It's been on my truck an functional for several years. Suddenly, high speed fans aren't kicking on. And when the low speed fans run, I have one of my relays clicking non stop. I was told to check my grounds, they're all tight. I cleaned any that looked even slightly suspect. Same condition. Not a real concern right now as it's winter, but once the weather gets warmer I am certain that I'll have overheating issues. Next step? I'm seriously about to install an 05+ OEM radiator, fans, and harness.

 

Have you tried relays just to be sure?
Have you inspected the plugs for corrosion or melted sockets?

 

Not yet, that will be my next group of things to look over more thoroughly.

 

If the relay is chattering, check the signal circuit (pins85/86). If the +12V and ground on those pins are good, the windings in the relay could be going bad.