I normally use heartbeat on my Pi, set with:-
$this->bbcode_second_pass_code('', 'dtparam=act_led_trigger=heartbeat')
I tried this with latest Arch on my Pi3, but it does not work.
$this->bbcode_second_pass_code('', 'cat /sys/class/leds/led0/trigger
none kbd-scrolllock kbd-numlock kbd-capslock kbd-kanalock kbd-shiftlock kbd-altgrlock kbd-ctrllock kbd-altlock kbd-shiftllock kbd-shiftrlock kbd-ctrlllock kbd-ctrlrlock usb-gadget usb-host mmc0 mmc1 timer oneshot [heartbeat] backlight gpio cpu0 cpu1 cpu2 cpu3 default-on transient flash torch input rfkill0
')
seems to show that is has been set