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.

The twin radiator fans of our 1995 Volkswagen Cabrio were running constantly at low speed unless the electrical fitting was unplugged from the radiator cooling fan switch. The replacement part was easy to locate and purchase on TRUCKiD.com. The price was right and the part arrived within the promised time-frame. I used the need to drain the radiator as an opportunity to flush the cooling system and replace the coolant with a more current VW product (still had original, antiquated coolant). The replacement switch installed easily and correctly, and the twin radiator fans now operate as designed.