Possible scheme for overheat warning system

(John) #1

Due to poor original temperature gauge design I was thinking for quite a while of a way how to create some sort of warning if engine is overheating. I think I have a plan in my head now :slight_smile:
My goal is:

  • Keep it simple.
  • There should be no additional mods necessary to cars interior (no additional gauges, no additional lights etc.)
  • Warning system should be easily installable.
  • Should warn about overheating in good time.

I think I could use existing low coolant warning system for overheating warning.
What if I could incorporate small NC thermo-switch into the existing probe. Existing probe is just a stainless steel pin which shorts to ground through coolant. If coolant level is too low, then no ground and you get a red warning light on your dash with radiator.
My idea is to fabricate stainless steel capsule that could fit into existing probe hole. something like this:
and then insert and JB weld NC thermo-switch with temperature rating of ~105c into the capsule.
Wire in such a way that wire from the low coolant module goes to one contact of thermo-switch and another contact of thermo-switch conected to stainless steel capsule body. In this case ground contact will be lost if coolant level drops or C thermo-switch goes open. In both cases warning light on your dash will light on to get your attention.

My concerns are:

  • Not sure if I can find small enough NC thermo-switch. should be smaller then original probe.
  • Of couce larger hole can be drilled to accommodate bigger probe but then I need a suitable rubber grommet.
  • Is JB weld a good heat conductor?
  • what temperature rating switch should I use? is 105c too high or too low?
  • any other concerns I should take into account?

(Richard Dowling, 1979 XJ-S HE+5sp coupe, 1989 XJ-SC, 2003 XJ8 3.5L) #2

Done that on the coupe V12.
Used a thermal cutout switch on A and B banks as in photo.
The switch is in the middle of the head.
These ones are 100deg C because I had a bunch of them in the factory
You have to take the throttle body off and drill a couple of small holes about 5mm deep for 2 small ST screws to hold the switch.
I did that with a right angle drill to get under the intake manifold.

The two switches are wired in series to a relay coil. With ignition on the relay is energised, and if a switch opens it causes the relay to drop out and sound a12V buzzer in the centre console.
I think this is a bit better than monitoring coolant temperature or level.
If those heads get to 100deg C I want to know about it.

(John) #3

Nice! Definitely gives you more confidence :wink:
Although I would prefer red warning light instead of buzzer, which might be confused with seat-belt buzzer or lights buzzer. Or even better use both :wink: light and buzzer :))
My idea is to make something really simple and easy to install, so everybody could do this upgrade and hopefully save few engines from overheating :slight_smile:
Another criteria is that it should warn even “untrained” Jaguar driver who might not be monitoring temperature gauge like a hawk :slight_smile: So I think red light with empty radiator should at least point to the right direction that there is something wrong with cooling system and hopefully force the driver to pull over.
I’m also came up with another location for thermal-switch which might be even better… radiator bleed bolt on the left side of the radiator. I could machine a special bolt where I would fit the the thermal-switch and wire it in series with low coolant probe. In this case this mod could be done even without draining the coolant! Also B bank radiator inlet theoretically should be the hottest spot. And the bolt should look something like this:

I think I’ll start doing first prototype :wink:


Me being no mechanical engineer, but I wonder if it would be simpler to work up some kind of gizmo using one of those laser remote thermometers to monitor the actual external engine temp, and whether that sort of monitoring would be advisable. I’m thinking in terms of mounting the laser part of the thermometer permanently in such a way that it comes on whenever the engine is on and is pointed at the exterior spot of the engine most likely to first give signs of overheating. Run a line from it to a gauge inside the cabin that shows that engine temp at all times (while the engine is on). :question:


B water rail has a bung for the vacuum thermal switch. Wouldn’t that be ideal? At least for anyone not running the air injection system.

As JohnJohn1 pointed out. The Above B should be A.

(Robin O'Connor) #6

Another option could be a discreet display utilising a thermistor to measure true themperature;

(John) #7

Everything is possible, but is it worth the effort? :slight_smile: Another problem is that those remote thermometers with laser pointer are not that accurate and their readings are even more off on shiny surfaces like aluminum.
Also it kind of contradicts one of my goals - keep it simple :slight_smile:

(John) #8

Vacuum thermal switch is actually on A bank but I have moved ECU temperature sensor to that location and installed temperature gauge sensor on B bank where ECU sensor used to be.
Another ideal place would be to tap into one of the blanking plates under the water rail. Then I could use 2 switches (one for each bank) similar to [Richard_Dowling3] setup. I’m considering this option too. Have to check how thick those plates really are and if I can tap directly or I will need to fabricate a thicker plate.


