Water temperature says cold, heat blows warm air

[Ryansmiller]

I have a 1998 Ford Explorer 4.0L sohc. The water temperature gauge almost always read right on the C and will read operating temperatures almost at random. I do have hot air blowing and the thermostat was replaced today and it did not fix the problem, any ideas?

 

[drdoom]

Check the sensors with a meter. And of course the connections too.

 

[SoNic67]

You have two sensors - one for the dial (gauge) in the dash, another one, more precise, for the computer (PCM). You can read the second one with an OBDII reader.

The sensor/sender one for the dash gauge sometimes has just one pin and in that case requires a good ground trough the threads and engine block.
On a 4.0L that type of grounding poses a problem with the one pin because the plastic case that is installed in, so they have a 2 pin for that application.