re: Car overheating while idle.

I once had to chase down the cause of that problem, and many frustrating hours later, discovered that electric fan motors sometimes don't break completely.

When I tested the fan with a wire from the battery, it ran just fine. However, straight from the battery provided a higher voltage, so the motor worked, but when the power was routed through the relay, it was a much lower voltage, which wasn't enough power to get the sick but not dead fan motor working!

I think I ended up replacing every component in that circuit before I figured it out!