How to fix Macbook Air 11″ A1465 no Wi-Fi issue?

Background:

MacBook Air 11″ fully functional except no Wi-Fi. Apple quoted $700 for replacing the whole logic board. Macbook model: A1466 Year: 2015 Logic Board number: 820-00164.

Diagnosis:

Remove the old Wi-Fi card, replace with our known good one. Boot into OS, and there is still no Wi-fi hardware found. It confirms that the fault is on the logic board.

Analysis:

The wi-fi card must co-operate with the logic board with these power rails and control signals:

• logic board provides power rails pp3v3_s4 and pp3v3_wlan
• logic board sends AP_RESET_CONN_L to initialize wi-fi card
• wi-fi card responds with AP_CLKREQ_Q_L to the logic board requesting a system clock
• logic board sends system clock PCIE_CLK100M_AP on request
• wi-fi cards uses a pair of PCIe bus PCIE_AP_D2R and PCIE_AP_R2D to communicate with the logic baord

Repair:

• Check power rail pp3v3_s4, 3.3 V, ok.
• Check power rail pp3v3_wlan, missing. It comes from u3550 WLAN switch chip.
• Check u3550 power supply pp3v3_s5, 3.3V, ok.
• Check u3550 enable signal smc_wifi_pwr_en, present.
• u3550 has been provided all the sufficient and necessary working conditions but still no output. So the u3550 chip must be not functional.
• Replace u3550 and problem solved, easy fix.

Note:

u3550 is the chip number on Apple logic board electronic circuit diagrams (schematic 820-00164).