When should the Aux fan come on? (83 300SD)

[83w126]

It was very hot here the last couple days, and I noticed after driving on the freeway that while idling the temperature would climb up to the 3/4 mark on the gauge and not go back down (I have been driving this car for 2.5 years and it has never gotten that hot). The aux fan didnt come on at all, so I think that it may not be working. I am a bit concerned because I need to be able to spend 30+ minutes in 5 mph traffic at 110 degrees without it overheating and I'm not sure the cooling system is up to that right now. Is it maybe time for a new radiator?