UK Car Repair: 93 XJS faulty temp gauge, line resistor, alligator clip

Question
Hi William,
I did some testing today and this is what I found. The back of the panel is a maze of circuitry like a circuit board so I traced the ground of the adjacent lights. When the car is running, the 'ground' stud on the gauge reads .1 volts, and .35v with the lights on. The other studs read 14v, 8.1v and 6.3v. So I assumed that the .1 volt stud (that also is in circuit with the light ground) is definitely the ground. I then shut the engine off, then attached an alligator clip to that ground stud and grounded to an exposed metal ground which I had tested earlier for ground and there was no change on the gauge when I started the car again. As I said earlier when I start the car fom cold the gauge reads cold and then after 4 to 5 minutes the gauge goes to the 'H.' I had previously taken the car to Jag service and they said the gauge was good and the sender was good. The service advisor had heard of this problem years ago and said someone had tried an in line resistor between the sender and the gauge.  I went to Radio shack and bought various resistors and I found that a 33 ohm 1/2 watt 5% resistor in line between the sender and the gauge lowers the reading to 'N' after the 4-5 minutes of warm up of the car.  My final question  is this: Does the gauge now read normal and is it reliable to indicate a hot engine if that occurs? Thanks for your help. I know this is lengthy. I really appreciate your help. Paul

Answer
I thought I answered this! Once again, something is wrong, but as long as the readings are now correct it doesn't matter.  Perhaps you ought to try getting the car overly hot ( disconnect fan or block radiator etc) and monitor with the pyrometer to see if the gauge reacts in cancert with the increase in temp.