How to diagnose integrated AAC/Radiator cooling fan assembly
 

On my 2007 GL450 X164, the radiator cooling fan works intermittently, and sometimes none at all. This causes the engine to overheat.
When I disconnected the fan assembly while the engine is running, then turn off the engine/ignition and reconnected the fan, it works at full high speed then retracted and shut off. However, at the time it came on, if I switched the engine/ignition and a/c on, the fan works perfectly if the engine and a/c remain on. It do not repeat if I turn off the engine and restart.
Whenever the cooling fan does not come on the a/c does not blow cold air. There are no fault codes shown when I ran a diagnostic. Can anyone help me to determine the cause of this problem? Has anyone experience and resolved similar issues?
During my research I came across similar issues caused by the a/c pressure sensor, fan module and, ECM, and coolant temperature sensor. The fan assembly is brand new - I changed it because I thought the original assembly was bad. I was wrong.
The vehicle is not throwing a fault code. I am confused and need help ASAP.

Thanks
Lanny