Temperature gauge question

Hi there

My temperature gauge is playing up. It starts increasing temperature and then drops to zero.
There is an additional component next to the gauge (blue cylinder in the photo) which seems to be the problem. If I remove it the gauge seems to work OK, it might be reading a bit low.

There are no markings on the blue cylinder, it has low resistance when checked at room temp. Any ideas what it is and what it does, or if I need it?

Car is 1966 MK2, 3.4 auto LHD


I can’t say I know what that blue thing is but it might be a resistance used with a wrong scale sender. I normally wouldn’t expect the sender to have that kind of connection. If the sender is wrong I hope at least they matched the correct thread into the manifold (water rail).

I suspect Tucurious is right. My 1965 3.8S has no blue resistor at that location. The correct Smiths temperature transmitter for your car is a TT4801/00 (Jaguar number C15472) The TT number is usually stamped in very tiny print on one of the flats of the transmitter. There were a myriad of transmitters made for various cars and each one was specific to the vehicle’s dash gauge. You need to find the correct one for your car and this will likely correct the issue.

Someone put a blue item just like yours on my XJ6.

I dont know much about electrics , but the temp gauge is 10v , if the regulator stopped working , maybe someone replaced it with the blue one , check the wiring at the back of the gauge , see if it comes out the loom !
Is the wire going into the blue part Green and black , as it should be !

Hi Gents

Thanks for your comments. I dont think it is the 10v regulator as when it fails that knocks out the fuel gauge as well. The blue thing has been there since I got the car over 20 years ago, and things have always worked. I was thinking it was a resistor to “match” sender to temperature gauge, but it has ~zero resistance, so that does not add up. Added to this on a short run yesterday with the blue thing removed, the temperature seems to be correct. I will try a longer run - between the torrential thunder storms - and let you know if anything changes.

I do have a fuse contact problem, which is intermitently knocking out fuel, oil, temperature and indicators - might be time to change that old broken fuse holder as well.


That blue thing was installed in the factory, it was on a wiring loom I put on my engine and when put in place (on the old sensor that previously worked perfectly) it corrected the high reading of the new loom to the old, good reading. You don’t need it if the gauge reads okay without it.
It’s a resistor. Maybe it has a bad contact inside. Or a fuse…

Hi all

So long run today and the temperature gauge was stable and looks pretty close, maybe a little low?

I dug into the blue thing (literally) and it’s just a 5 ohm resistor with a broken leg which was obviously making an intermittent contact.
I wonder where I might find another one, any ideas? Probably this is beyond my soldering skills. Davidxjs was spot on.


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It’s a 4.7 ohm 5% (color code yellow violet gold gold) 1/4 watt standard (carbon) resistor. Any mail order vendor should have it (eg Jameco, Digikey). You can use crimp terminals and skip soldering if you want. Same vendors sell the male and female crimp spade connectors.

If it’s within reason without it leave it off. If it was far off I‘d shrink wrap the new one. The blue tube looks out of place.

Soldering is easy! Clean everything, solder must be on the tip of the hot iron, always use flux, and the part (here the spade)has to melt the solder. The wires etc. should be pretinned.

Similar intermittent gauge readings from nil to normal to ‘pegging’ on our car. The sender had been (a’hem) ‘modified’ (botched) at some point - replaced with original type (10 minutes) and all good. - If required part number is Lucas-SNB105.