Engine fan kicking in too often when using A/C when ouside temp is above 75 F.

Hello All, thanks for your help in advance! I apologize for the long post, buts it needs proper explanation.

First off, I have a 2005 Western Star 4900EX with a Detroit 60 Series (vin N95287 for those who might ask for it).

Ok here's my problem... When driving and my A/C is off, the engine fan is working as supposed to. In other words rarely comes on. Now... when driving and USING my A/C with the outside temp below 75 degrees F all is working fine and engine fan rarely comes on. When the outside temp is above 75 degrees F the engine fan comes on too frequent. The engine fan turns on... runs its cycle and shuts off... 30 seconds later it turns on again... runs its cycle and turns off and 30 seconds later turns on again... and keeps doing that till I turn off the A/C or the outside temp drops below 75F. I have gone through a clutch fan and just replaced it because of the frequent on/offs and do not feel like replacing one again so soon.

What I have done to try and remedy this... I went to an A/C specialist and system was evacuated... vacuum held well... recharged 4 lbs. 4 oz... Trinary switch replaced (twice)... condenser is clean... cab and sleeper filters are new... no freeze up on evaps. I thought maybe it's the air ambient temp sensor... cause my datastar outside air temp is stuck at 69 degrees F, regardless of outside temp.
So I looked for an Air ambient temp sensor behind bumper, near head lights, in mirrors... nothing. Apparently there is no such sensor on my truck according to 3 WS dealers.... yet when I had my truck in for the top end adjustment at my Detroit guy, the tech ran diags on engine as well... on the print out it shows the faults, the settings and readings.... and shows that the "Air Ambient Temperature" is 69 degrees F. how could there be a reading if there is no sensor???
So stummed... please help.

Thanks!

 

There's no High/Low Presure switch on this system. It has a Trinary switch which incorporates the two switches into one... and I have replaced it twice.

 

In town or freeway.. at idle it just stays on.