Worked on the Ex again this weekend to attempt to fix the 'non-working' Temp gauge. Having experience with this sort of thing, I pulled the wire and grounded the lead and the gauge maxed out as it should.
I checked ohm's on the temp sender and I was not getting anything on the meter. So I picked up a new one and installed it...nothing...not even any resistance on the single pin output for the gauge.
I re-installed the old unit and checked resistance on the single pin (used by connector) again, still nothing. So, I swapped over to the other un-used pin, and got roughly 2.3 milliohms on the meter. That seemed very odd to say the least since nothing (no factory wire) connected to that pin. I checked resistance between the pins and got nothing as well. So I assumed the sender I got was bad and returned it for another - different brand.
Got home, installed it.....The same thing. No reading...except, i did notice the gauge needle moved 'colder' (below cold) and did not move to indicate any heat in the block. I turned the heater on to ensure the water was heating...Yes, there was heat, but no movement on gauge needle. I re verified the gauge again, by grounding it...still functions as it should.
(see fix below)
I know there is a 2nd temp sending unit for the PCM (??) on the left. (see picture below) It has 2 wires on it. So, I thought maybe the temp sending unit issue might be with it. I swapped the new sender into it and made the connection....still nothing.
Notice the connector differences on the senders - My question is, are these 2 senders different in functionality because they feed 2 different sources and/or do these work in conjunction to make the gauge function properly....I'm stumped...Im assuming the wire to the gauge has to be good since it responded to the ground I applied.
(Now, I did not pick up a 2nd sender to ensure 2 new units were installed. Since I did not get any changes, I held off to see what you guys thought) The new 'Generic' sender I got from Oreilly's did not have the 'tang' or slot on the top side of the connector, so it 'interchanged' between the two connection points easily.
****This factory one below goes to the left (PCM) has the slot in the middle (left side of connector)
This is a close up of the sender the gauge connects to - Notice the slot is located off center to the right.
Fix: I had this same problem on my 1999 Mercury Mountaineer. Same engine and electronics to dash. Both had bad thermostats. There is a picture of the bad thermostat in the 1997. Once I got the engine warmed up again after all the repairs, the temp gauge started working as it should.
) thinking of each aspect of the system to ensure I didn't overlook anything...Im telling you, i checked everything next to pulling the box and cutting it open!....Then it hit me - the only thing it HAS TO BE is maybe a rodent built a nest in the box (remember I pulled a ton of acorns from the fan area when I got it)...there wasn't anything flying out of the vents...but hey, what the heck....its got to be the only reason why air was not moving in the box. 
