Ok long one but here it goes. Some help would be great and I have warranty but wanna get it done right first time.
It's a 2006 F-350 6.0 Harley Davidson model....has 60k on it. Had it for month so far love it. When first got it noticed temp gauge would drop like truck off and the engine fan would engage and the ac would shut done. Looked at ECT sender and housing was cracked and no idea why. So I replaced sender and problem got a little better but not gone. Now when driving sometimes I hear fan kick on look down and see the gauge drop about 2-3 sec later like truck off. Takes 15-20 sec at least for gauge to get reading again. The ac will kick right back on and eventually clutch fan will disengage. But it is doing it so often the fan won't shut of for a while cause gauge heads to normal point then drops like truck off again. When cold it is much worse when truck is warm it was better until past couple days. U can see gauge come up and drop quick when truck cold. Some days it is. It bad some days it is. I do notice when everything working correctly then I stomp on it the ac will shut off fan turn on and gauge drop no matter what. Any ideas. Since sender is new I doubt new one is bad. Just seems like a connection somewhere is loose and any load makes the problem worse. So how does the sender control the fan and ac. It's like it goes I to a protect mode and shuts down anything that can create heat and makes fan spin to keep it cool due to no reading. All connections look good and clean. How can I chase and test electrical to find problem. I will have a dealer fix but want to tell them what it may be so they don't go replacing things and I keep going back till they replace right part. Thanks josh.......great site lots of info!!

 

check all your main connections to engine harness, on drivers side fenderwell kinda below the brake booster, think it's 3 of them. push on the connectors and see if any of them click, if one does then that was probably your problem

 

Could be the fan clutch, I had issues chased them all over!! Then changed fan clutch and it is gone. Fan clutch is has wires that may be damaged?? Check them to?

 

It sounds like his fan is working though if he can hear it come on, just sounds like it's being commanded to come on at the wrong time, due to the temp sensor dropping off. Maybe check the wiring going to the coolant temp sensor?