibility of a CPU issue, or an issue with the display cable/connector.

③ Check the 5v inputs to U8400 (PP5v_S0_BKLT_D and PP5v_S0_BKLT_A).

PP5v_S0_LCDBKLT_D can be measured on R8444 and PP5v_S0_LCDBKLT_A can be measured on R8445. Be sure to check voltage on both sides of the resistor. If voltage is lower on one side of the resistor, R8444 or R8445 is likely blown, probably due to a short within U8400.

If PP5v_S0_LCDBKLT_D or A is low (Substantially lower than 5v), replace U8400 AND R8444/45 as R8444/45 acts as a current limiting resistor to protect PP5v_G3S, and likely blew as a result of an internal short within U8400.

If you have low voltage on pin 2 of R8442 with normal voltage on pin 1 of R8842, replace U8400.

If VIN, EN, and PP5v_S0_LCDBKLT_D are all present at normal levels, and you do not have backlight, replace U8400.

If EDP_BKLT_EN is missing, U8400 is NOT your issue.

➤ Repair Steps

Backlight output shorted to ground:

- Find and replace shorted component.

''Ensure the short is not caused by the display. Be sure to measure with and without the display connected.''

Most commonly, a short on backlight output is caused by a shorted capacitor. If you have a short on backlight output, solder a wire to a sturdy component that can handle a good amount of current, usually one of the backlight output capacitors (C8471 or C8461 is usually a good spot.) Inject 10v at 5 amps and thermally image the system board or deploy comparable short detection methods. For more information on short circuit detection, please see the following page - [10]

Device has backlight output on a known good display, but not on the originally installed display

- Replace the display assembly.

Backlight fuse (F8400) blown in the absence of a short to ground

- Replace the backlight fuse with a fuse from a donor board or a compatible replacement. (0603 package size, 3 amp, 32v fuse. Brand does not matter.)

''Before applying power be sure to double check for a short to ground. Check for a short to ground on backlight output and backlight input''. • Failed T2 or NAND (Most likely diagnosis for most cases in the absence of the below causes.
 * No Power, 20v, Static ~0.03-0.06a
 * Differential diagnosis for no power, with 20v and ~0.03-0.06 as measured on a USB-C amp meter.
 * Differential diagnosis for no power, with 20v and ~0.03-0.06 as measured on a USB-C amp meter.

• Device stuck in Recovery mode due to prior failed DFU revive. (Most common.)

• Short to ground on PP3v3_S5.

• Corrupt T2/BridgeOs firmware.

• Failed PMIC (Rare.)

• Shorted or Absent SSD voltages.

• Corroded Ocarina PMIC. (U9000)

➤Diagnostic Steps

1. Check if the device is stuck in DFU or Recovery mode.

- Plug the MacBook into another Mac or MacBook via its master port (Left side, bottom port, closest to the trackpad) and open Apple Configurator 2 to verify that it is or isn't in DFU mode. If the device is in DFU mode, Proceed to "Device stuck in DFU or recovery mode due to corrupt T2 firmware" in repair steps below.

2. Check for a short to ground on PP3v3_S5.

- If a short to ground is found, proceed to the "PP3v3_S5 short to ground" repair steps below.

3. Check resistance to ground on PP2v5_NAND_SSD_0. (Normal values can be as high as >1MΩ or as low as 60Ω. In the case of SSD/NAND failure, resistance will usually be between 1-3Ω.

4. Inspect U9000 for corrosion. The location of the chip on this board makes it prone to corrosion, either from liquid ingress or from condensation from dust collection. U9000 can be reballed if corroded which will likely resolve the issue if it is corroded.

➤Repair Steps

Device stuck in DFU mode due to corrupt T2 firmware:

- Revive or restore T2 firmware via Apple Configurator 2.

NOTE: You must be running the latest version of MacOs for this to work consistently. Check for MacOs updates prior to reviving/restoring T2 firmware.

How to revive T2 firmware: Plug the device you are working on to another Mac or MacBook via its master port. The master port on the A2251 is the bottom left side USB-C port (closest to the trackpad.) If you are confused, please see the diagram listed on Apple's how to page — https://support.apple.com/guide/apple-configurator-mac/revive-or-restore-an-intel-based-mac-apdebea5be51/mac

Once plugged in, open Apple Configurator 2. You should see a big square icon pop up that says "DFU" or rarely, "RECOVERY". Click the icon, Navigate to the top menu bar click "Actions" then "Advanced". Select Revive device. You will see a progress bar appear. This process can take anywhere from 2 minutes to over 30 minutes in some cases.

