Missing PM_SLP_S4_L

This is an advanced repair guide for technicians dealing with MacBooks that fail to progress past the S4 sleep state during power-up.

Initial Troubleshooting

When diagnosing a non-functioning board traced to PM_SLP_S4_L, start by verifying PP5V_S5 and PP3V3_S5 presence. If both exist, test PP5V_S4 and PP3V3_S4 for shorts. If no shorts appear, proceed with microscopic board examination.

Common Corrosion Points

MacBook Air models (2013-2017) frequently exhibit corrosion near these chips:

• U1900 — Generates the 32kHz clock signals essential for processor and chipset operation. Any corrosion requires replacement.
• U1950 — Outputs PM_PCH_PWROK. Corroded PP3V42 pins may require jumper wire bypasses and chip replacement.
• U6100 — BIOS chip. Rarely sustains direct liquid damage, but surrounding SPI termination resistors and traces frequently corrode, requiring careful diode-mode testing.

Power Sequence Overview

The signal sequence progresses through RTC rails, S5 power states, suspend rails, and sleep states before reaching SLP_S4#. Boards may power-cycle down to these sleep states when encountering upstream problems.

Diagnostic Notes

Standard multimeter speed may miss brief signal spikes during the power sequence. Oscilloscope verification provides superior SLP_S4# pulse observation alongside VCore activity and ALL_SYS_PWRGD signals.