At one point I saw the infrared thermometer temperatures that should correspond to N and H on the temperature gauge, but I can't seem to find them now. Can someone please provide me with these temperature values? I got a great deal on one of these thermometers and I am anxious to see if my temperature gauge is the source of my over heating worries.

In addition, I was thinking that if the gauge is at fault will it work for me to measure the temperature and put a small mark where N and H should be?

Thank you

Lionel

 

Water temperature should be in the range of 90-110°C. Radiation Pyrometers (AKA Infrared thermometers) can differ in readings depending on the surface you are measuring and the distance to the item. Read the instructions, they will have a distance to spot ratio (DTS) this is distance the unit is calibrated at.

Bear in mind cheap IR thermometers can very upto ±8°C, What I have seen. If you spend over $80 they should be better than ±2°C at 100°C

I bought a cheap one ($40) but I am lucky enough to work in a calibration laboratory, so I could adjust it so it read accurately.

Poor surfaces = anything shiny, the unit will read low, how low depends on the quality of the thermometer.
Good surfaces = mat black is ideal, anything that is dark is ok too.

 

Thanks Warjon, this was regular $100 and I got it on sale for $20 and it is plus minus 2 degrees.

The distance to spot ratio is 8:1....in practical terms what does this mean?

Any idea what the N and F temps should be when measured at the thermostat?

 

DTS is the ratio of the distance to the measurement surface and distance from the surface. For eg if you hold your thermometer 8" from a surface the measurement will be averaged over 1" of that surface. THIS IS IMPORTANT. If you measure the water rail and hold the thermometer too far away the thermometer will also measure the temperature of the exhaust heat shield.

With the Temp Gauge the sender has an effect on where the needle will be on the gauge. My car used to sit just above 1/2 way between C and N, I recently replaced the sender (broke the terminal off the old one) now my gauge sits just above the N. So the gauge is not a good indication of engine water temperature.

Best way to take the engines temperature is to use a multimeter and K type thermocouple. Insert the end of the thermocouple in the hose on the outlet of the thermostat and tighten the hose clamp.

The IR Thermometer could read ±2-20°C depending on how you use it. The K type method will be far more accurate than the IR thermometer. It would be ±1°C depending on your multimeter.

Use this type of thermocouple. It is cheap and very thin.
250C 1M K Type Thermocouple Wire Surface Temperature Sensor | eBay

Then use this table to convert the Voltage to temperature.
Type K Thermocouple - Type K Thermocouples - K Type Thermocouples - K Type Thermocouple

 

Do you just hook the multimeter leads up to the prongs on the thermocouple and then read voltage?

 

Yes, If the probe has the yellow plug (yellow is for K type) you can either buy the adapter to fit the meter or connect the wires to leads with alligator clips.

Any of these type thermocouples are very accurate and read a certain voltage per degree.

 

What would be appropriate values for N and H

 

This is not important and is not adjustable without making changes to how the gauge works. or installing an aftermarket gauge.

What you want to know is where your temperature gauge needle is when the car is in its normal temperature range.

 

Normal operating temperature is 90-100°C, in NASCAR they run their engines around 99°C I have been told as this is where they make the most power.

My engine runs around 95°C and I have seen 100°C in traffic in hot weather. Hope this gives you an idea

 

Thanks for a ton of information

 

i very regularly check mine with an IR as it sits over the N and some times goes a good bit higher. Over the N mine is usually around 77-82c measuring at the Tstat housing and hoses surrounding them.

Im assuming my IR could be off a little and my temp gauge is waaay off, but its good to have one for peace of mind

 

I would think that 195 degrees is a good reading in the center of the N or at the bottom of it. Of course, you are relying on the accuracy of your gauge.

 

Read my post #4 IR thermometers in a confined space can be inaccurate. You need to get very close to keep the spot small enough not to measure surrounding items.

If you want a measurement with a small uncertainty use a thermocouple and DVM.

It does not matter if the Temp gauge needle is on/above or below the N as long as you know where it should be at 80, 90 and 100°C.

 

Lionel,

You probably have nothing to worry about. In 93 or 94, Jag changed the graphics on the temp gauge to show that Normal is not really a mark but a range that extends almost from the C mark to the H mark. They did this because of many customer concerns about overheating. I can say that on my 94, the temps at the thermostat housing are around 195-200F (90-93C) when the needle is on 'N'

Coolant temps can and will often get hotter than that which is fine according to Jag. The design specification and the mounting position of the radiator fan switch will not activate the auxiliary fan until temps at the thermostat reach about 215-220F (101-103C). The attached photo shows the needle at the point that the fan comes on (217F in my case) in my car. As you can see it is well above 'N' but well within the 'Normal' range.

