Series 3 Temp gauge reading high

Series 3, 1974 with a 3.5 Rover V8. Changed the temp sensor as the connector had snapped off (garage did it when messing with the SU's) so bought this, which is compatible with the 1971 P6B engine it on.
http://www.ebay.co.uk/itm/191598899367
Now reads just under the red and creeps into the red but the temp when just under the red is 75-82 after taking readings on the rad (top and fins), thermostat housing, stainless connector pipe, hoses etc. On the TVR the fans kick in about 90 so 80ish isn't hot. Do I add a resistor or get another sensor. I assume the one I have is a different rating to the one that was on originally? The original had a yellow plastic casing below the connector so this must mean something. Lots of Cambiare on ebay but all different part numbers?
FFG

You just need the right one,but why not get an after market gauge/sender cappilary type that reads in numbers and know what the temp realy is.
Well worth doing.

Checked the resistance readings yesterday and when cold (12.5c) the reading is 1350 ohms, at 20C it reads 370, at 30C 13-127 and temperatures above that the readings fluctuate wildy, changing every 1/2 second from -117 (yes minus) to 32 at 58C for example. Is this right or is my "new" sensor faulty?
FFG

Your sender needs to match the gauge to read correctly , what are the threads where it fits the engine?
Table of some senders resistances below
Temperature 13 15 18 56 100C
series sender 460 385 323 118 56
300tdi 770 587 511 150 98
200Tdi 4100 2800 2000 600 300
Attached some part numbers and other info

Thanks. I believe the thread is 5/8" and uses a 19mm spanner to get it in/out. The part number for the P6B (which is the 3500 V8) is 560855 which the Intermotor part I bought is. The old sensor worked fine but the garage snapped the end off but I cant recall what they did with old one. All I recall is that it had yellow plastic on the end but the only ones I can see now are red or black. Its feeding the original S3 gauge as it was originally a 2.25 petrol.

Is it an ex auto engine or a manual engine as I think the two types had different senders think mine was an orange one as it was an auto yours could have faded looking yellow

Auto I believe. Engine number starts with 425 and has a "c" on the end.

Drove the truck yesterday and noticed the temp gauge went down into the black but the fuel gauge also dropped from 3/4 to 1/2 so they must be linked somehow. Is there something on the back of the gauge that could be my problem? When it went back into the red the fuel gauge also went up.
Just looked at a wiring diagram and see there is a voltage stabiliser that feeds both gauges. Where is it located and where can I get a replacement?

Poor connections at the fuse in the box under the colume can resuly in fluctuating gauges.

Would it be the wire feeding the voltage stabiliser as its affecting both gauges? Will check earths too.

Voltage regular is mounted on the bulkhead to the right of the clutch master cylinder.

Thought the voltage stabiliser was on the back of the speedo, part number 148876. I can see it, just trying to work out how much I have to remove to get access to it. It has a darker green and a lighter green cable attached to it but only those 2. I think the darker green comes from the fuse box and the lighter green feeds the temperature and fuel gauge.

That is correct .
You can buy electronic versions of the voltage regulator on ebay best option in my opinion.

I think the mention of bulkhead mounted voltage stabilisers perhaps was thinking about the regulator box for dynamo charging systems on older Land Rovers. The voltage stabiliser for the instruments is a different device, and is generally mounted directly on the back of the instruments.

It's worth checking the voltage stabiliser is well earthed, as without an earth, the winding in the stabiliser will never pass current, get warm, and open the points within, so, the instruments will be fed full battery voltage, and will tend to over-read.

With a meter like an AVO, you should be able to see the output voltage from the stabiliser to the instruments flicking/pulsing as the points within open and close - as the dashboard instruments can't react quickly, this gives an acceptable approximation to a constant 10 volts.

There is a subtlety in the operation of this circuit which is lost if you fit an electronic regulator. The instruments are thermal devices - relying on the heating effect of the current flowing through the instrument to bend a bi-metal strip, and thus to deflect the needle(s). Being thermal devices, the gauges are also sensitive to ambient temperature. However, the voltage regulator is also a bi-metal device, and its output is sensitive to temperature in the opposite direction. So, if the ambient temperature behind the instrument panel increases, the instruments would tend to over-read, but the voltage regulator would compensate by reducing its output, and partially cancelling the error - this error cancellation is lost if an electronic stabiliser is fitted.

http://www.richer-engineering.com/Series/gauge_tutorial.php
It would be interesting to know the range of the temperature of the bimetal strip to compare to the range in ambient temperature.
My gauges do not seem to be affected by the ambient temp.

I've cleaned up an earth I found by the steering column inside the engine with 2 black cable which feed back into the cabin so this may help.