This is not for all but for those where using refrigerators because sound it follows the scania engine if engine is off unit is off.
Still when Trucks is alone unit works like with trailer.
Now the unit not start immediately with truck engine it has low rpm, high rpm and again low rpm afterwards it stops like an automatic unit and start again after a period when it loses temperature.
But still when tractor engine is off unit is off too.
Credits:
Volidas