Hi All, well after a week of tidying up the engine bay (painting, polishing etc) on my 74 series 2 I have just taken it for a drive and the temperature gauge has stopped working, it was fine before I started cleaning and polishing. I’m pretty sure that I will have dislodged a cable somewhere during my over zealous cleaning. When I got the car there were 4 cables under the bonnet that were not connected to anything at all and I assumed that the cables had been re-routed at some point in history and despite my best efforts I can’t track down the cable that goes to the gauge from the engine. Could someone please advise where on the engine the gauge operates from so that I can track it from the source?
Hi Robert, it should be a green and blue wire and it runs from the sender to your gauge.
Sender is mounted in the inlet manifold water jacket by the thermostat housing.
Wire just clips on sender.
Thanks Gary, I’ve located what appears to be a green wire running from (a sensor?) underneath the inlet manifold. I’m guessing that this is the wire.
I will follow this back, but I’ve got to be honest I seem to have a real spaghetti mess with wires that don’t seem to connect to anything else after I follow it down from there.
Many thanks for pointing me in the right direction,
I’ve tracked the wires as far as I can and all seems fine, so I’ve taken another drive and I think that the gauge is just very slow to react! It has only ever moved about a third of the way to being perpendicular so I think that all is good. Thanks Bob
That is not good, Bob…
The gauge should reach engine operating temp in about 5 minutes of driving - about 11 o’clock from vertical.
If longer time is required or the gauge fails to reach operating temp; either your thermostat has failed…or the gauge/sender has.
I suggest you get an infrared thermometer to check water rail temp at the sender. It’s a worthwhile diagnostic tool for a lot of purposes…
xj6 85 Sov Europe (UK/NZ)
And there was me thinking that it was good news that the engine doesn’t over heat!
But I get your point, it could overheat and I wouldn’t know.
I will have a look around for a thermometer as you suggest.
Thanks for the tip,
Sorry, one further question, what should be the normal temperature range of the engine?
It’s sort of decided by the thermostat fitted, Bob - ideally, the gauge should show rated thermostat opening temperature at all times.
Some prefer the 82C, or 85C - lower than the initial factory fitted 88. Principally; the engine works better at some 88C - but the cooling capacity of the xj is less than excessive…
As an aside; a stuck thermostat cause extended warm-up time - and may cuase the engine to run too cool. However, when high cooling is required; a stuck thermostat may cause overheating - as it then restricts coolant flow…
xj6 85 Sov Europe (UK/NZ)
Thanks for the information Frank, sounds like a good starting point will be to remove the thermostat and check it’s operation. I didn’t realise that they have different temperature ratings!
On a system in good working order the engine should be running at the thermostat’s temperature value and the gauge should read the same.
Disconnect the wire from the sender and connect it to ground, the gauge should go to max.
If not, your gauge or wiring has a problem.
To be safe, you should also take the sender out and measure its resistance versus temperature (plenty of info in the archives).
You must be absolutely sure that your gauge is reading correctly if you don’t want have some very nasty surprises…
If your gauge is indeed reading correctly and you engine is running cold, it’s not good thing either.
In this case most probably your thermostat has a problem.
Also check your coolant level, if it’s seriously low the sender is exposed to air and not reading the actual coolant temp, you might be overheating and not knowing it…
Whatever the cause is, I would investigate further.
Thank you for this advice Aristides. I will try what you suggest in the morning as it’s getting a little late at night here.
Another strange thing has been happening with my cooling system - it has been losing coolant, but I don’t know where it has been leaking. There are no obvious signs anywhere of the tell tale green colour of the coolant but I suspect that it may have something to do with the header tank which on my car has been replaced with what looks like a custom made tank. There are 2 rubber pipes coming out of the part near the thermostat (please excuse my ignorance of correct name) where the ‘radiator’ cap is, 1 pipe goes to the header tank and the other to the radiator itself. Obviously as the water heats up and the pressure rises the coolant is going to go down one or both of these pipes.
My question is what determines which pipe the overflow goes to?
Also, the ‘radiator’ cap on the engine has no spring pressure relief valve and neither does the cap on the header tank. This puzzles me - is this correct?
I have only had the car 6 weeks or so and I’m desperately trying to catch up with 46 years of previous owners modifications!
Thanks and regards,
HI Bob the cap at the thermostat housing is just a sealing cap and should be tight to move. The cap at the header tank on your mud guard should be a pressure cap of 13lb
I’ve just been learning (on YouTube) about header tanks and expansion tanks and the difference between them and from what I have learned the tank on the mudguard is an expansion tank and it should have a small hole in the cap (mine doesn’t) to allow the water to go into it (and also to allow it to return back into the cooling system as it cools down). From what I understand the cap on the header tank (which is next to the thermostat) should be a pressure cap at either 13 or 15 pounds.
I have removed the (8mm?) rubber pipe from the thermostat housing and I cannot blow any air at all into the expansion tank when the cap is in place on the expansion tank, so how could the coolant enter the expansion tank?
Or have I got this all wrong?
On the Jag engine the tank by the Thermostat does not have a expansion cap it is only a sealing cap. Only the tank on your mud guard should have the expansion cap of 13lbs the expansion tank is the one on your mud guard. the one by the thermostat is the header tank.
Your header tank should be full to the base of the filler neck your expansion tank should be approximately half full. your anti-freeze should be a 40% solution and must be used to top up or for renewing your coolant.
The Two small hoses that come from the header tank they go along the top of the cooling fan together the top hose goes to the radiator the bottom hose goes to your expansion tank lower fitting.
hope this helps you.
Thanks Gary, that helps a lot. Should the expansion tank on the mudguard have a hole in it (or the cap) to allow the coolant to flow?
Yes the tank should have a over flow hose on the neck of the filler opening. If the pressure exceeds the 13lbs of this cap the hose lets it drain out.
The more I think of what I’ve got on my car at the moment, the more I think that what I’ve actually got is 2 header tanks instead of 1 header tank and 1 over flow tank.
I’ve had a look at the electrical side of things this morning and, well as I’ve mentioned before it’s all very old spaghetti which is outside my knowledge/expertise so I’ve called an auto electrician to sort this out for me.
Thanks for all your help so far
Hi Bob have you got a Haynes owners workshop manual it would help you to see if your car is the same as the manual shows you may have had a after market overflow tank fitted by a previous owner and this may be confusing the issue.
Picture of over flow tank is yours like this?
I have a Haynes on the way from the US, I’m in New Zealand so could take a while.
I have attached 2 photos showing my overflow tank attached to the inner wing, as you can see nothing like the original! My logic goes like this. The coolant heats up and then and only then does it pop the spring on the radiator cap (the one by the thermostat) allowing the coolant to flow into the overflow tank, but it can’t do that if the overflow tank is not vented to atmosphere and neither can it allow the coolant to go the other way as the whole system cools down. It just sounds like basic physics to me but then I was never brilliant at that stuff! If you don’t have the correct cap by the thermostat then it will try and force coolant out from the get go instead of when the pressure increases.
As you can see the cap on the overflow tank is a pressure type which does not allow the coolant to flow, hence my thoughts about having 2 header tanks.
Anyway, I’ve decided to get the electrical side fixed first so that I know what I am dealing with from a temperature point of view.
The cap at the thermostat is not a pressure cap it is just a sealing cap the pressure cap is only on the overflow expansion tank and your overflow expansion tank is not the jaguar one which wont help you it is a aftermarket one.