Recently I rigged up my 2 channel temperature probes to measure top hose and water inlet temps. I was interested to know the relationship between the gauge position and the actual coolant temperatures.
One probe was taped to the water pump inlet spout next to the OEM temp switch; the other taped to the B bank top hose. Then I went for a drive. The temps on the move were not accurate (they were lower than the actual coolant temps) as the airflow affected the probes and cooled them. Once the car was fully hot and warmed through after about 10 kilometres, I stropped, turned off the aircon fan and waited, engine running.
My main electric fan (mechanical fan removed) cuts in at 85 C. This is what I found:

• The radiator reduces the water temp, inlet to outlet, between 10 and 12 degrees C.
• In normal running, regardless of speed, top hose temps are about 85 to 86C; pump inlet temps 78C or so. In these conditions my dash gauge is somewhere in or just below the N.
• At the point that the dash gauge is about 1/3 of the way into the area above the N, top hose temps were 91/92C, engine ticking over with no fans running
• At this point the main electric fan kicked in, switched from the water pump inlet 85C switch, and temps stabilised and slowly reduced. Aircon fan still off.

From all this I deduce that even when the temp gauge is well above the N water temps are still well within the system's safe range. And that providing the system is sensibly maintained, rad fins clean, thermostats in good order, auxiliary fans working, even quite noticeable gauge movements indicate actual coolant temperature changes that are not that significant.
In normal use the aircon is always running and thus the auxiliary electric fan is too. Under these circumstances the main electric fan has never cut in on its thermostatic switch, except once in an hour long Paris traffic jam on a really hot day.
I have not measured this, but I think from these readings, for the gauge to be at or close to the H, top hose temps would have to be around 100C or higher.

 

I installed two Auto Meter (AM) gauges. Sensors are located in the water rail's about 3" from the back end. I also still have the stock sensor & barrel gauge running 180F states. In Florida temp's I found the AM's read around 220F in traffic, 90F temp with high humidity. In comparison the barrel gauge is almost in the lower middle part of the "N". Both electric fan's running, AC on. I really have my doubts about the stock barrel gauge's being that accurate.

 

As you should.

Many, many years ago, on a different Jag board, several XJS compared gauges readings and discovered large disparities. The "middle of the N" might be 180ºF on one car and 200ºF on another, for example.

Every XJS owner should check this.....especially since a needle rising a hair's width above the "middle of the N" puts so many XJS into panic mode.

Cheers
DD

 

Absolutely true Doug. The importance of the gauge is its relative movement from whatever it's usual point is, rather than it's absolute position on the gauge scale. My own findings, above, seem to indicate that even relatively large movements from the usual position may be caused by only a few degrees rise in temperature, thus the panic need not start until the needle is very close to the H!

 

Yep this would be true for any analog gauge. There are uncertanties that effect the reading of the gauge.


These are
- Resistance tolerance of the sender.
- Tolerance of the gauge itsself.
- Voltage, as the system voltage varies the gauge reading will change, there is no regulator on the instrument cluster.


I broke the spade terminal off my gauge sender so purchased a new sender, this sender had the needle 1/2 way between N and H with normal temperature.

Greg surface temperature measurement are notoriously difficult to get accurate without a contact probe, good ones are very expensive, the one we used in the Calibration Lab was nearly $2000. Best way to do this is with a K type thermocouple under a hose clamp then wrap it to prevent convection errors.

 

Thanks Warren. I used K type thermocouples, tightly taped into contact, but they were only accurate-ish when the fans were off and the car stationary.
Will the T/C be Ok under the hose getting wet? Thanks
Greg

 

The tests I did a few months ago with laser/infrared thermometer gave me readings of about 110-115C when the needle was at the top of N or a hair above it.

Granted, this test was done when the car was idling in a garage, and the devise likely had some margin of error, but after these tests i am in “panic” mode every time the temp gauge is approaching the top of N

 

i never thought much of any type factory temp gages, or any other dash gages,for that matter!

i use a custom ECU, sender mounted in cylinder head(rear left bank) that has a digital numbered reading, in 4*F increments, from 0*F to 300*F, i think that should cover most of my needs, altho some engines can run higher,kinda scary tho!

average temps today 98*F ambient, running around 185*F,digital cruising around town, moving slowly, on hiway at 65/75 mph reading drops to 178*F,( I consider perfect).

now after getting back in garage, shut off 10 minutes temps rise to 194*F,.

never had a COOLANT temp over 200*F, interestingly oil temps have reached 220*F, it is just the design of the engine and components!
i have put this on JF before, the reality is Jag V12 in hot climates is always a border line overheat , hoping it never goes over the line!

 

 

Jaguar jumped on the bandwagon started going to the faux gauges 1995-ish.

The reason manufacturers don't 'trust' us with the real info is because a great many owners/drivers don't really understand what they're seeing.

Back in my dealership days we would frequently (almost constantly, really) get calls from owners who worked themselves up into a lather over what they were seeing on their gauges. In 95% of cases the lather was totally unwarranted.

Anyone on this site (or Chevy, BMW, Audi, Mustang....whatever the case might be) is here because he/she is more enthusiastic about ownership and either understands these things or wants to understand them. We're a minority.

Cheers
DD

 

They will be fine submersed and a far more accurate. Be careful if you insert it between the hose and barb, if you short the wires then the measurement will happen where the wires are shorted.



These TC's are not really designed for contact measurement as they take some time to stabilize and in an engine the temp can change faster than the TC can measure. I have used them under a hose clamp on the rad hose.



K type TC is just 2 wires twisted together soldered, Chromium and Alumel, when they touch they generate a specific voltage per degree which is pretty linear from -270 to +1260°C.