 

Hi Allan,
Your photo shows where my gauge sits on a warm day (20c) sitting in traffic. I only bought the car in the fall and I am somewhat concerned that it is going to overheat on a hot day.

 

Lionel,

Yes that is probably quite normal and is well within the 'Normal' range for the car. The key is that it does not go much further. My guess is that the coolant temps have not quite reached the point where the auxiliary fan switches on. When it does so, the temps (and needle) will drop to around 200F before it switches off.

Unlike the V12 engine, the AJ6 engine does not have a reputation for overheating. Of course, like any other engine, especially in a car that is almost 20 years old, things like thermostats sticking, radiators plugging, fans and fan switches failing are all possible.

So for peace of mind, I would do one of the following.

A) Super safe route
1. Drain and back flush the radiator and test/replace the thermostat (this should be done every two years anyway)
2. Locate the fan switch (it is towards the top left of the radiator and you will need to remove the air intake tube to get to it) Disconnect the PM5 connector for the switch then using a jumper, jump the two wires on the engine side of the connector. This should immediately start the auxiliary fan. If not you will need to check the fan relay, the wiring or the fan itself to find out why.
3. The fan switch itself is very reliable but if you want even more peace of mind, you can swap it for one with a lower on/off switching point. (The standard switch is 95C on and 91C off. You probably won't find one with the same connector but it is easy to cut and solder/connect the two wires. The switch itself has a standard M22 x 1.5 thread and there are dozens of options available.

B) Very slightly less safe but quick and easy
1. Drive the car around a bit to get the engine up to temp.
2. Stop and let the car idle until the needle rises passed the 'N' mark. You may need to rev the engine a bit to get the temp up.
3. Once the needle is at the point as shown in the photo, pop the bonnet (engine still running) and using your infra-red temp gun, measure the temp of the top hose that goes from the thermostat housing to the radiator.
If the temp is below 215-220F and the auxiliary fan is not running, so far so good. (If the auxiliary fan is on and the temps stay at or above that range, stop the engine immediately and go to step A as you may have a sticking thermostat or slightly plugged radiator)
4. Now close the bonnet again and let the engine run. If the needle moves towards H some more, the fan should come on and the temps drop very quickly. (You may need to rev the engine, put on the A/C or even put some cardboard in front of the radiator to get the temps up). If it does, all is well, if not, stop and go to step A.

As the car is new to you, I would do step A just to get a baseline, along with changing all the other fluids/filters too.

 

Thanks Allan, it will be a lot more fun to drive it with the added peace of mind.

 

One thing I forgot to add with the K Type thermocouple is to add the voltage from the table for abient temperature to your reading. So if the amient temp is 25°C you need add 1.0mV to your meter readings.

So 95°C will read 2.9mV on the meter and add 1.0mV for 25°C ambient.

 

Hi all,

I bought my 90 XJS last summer and have worried to death about getting the temp guage to ready in the middle of the N or lower. I have read in other places that it should NEVER go above the N.

I got radiator boiled and rodded, replaced the aux fan swicth with a 182 F, put 180 thermostats and put new hoses all the way around.

During the winter months here in Albuquerque, NM (USA) the needle never got up to the "N". Over the last few days, however, the outside temp has been up. So I bought an infra therm and gave it a try.

The temps looked within the safe zone and the needle was at the top of the N and a bit higher. SO, I will keep checking, but I assume I've worried or nothing about the needle.

Does anyone know what the temp of the water coming out of the bottom hose should be (assuming a perfectly working radiator - which I doubt I have even after the servicing)?

Thanks,

Tony in NM

 

A very interesting thread and particularly warrjon's suggestion about buying a thermocouple to accurately test the temp.

As most owners know, there are 2 issues relating to facelift temp gauges:

- the gauges themselves vary in accuracy. N on one car can equal between N & H on another car at the same coolant temperature

- the electrical connections on the facelift cluster can cause variations in gauge accuracy, in particular the earth screws on the cluster can cause inaccuracies.

Measuring the temp, even with a IR thermometer, can give some peace of mind that the car is behaving as expected.

The biggest single cause of temperature variation on 4 litre cars is corrosion of the cooling fins in the lower third of the rad. Usually hidden from view, they disintegrate due to road dirt continually hitting the rad at the bottom. In addition, any oil leaks from the engine cooler tend to spray onto the radiator fins and then accelerate the disintegration.

The most noticeable indication of this is a rise in the temp (& gauge) when sitting in traffic followed by a drop in the gauge when you drive off as the airflow is increased and compensates for the lack of fins.

So, check the rad first and then check the temp as accurately as you can. (I'm off to buy a thermocouple wire next week!)


Paul