MacBook Air A2179 Not turning on, either 5V or 20V on USBC but with low current draw repair

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MacBook Air A2179 Not turning on, either 5V or 20V on USBC but with low current draw repair
Device MacBook Air A2179
Affects part(s) Motherboard
Needs equipment multimeter, soldering iron, soldering station, thermal camera
Difficulty ◉◉◉◌ Hard
Type Soldering


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Problem description

When connected to the USB-C meter, the 820-01958 board draws very little current at either 5V or 20V. Not turning on and SLPS2R and AWAKE voltages/signals are cycling.

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Symptoms

  • MacBook A2179 not turning on.
  • Little to no current draw as measured with a USBC ammeter.
  • when measured, SLPS2R and AWAKE voltages/signals are cycling.

Solution

Diagnostic Steps

Check voltage and resistance to ground on PP5v_G3S

  • When measured while the board is off, the resistance to ground on PP5v_G3S is typically large, well within the kilo-ohms.
    • If you measure less than 100 ohms, then you most likely have a short on this rail. See repair steps below.
  • When the voltage is measured while the board is on, you should expect 5v.
    • Values less than 4.9v should be considered abnormal. If that's the case, see the relevant repair steps below

Check voltage and resistance to ground on PP3v3_G3S

  • When measured while the board is off, the resistance to ground on PP3v3_G3S is typically large, well within the kilo-ohms.
    • If you measure less than 100 ohms, then you most likely have a short on this rail. See repair steps below.
  • When the voltage is measured while the board is on, you should expect 3.3v.
    • Values less than 3.2v should be considered abnormal. If that's the case, see the relevant repair steps below

Check voltage and resistance to ground on PP1v8_G3S

  • When measured while the board is off, the resistance to ground on PP1v8_G3S is typically large, well within the kilo-ohms.
    • If you measure less than 100 ohms, then you most likely have a short on this rail. See repair steps below.
  • When the voltage is measured while the board is on, you should expect 1.8v.
    • Values less than 1.7v should be considered abnormal. If that's the case, see the relevant repair steps below

Check voltage and resistance to ground on PP3v_G3H

  • When measured while the board is off, the resistance to ground on PP3v_G3S is typically large, well within the kilo-ohms.
    • If you measure less than 100 ohms, then you most likely have a short on this rail. See repair steps below.
  • When the voltage is measured while the board is on, you should expect 3v.
    • Values less than 2.9v should be considered abnormal. If that's the case, see the relevant repair steps below

Repair Steps

Short circuit on one of the rails or less than 100 ohms measured

How to find short circuits

  • Inject 1V 5A to the shorted rail.
    • With either a thermal camera or your hand feel for the area that is heating up.
    • Identify and replace the shorted component.

No short is measured but voltage is still missing

Replace U7800.

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