Cooling fan woes

Greetings, all.

I have been having cooling/cooling fan issues with my car for some time now. It's a 1997 3.8l

I changed head gaskets a month ago, becuase there was air constantly in the cooling system. So far, this repair was successful. During the repair however, we changed the thermostat and broke the engine temperature sensor. New sensor in and new thermostat, and we had a problem wiht the engine not warming up (It never went further than O in normal, unless you gave it a good thrashing on the autobahn at 100+mph.) We swapped the thermostat, and the temperature then stayed between R and M, which I found a bit on the warm end. Also, during stop and go the engine temperatures would rise (I never allowed them past M) and we had to shut off the car. I checked under the hood and realized the cooling fan was not cutting in. It cuts in for a few seconds after start-up, and then refuses to start, unless you turn on the AC or unplug the temperature sender. I've now swapped the thermostat with a genuine ford item, and changed the temperature sender, which I assumed to be faulty, but so far, nothing changed. What could be a reason for this behaviour?

Also - we have been adhering to the filling procedures for the cooling system and the temperature gauge does not bounce, sway or suddenly read low like it did last time when there was air in the system.

Never had these sort of troubles with my other cars, grumble.

Conor

I have now aquired a working OBD scanner, and I've checked the temperature, when I unplug the sensor it goes down to minus -40(fan comes on) and when I jumper it it goes up to 338° and the fan comes on, so that seemingly works.

I've watched the temperature while letting the car gradually warm up, according the the OBD sensor 190 is pretty much where the gauge is in the middle of R, and at 208° (pretty much at the end of M) the fan actually does cut in, which corresponds with the information i found that there's a 10°C gap between the thermostat opening and the fan cutting in.

however, I went and measured the little plinth into which the temperature sender is screwed with an infrared thermometer(five minutes after I had turned off the car for the fan test) and it still measured 213°, roughly 10°f more than the sender told the OBD scanner, which, at the same time, read at 200.

From how it seems, my sender is under-reading.

Or is this the norm?