XJ40 temperature gauge shows N (normal) even when overheating

My 1991 XJ40 had a leaking radiator that was repaired unsuccessfully with epoxy. The rubber flaps in the fan shroud were as hard as plastic and broke off when touched. I was given the car because of the overheating problem. I found that the harmonic balancer was sheared and the radiator fan viscous coupling had leaked all it’s silicone oil out so was in freewheel mode only. All these cooling related faults should have cooked the motor but miraculously the motor is undamaged.

The quote obtained for making a new radiator was R12,000 (South African Rand). I decided to try plastic welding to repair the inlet pipe which was cracked nearly all the way round with a big 1cm hole at the top, and it looks like I got it right on my second attempt. I used a 25 Watt soldering iron and black cable ties and made sure all the edges were blended in properly. On the shroud I used 1.5 mm sheet rubber to cut the pieces needed to replace the flaps and fitted them with small bolts and nuts rather than pop rivits.

On the harmonic balancer I removed the trigger wheel and in the land behind the pulley I drilled and tapped four holes and fitted 6 x 25 mm bolts to make the pulley solid (no rubber damping). The fan coupling I also drilled and bolted but as a temporary measure until I raised the R2000 that a new one cost.

All these repairs I carried out have changed the driving experience from a nightmare to a pleasure but the last problem was the temp. gauge. I was not happy with a temp. needle fixed on N in the middle of the gauge. I want a temp. gauge that tells me that my motor is getting hot, not a temp. gauge that tells me that my motor is overheated. I checked the resistance range on the temp. sender unit and it appeared about right. The fuel gauge appears to read correctly so I assume the voltage stabilizer is putting out it’s correct 10.5 volts.

Using a few resistor values across the sender unit to earth I established that a 200ohm minipot from the sender unit to earth would enable me to use the range above N to show actual increases in temperature so now the needle on N is cold and needle at 3/4 is operating temp. and an overheat condition would put the needle in the red and actuate the red overheat warning light on the dash, long before your engine has burnt out.
Adjusting the potentiometer allows one to move the needle to the 3/4 hot position and makes small corrective adjustments easily possible


Have you tried replacing the sender?

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Hi David, I tested the sender in hot water and the resistances appear to be within range. There are other posters on the forums who have changed the sender and found that it doesn’t solve the problem. I first fitted resistors to see if I could get a sender from another car with a resistance range that would move the needle higher than the N (normal) it was showing when the engine was overheated to the point where I was stuck on the side of the road. The expansion tank still had water in it so there was no damage done to the motor. The overheat warning light is operated by the needle of the temp. gauge moving into the red band on the gauge. If my motor is boiling showing normal on the gauge I don’t want to see, can’t afford to see, how much hotter it must get to activate the overheat warning light.

If the sender, the gauge and the voltage stabilizer are all correct then this appears to be a factory fault and I am not in a position to spend the money to try to solve the problem, so fitting the potentiometer is the easiest, cheapest way to do it. Yesterday, when I drove through peak Pretoria traffic the gauge edged up toward the hot band, but when I hit the open road it dropped back down to the 3/4 mark, which is just the way I want it. When I stopped and checked the water level it was still full. Now I will drain it and fill up with proper coolant knowing it won’t end up on the road.

I hve digital gauges and once warm it never moves from mid range, I would be more concerned with why it is running hot at times.

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Hi David, the r/h side radiator tank was badly cracked at the inlet pipe which was why the engine was overheating. The nylon welding I did is holding and not leaking so the motor is no longer overheating but I don’t trust it 100%. The temperature gauge as I have rigged it now will show a rise in temp. if I go up a hill and a drop in temp. going downhill, much like a capillary operated gauge will do. This means that I have an early warning should my radiator repair fail. Also, I know the overheat warning light will come on before the motor is cooked.

The potentiometer mod should work with a digital temp. gauge also, I think. If you are concerned with the temp. gauge staying at mid range you could slip a piece of cardboard between the aircon. evaporator and the radiator and run the motor till it is boiling then see what the gauge shows. If it is still in the mid range then it is a problem, in my opinion.

I drained and filled my cooling system with 50% MEG and it took 10 litres of coolant. Before I filled with antifreeze I checked if the low coolant level light was on, but it wasn’t. Someone had bared the wires and twisted them together. I removed the unit and tested it with my multimeter and a magnet. It is working so the magnetic float in the tank is stuck, or waterlogged. May have to get another tank, not easy this side of the world.

A few years ago my rad partially blew the hose off the compromised top inlet (luckily just a half a mile from base) causing the temp gauge to climb up toward “H” - as well as steam pouring out of the front grille!

In your case, you said the header tank was still full after the problem of the rad pipe cracking - that’s probably why the temp gauge didn’t climb …you still had plenty of coolant in the system.

So I don’t think there’s an issue with the factory sensors or gauges sticking at “N”. Like David said I’d be more concerned if my engine temperature fluctuated up and down hills - did you take the stat out by any chance?

Hi Larry, I was running the car with the radiator leak. I kept 10L of water in the boot. I could do about 30 km and then I would top up for the return trip. The gauge always rose to N from startup and just stayed there. I had to travel further than I normally would have one day and that is when I got stuck on my way home. The car would not pull away. I moved out the road and opened the bonnet. The water was boiling and leaking out the crack in the radiator side tank. I slowly opened the cap till the pressure dropped. The motor was boiling hot but the gauge was still showing N.
If you Google this Jaguar temp. gauge showing normal problem you will find maybe hundreds of posts where a motor was overheated and the gauge shows normal, not just XJ40’s but other jag. models as well, so I do think this is an issue.

A temperature gauge fluctuating up and down hills is not a concern because an engine’s temperature does fluctuate going up and down hills and having a gauge that reflects this is an advantage, not a concern, as long as the temperature is within the engine’s normal operating temperature. It is comforting for me to see my temperature gauge doing down after heavy traffic or a steep hill. It shows all is well with the engine. I would rather have my temperature gauge as an early warning system rather than the vague hit or miss Russian roulette gauge that it is.

I did not take the thermostat out. It is a vital part of the cooling system and if it was faulty I would replace it immediately, as I am sure you would too.