It is important to note, if the device is in Recovery mode, the end user may have brought the device to another repair shop or Apple first, who attempted a DFU revive which failed. You may have a secondary issue if the revive fails again.

Possible causes for a device to fail a DFU revive:

- Short to ground on PP2v5_NAND_SSD_0

- Absent U9000 voltages. (PP0v9_SSD_0, PPVCCQ_ANI_SSD0 (1.8v) )

- Failed Trackpad (Will almost always show the Apple logo before failing the process.)

- Failed NAND

- Failed T2 or T2 RAM.

- Cracked solder joints under T2 chip (Uncommon, usually only suspected if device was dropped prior to initial failure.)

If no secondary causes are found, a DFU Restore may resolve the issue.

SELECTING RESTORE WILL WIPE ALL USER DATA!

PP3v3_S5 shorted to ground.

- Locate and replace the shorted component.

PP3v3_S5 is a tricky line to inject voltage to on this board, as most components are small 0201 parts. Be sure to use a small gauge wire to inject voltage. There are many places to inject voltage to on this line, and unfortunately, none are ideal. R7481/R7485 is likely the best, as you could solder the wire onto both resistors allowing more current draw into the line.

1v at 5 amps is an appropriate voltage to inject into the line. Remove the heatsink prior to injecting voltage as occasionally, the CPU will be the cause of the short to ground.

With voltage being injected, perform thermal imaging of the board. If thermal imaging is not available, feel around the board to see where it is getting warm. Once the area is localized, add a small amount of isopropyl alcohol to the area to localize the shorted component.

Once the shorted component is localized, replace the shorted component.

For more info on finding short circuits, please see - [4]

U9000 visibly corroded or damaged.

- If corroded, reball the IC. Replacement is usually not necessary.

- If physically damaged or burned, replace the IC. The IC is not programmed and can be taken from any board which utilizes the same part. (338S00410)

If no short is found on PP3v3_S5, and the device is not in DFU or recovery mode, and all other findings are negative, you can try forcing a firmware revive.

• Attempt to restore BridgeOs firmware via Apple Configurator 2 by placing the device into DFU mode. Solder a wire across RE032 (Omitted/No Stuff) to pull SOC_FORCE_DFU high. Wire should be across pins 3 and 4 or 1 and 2.

- STOP! Before you begin, is your Mac on the LATEST VERSION of MacOs? If not, update your system before proceeding. Forcing a MacBook into DFU mode, and attempting to restore BridgeOs firmware on a old version of MacOs may result in a bricked device.

NOTE: Selecting restore will wipe all user data.

➣ Follow the instructions on this Apple support article on how to revive or restore T2/BridgeOs firmware, including on how to force a Intel based MacBook into DFU mode by using a key combination. https://support.apple.com/guide/apple-configurator-mac/revive-or-restore-an-intel-based-mac-apdebea5be51/mac

If the device fails the firmware revive or restore, the T2 chip or one of the NANDs is likely dead or is receiving unstable power from the PMIC (U7800) causing it to crash. If the device fails the firmware revive, you can try empirically replacing U7800.

Replacing U7800 should be seen as a last ditch effort, as it only works in a small amount of cases, and does not have a definitive diagnostic test to determine if it is bad. ➤Diagnostic Steps
 * No Power, 20v, Fluctuating~0.03-0.10a
 * Differential diagnosis for No Power, 20v, and Fluctuating~0.03-0.10a on the 820-01949 logic board:• This behavior is nearly always caused by a short to ground on a subrail. High Resolution Thermal imaging is the go to diagnostic method for this failure presentation.
 * Differential diagnosis for No Power, 20v, and Fluctuating~0.03-0.10a on the 820-01949 logic board:• This behavior is nearly always caused by a short to ground on a subrail. High Resolution Thermal imaging is the go to diagnostic method for this failure presentation.


 * 1) Perform High Resolution, High Sensitivity thermal imaging of the entire system board. Low resolution thermal imagers will likely not pick up the subtle temperature changes failures like this often present with. The failure will often present as a component quickly heating up and cooling down in sync with the amperage changes ; Occasionally, you will find a resistor heating up/cooling down in sync with the amperage changes. The short is likely caused by something on the output side of the resistor.

➤Repair Steps


 * 1) Remove and Replace the shorted component based on thermal imaging findings.
 * No Camera Detected
 * Differential diagnosis for camera detection issues on the 820-01949logic board:
 * Failed webcam on display assembly.
 * Failed webcam on display assembly.


