When the truck is idling at normal operating temperature the needle stays in the middle like it should. But when I turn the fan switch on the needle drops down to 0, while the engine continues to run normal temps. I've shorted out the sending unit wire and this caused the needle to rise then I turn the switch on and the needle stays in same position.

I've replaced the sending unit and fixed the wire connector but the needle still drops to 0?

I'm not sure where else to look right now? Where are the grounds located maybe I missed one?
Thanks

 

anybody figure this out yet mine is doing the same thing