MacBook Air M1 (820-02016) Stuck at 5V - Power Cycle Loop 0.04A to 0.12A - Low resistance on PP1V8_S2

[Ictc]

1. Sensing Circuit Repair (Completed): I found clear evidence of degradation in the voltage sensing section. I have already replaced RD538 and RD539 (PPBUS Voltage Sensor) because they were completely out of spec:

• RD538: measured 195kΩ (target 78.7kΩ) replaced with 82kΩ
• RD539: measured 59kΩ (target 31.6kΩ) replaced with 33kΩ.

2. The Root Cause - PP1V8_S2 Short: Continuing the analysis, I identified the reason why the PMU is not releasing the enables: there is a 4Ω short to ground on PP1V8_S2.

• This rail is critical for the early boot stage. The PMU detects the overcurrent and resets the sequence immediately, which also explains why PPBUS is cycling between 8V and 12V.
• Current Status: Injected 1V into the rail, but with 4Ω the draw is only 0.25A. It’s a "cold short" (0.25W), making it difficult to spot via thermal cam. What do you suggest in order to isolate the failing component?
• Should I remove UD550 and UD560 (INA190) to see if the 4Ω short on the 1.8V line clears, as they are directly powered by this rail and are in the area where I found the degraded resistors? Do you find this apropriate?
• Should I investigate the PMU (U7700) or potential decoupling caps around the SoC?

 

[2informaticos]

RD538/9 can only be properly tested out of board.
Onboard measurements over high value resistors are influenced by other components.
I personally keep the small components soldered with one pad; being easy to check without losing them.

You can inject 2V into 1V8 rail without problem; 0.5W hot spot can be detected with a good thermal camera.

 

[Ictc]

The resistor value was measured out of circuit, which is why I decided to replace it anyway. I already used freeze spray on the board before injecting 1.8 V and checked it with my FLIR One, but I wasn’t able to find any hotspot so far. Half a watt on a 0201 component should probably show some sign of heat, but I’m afraid it might not be enough to really spot it. I'll definitely give it a try anyway and let you know. Thanks for the prompt reply.