 * Failed TCON board, resulting in PP5v_MAIN_ALSCAM being shorted to ground, which usually presents as a catastrophic failure of L8504. Usually, when this fault occurs, the TCON board is seen with a burned camera connector with occasional PCB layer damage.


 * Corrupt T2/BridgeOs firmware. This is rare.


 * Failed camera secure disable chip (U8502). This is very rare.


 * Corroded display connector/cable.

➤ Diagnostic Steps:

1. Check continuity on L8504.

''- Place your multimeter in continuity mode and measure across the component. L8504 is a filter, so it is essentially a wire. Your reading should be close to 0.01. You will often find this component visually exploded/burned with adjacent trace damage. If L8504 measures normally, proceed to the next steps. If L8504 is blown or burned, proceed to the "L8504 blown or visibly burned repair steps below."''

2. Check camera function with a KNOWN GOOD display.

''- You don't have a board issue until you know you don't have a parts issue. If L8504 measures normally, test with a known good display, or place the board in a known good housing. If the camera works normally with a different screen/enclosure, the display is the issue and will need to be replaced. BE SURE TO CHECK L8504 FIRST!''

➤Repair Steps

Camera works with a known good test display

• Replace the display assembly.

L8504 blown or visibly burned

Understand that this fault occurred because of a catastrophic short within the TCON board of the display. or from a corroded display cable/connector which melted and shorted the 5v camera line to ground. If you repair L8504 without addressing the display issue, L8504 will fail again.

• Generally, when L8504 blows, there will be corresponding pad and trace damage and jumper wires will usually be required.

- 36 or 37 AWG enamel coated copper wire is recommended for jumper wires in this application due to the power requirements of the webcam and ambient light sensor, which are both powered off this filter.

- The closest recommended jumper point for the input side of the filter (PP5v_G3S) is R5610, which the input side of the filter can be directly connected to.

- Using a conformal coating on the jumper wire is strongly recommended.

''- Use a 120 Ohm ferrite filter rated for 1.5 amps in a 402 package size only, preferably from a donor board. Do not substitute a resistor, wire or fuse.''

- After L8504 is replaced, and jumper wires are run as needed, replace the display assembly, or repair the display assembly by replacing corroded/burned cables and connectors as necessary.

No camera function, L8504 measures normally, and the camera still does not work when tested with another display.

• Attempt to restore BridgeOs firmware via Apple Configurator 2 by placing the device into DFU mode.

''- STOP! Before you begin, is your Mac on the LATEST VERSION of MacOs? If not, update your system before proceeding. Forcing a MacBook into DFU mode, and attempting to restore BridgeOs firmware on a old version of MacOs may result in a bricked device.''

➣ Follow the instructions on this Apple support article on how to revive or restore T2/BridgeOs firmware, including on how to force a Intel based MacBook into DFU mode by using a key combination. - https://support.apple.com/guide/apple-configurator-mac/revive-or-restore-an-intel-based-mac-apdebea5be51/mac

• If you still have no camera function after a T2/BridgeOs firmware revive/restore, and you have ruled out other causes listed above, the camera secure element (U8504) may be bad, or you may have a non-specific trace/resistor issue between the camera connector/U8504 or between U8504 and the T2 chip. A hardware issue with the T2 chip can also cause no camera detection issues. • Failed PMU
 * 20V on USB-C, 0.00-0.01A current draw
 * Differential Diagnosis for 20V on USB-C, 0.00-0.01A current draw• PMU (U7800 missing 12v input (PMU_VDD_HI from R8050 being blown or corroded.)
 * Differential Diagnosis for 20V on USB-C, 0.00-0.01A current draw• PMU (U7800 missing 12v input (PMU_VDD_HI from R8050 being blown or corroded.)

➤ Diagnostic Steps:
 * 1) Measure resistance on R8050. (Normal value should be around 877kΩ) Important! R8050 is part of a voltage divider circuit. Voltage will be lower on pin 2 of the resistor. This is normal.

➤Repair Steps

If R8050 is found to be blown, check for a short to ground on pin 2 of the resistor. If a short to ground is identified, replace U7800 and R8050.

If R8050 is found to be blown and there is no short to ground, replace R8050. . 20V on USB-C, 0.000mA static R7900 has 26 Ohm (should be 0 Ohm) and C7802 was corroded/shorted Thanks guys for this wiki!!!
 * Daniel:
 * Daniel:
 * Board 820-01949
 * }