Fan control is a no-op on Intel T2 Macs (2018+): uses FS! instead of per-fan F0Md key

[leitaoodavid]

Summary

On Intel Macs with the T2 chip (2018 and later), manual fan control silently does nothing. The app reports success and the UI shows a manual target, but the SMC ignores the write and the fan stays on automatic. Reproduced on a MacBook Pro 13-inch 2018 (A1989), macOS 15.7.

Cause

In HelperService, the Intel (#else, non-arm64) path forces manual mode via writeForceMode, which writes the legacy FS! force bitmask:

#else
try smc.writeForceMode(fanIndex: fanIndex, forced: true)
#endif

On T2 Macs the FS! key is absent / ignored (see hholtmann/smcFanControl#86), so the fan never enters forced mode and the following F{i}Tg target write is ignored by the firmware. The write returns success but nothing persists.

T2 Macs use the same mechanism as Apple Silicon: the per-fan mode key F{i}Md = 1. The code already implements that (writeFanModeKey), but it only compiles into the arch(arm64) path, so Intel T2 machines never use it.

Verification

Writing F0Md = 1 then F0Tg = 3000 (float32) on the affected machine works, both directly via SMC and through the XPC helper:

[before]   Fan0  Md=auto(0)    Tg=0     Actual=auto
[after 7s] Fan0  Md=manual(1)  Tg=3000  Actual=2936

The fan physically spins up to the target. With the old FS! path, F0Tg/F0Md never change.

Suggested fix

On the Intel path, use the per-fan mode key (works on T2) and keep FS! as a best-effort fallback for pre-T2 Intel Macs. Same change applies to both branches of setFanMode:

#else
// T2 Macs (2018+) use the per-fan mode key, like Apple Silicon; older Intel Macs use FS!.
try? smc.writeForceMode(fanIndex: fanIndex, forced: true)  // pre-T2 best-effort
try smc.writeFanModeKey(index: fanIndex, forced: true)     // T2 primary
#endif

Environment

• MacBook Pro 13-inch 2018 (A1989), Intel, T2 chip
• macOS 15.7.7
• ChillMac 1.7.0 (built from source with Xcode 16.4, macOS 15.5 SDK)