volvo s60 2.4 auto

i towed the caravan with my volvo s60 the other day and its the first time ive towed with it, i noticed that when i was climbing hills there was a warm smell it didnt smell of anything like water or oil so i'm wondering if i may need to add a transmission fluid cooler have any of you had experience fitting one? i know quite a bit about cars but auto boxes are alien to me.