The original instrument is reading about 20 deg too much (compared to the laser thermometer). Another instrument showed tge same. I changed sender (the older screw on model rather than the 1967 introduced flat contact model).
No reading to start with. I then opened the nut and turned the cable end. This caused the reading to fall towards zero. I noticed that the cable is very thin with just a tiny amount of thin copper threads entering the clamp with the ring to sit on the sender.
I think that by cutting tge frail end off and redoing the clamp end I will get normal stable reading on both the old and new sender…
Make sense???
Can but try…
Ok, so with fresh metal at the end of the green cable, attached to the heat sender, I still get readings of 90-95 when it shoild be 75-80.
And the same on another water temp gauge…
Can it be a resistance/Ohm issue?
Can the gauge be adjusted?
As I know the true temp it doesn´t really matter, but itś annoying to look at the instrument when driving
And today I tried with a direct wire from the sender to the gauge, hoping that I would get a correct, and normal reading. But no, still the 100 deg on the gauge going to 110. The dig laser thermometer is at my summer house and I have measured it many times and it reads around 84.
What is going on here? 2 senders and 2vgauges tried, incl the direct wiring today…,
I suspect you have the wrong temperature transmitter. Smiths made many of these, often with different ohms resistances, I guess this was to suit various dash gauges. My official Smiths catalogue shows their number for the one that fits a Mark 10 3.8 liter, Mark 10 4.2 liter, or 420G as part TT 4201/00 or Jaguar part number C16895. Strangely, this is a different number thant the one for a Mark II, 3.8S or 420. The Smiths number is stamped on one of the flats of the hex part of the sender. These are very tiny and often hard to read but have a look and report back. And my constant plea to posters, please update your information to show your location on the globe and the year and model of the car you are dealing with.
I believe you have a bi-metallic gauge and as far as I can see they all have the same internals. Differing part numbers are probably just related to external differences such as chrome or black trim rings.
I have a Smiths Tester SR/D380 which applies test resistances in place of the sender so you can check the meter deflection.
For the Temperature gauge, a resistance of 66.4 ohms should give an indication of 70C. A resistance of 20.9 ohms should give a deflection to 110C.
It is possible to adjust the meter zero and deflection via 2 small ports in the back of the gauge but I don’t think this is a good way to go since the internal pivots are very tricky to adjust.
I suggest a series resistor in the sensor line to get the reading you want.
Wow, now we are getting somewhere…! I checked the Spare Parts List, and old and new sender, and they are all the same, as you can see in the pictures.![image|375x500]
(upload://qbnhKwfLkjmtCBtQyVWIA1ytyXN.jpeg) ![image|375x500]
(upload://iutOL1As8IBSKCu4ZkOBIkTMADt.jpeg) ![image|666x500]
(upload://A7PKfPH1CVRQhjCZ9WwdeYGQFR2.jpeg)
The suggestion to get a series resistor somewhere between the sender and fayge seems to be the solution.
Where do I find that? Abd to 66,4 Ohm…!?
How has this happened? Original sender and gauge. 1966 Mk10 4,2
I’m the 3rd owner. Found it in CT, USA.
I live in Sweden. None of my other 4 60s saloons, or 4 E-types suffer from this anomaly.
Peder
![image|640x480]
(upload://u9UuQB2uTXKp7V8gfnOaQeow4gX.jpeg) 
I would start by trying a few resistors around 10 ohm resistor say 1/8 watt rating.
You probably have to get them from an electronic parts store.
The 66.4 ohms value is what the sender looks like to the gauge at 70C
I bought one 75 Ohm resistor for AC and one for DC.
Which should I use?
No W specified. Might as well just try and see what happens to the gauge reading.
Will put it just behind the gauge spade contact, right?
I don’t know what you mean by an AC and a DC resistor. A resistor is a resistor.
Try a 10 ohm resistor at the sensor terminal lug.
Bruce
Alternating and Direct current.
He didnt have 10 Ohm resistors.
What will 65 do? Bring down the gauge to 30 instead of 75…!?!
Is there no other way to correct the reading (not counting fiddling inside the gauge)?
Tin foil around the spadd contact…?
Fraction of the copper cable attached to the sender…?
Someone linked an Ebay ad for a small resistor, which I bought a while back. Tonight I installed it, just adjacent to the gauge. I now have a steady 70deg reading, going up a few deg when parking and engine running for a few mins.
Very thankful for the advice and solution. (But why did the original setup work and later in life change the reading to 95-100 instead of the correct 70?)