UK Car Repair: 93 jag xjs temp gauge, line resistor, resistor type

Question
QUESTION: Hi William. Thank you for your help. I have a 93 xjs. 135K miles. Straight six, 4.0 liter manual tranny. The temp gauge now reads hot all the time after 3 minutes of starting the engine. Jag service says the sender is good, the temp is normal, actually lower, 160 degrees after warm up and should be 180 degrees. They are suggesting an in-line resistor? Your thoughts? Size of resistor, type, look at other issues? Thanks again. Paul

ANSWER: Either the gauge is faulty or the car is running hot, from what you say it is the gauge. I'm presuming you've filled the system properly. Did you check the temperature at the bottom of the radiator?

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QUESTION: Jag service has said that the gauge is good and the temp of the car is normal. Thanks again. Paul

ANSWER: I did not answer fully before, I said "the gauge or it is running hot" I left out the sender unit! I know you said you changed it. The fact is, something is amiss, it is warming up too fast or there is a problem with the sender or the gauge! This answer is reminiscent of the 70's Rolls and Jag solution to the gauge reading hot in California weather. The official answer was to fit a 10 ohm resistor in the sender line.  It is as ridicoulous now just as it was then.  Did you check the temp's or they did?

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QUESTION: Hi William,  The Jag service checked the temp and the sender and the gauge. How would i check the gauge, sender and temp? How do I measure the temp of the coolant? Where is the best place to check it? You mention the bottom of the radiator. How? With what instruments? What would be readings across the contacts on the back of the gauge? Thanks again.

Answer
I think this is an earlier Q. The only way to check the temp is with a pyrometer or a thermometer,  when the engine gauge gets warm -very fast you said- check the temp at the top hose.  Now, let the engine fully warm up and try it again both at the top hose and the bottom.  Switch the engine off for safety when you do it. Let me know the results.