Semi Truck Engine Cooling Fan Switches

I installed 2 of these units in my RV last year for a dual fan setup. I used wire larger than on unit and soldered all connections. Units worked fine for many cycles one afternoon was driving home from vacation and stopped for fuel. When started vehicle i notice engine temp climbing rapidly. Turned off engine and had one fried fuse totally melted? Fortunately I had a spare relay and fuse onboard I changed out and Walla it work perfectly and have not had any more issues? If anybody has an answer it would be much appreciated.