Overheating ? Or suspected faulty temperature gague

Hi All
I have an ex military 90 300tdi Auto ( engine from disco).
I had to replace the fan motor for the heater which is all in and working, however my temperature gauge now jumps to hot why would it do that?
I'm pretty sure I've bled all the air out 90 drives ok.
Investigating further I was checking the sender with a multimeter and all seems ok there, moving to the temperature gague during removal I removed the earth wire from the rear and the gague appears to function properly it still climbs to the red but should I trust it, as far as I can tell 90 is not overheating.
I'm a bit baffled really has anyone experienced similar and how best to correct the fault your help would be most welcome.

Hi, There are some threads on this on here somewhere. I'm working from memory, I think some people had problems matching up the senders from Disco's engines to Defender gauges. I know senders can be temperamental. I went through three Paddock senders in less that three years. I ended up getting one from Bearmarch which I have now had a couple of years.

Hi Mikesuba
Yes I have read this and other forums my gague was working ok before I replaced the blower motor.
I was considering draining the system and refilling it again.

If your gauge was working before and you have done nothing to it,then it is very likely you still have an air lock.Driving any distance in this condition can result in damage of some sort.[ have a motor in my junk pile where number one piston has just melted because of this,he did drive it a long way before it gave up] Bleed again with radiator plug open,fill here not at the expansion tank, then heater on and run until thermostat opens and rad is warm,heater is warm and no air coming out of radiator.