I need help troubleshotning defrost on SL 200 TG-VI

Startning At setp -20 C and pulling down,
after ca 90 min starts first defrost "demand".
When defrost exited the unit runin in high speed cool
aprx 20 min then defrost start again.... a

What`s trigger the defrost mode beside the 2 timers

dFI and dFn

Help me with the logic on this

Best regards Bessershmit