Wow. First I head of this. I assumed they were very accurate. How far off (+ - %) are they, typically? :worried:

(Mark SF) #10

The V12 has a flaw where the temp gauge is only on one bank. But there are two separate thermostats. This allows for a unique failure mode where one bank can overheat while the temp gauge reads normal.

Therefore, if I were going to add a warning system, I’d put it on both banks.

(chris gruchawka 1988 XJ-SC) #11

Anyone tie in a per bank thermostat with a per bank electric water pump?

(Greg) #12

My simple plan is fit water temp gauges for both banks in car, where trip computer is, and monitor them constantly as I drive. Easy.

(John) #13

I think main problem is not that there is only one sensor, but it’s location. As a rule of thumb B bank is always the first one that starts overheating.
Another flaw is uninformative temperature gauge. I have addressed this issue by installing two temperature sensors on both banks and wired them in paraller. Which. Gives me somewhat of an average temperature reading. Now my gauge is much more sensitive to temperature changes and when in normal operating temperature needle sits a bit above N to middle between N and H. If temperature creeps above normal, needle creeps close to H, which I think makes more sense.

(John) #14

You should keep your eyes on the road and not on the temp gauges :))) just teasing :wink: would be interesting to hear your feedback if there will be any acual temperature difference between the banks in daily driving.

(Greg) #15

Will be a couple months until I get to it, but will report back. Got the idea from another member here who has already done it. I have taken readings of both rad hoses with my IR temp gun, and they were usually within 2F of each other.

(Robert King) #16

What I did on my ‘88 V12 XJS was to replace the stock temperature sending unit with one from a MGB and added an additional ground to the cluster at the temperature gauge. The sender has the same thread (5/8-18) but tapered, so don’t use the washer when installing.
Gauge now reads in the middle of “N” with 88C thermostats, and rises more quickly if coolant temp increases above normal.
You could also move the sensor to “B” bank if it eases your mind.

(Robert King) #17

I installed a K thermocouple in the left radiator bleed plug; I have never seen a reading over 205F, and that is the outlet of “B” bank.

(Greg) #18

BobPhx is the member that I am copying. Here is his link. http://goflyrc.com/projects/XJS/xjs.htm, scroll to the bottom. I really like his set up for monitoring both banks.

I will tap the B bank by ditching the temp sensor for the charcoal canister. I will tap the A bank by ditching the temp sensor for the air pump (which I have removed). The only hard part is dealing with the threads.

(Richard Dowling, 1979 XJ-S HE+5sp coupe, 1989 XJ-SC, 2003 XJ8 3.5L) #19

Two electric water pumps would be ambitious.
I have one in the coupe you can see on the RHS of the photo, and that is a bit of a struggle. A major problem is trawling thru the stock of hoses in the local auto shop to find ones with the correct ID and with bends in the right place so they will couple to the pump, the rad and the engine.
Next time around might be able to use flexible hoses to some extent
The plan is to put the same setup in the spare V12 being rebuilt for the convertible, although with a bit more sophistication for mounting the idlers.
You can spot there is no mechanical fan here, replaced with twin electric fans.

In any case no need for 2 pumps, one does the job and there is not exactly plenty of space in the engine bay to add extras.
The pump used here is a Davies Craig brand made in Melbourne. It is one of the earliest in the market place, no doubt plenty of others available now.
Davies Craig offered an electronic control module with the pump, the idea being to ditch your thermostat and have variable speed of the pump to control coolant temperature.
I did not go for that idea, and it does not lend itself to engines with 2 banks and 2 thermostats.

So why would anybody go to the trouble of putting a single PK6 belt onto the front of a V12 plus an electric pump to cool it ?
( It is a fair bit of trouble too ).

  1. You can change the belt from the top of the engine without moving anything else.
  2. You can change the pump in about 45 minutes.
  3. The rusty crossover pipe easily replaced with a home made copper one,
    and the OEM overflow tank replaced with a SS one up near the firewall.
    That tank higher then any coolant in the engine and easy to bleed.

The drawback ? A lot of work to do it all, and you need a lathe to make the adaptor for the front of the crankshaft. No problem for me since it is all Saturday morning therapy and meanwhile no pressure to have the rebuilt V12 ready anytime soon.
Not recommended for most V12 owners.

(chris gruchawka 1988 XJ-SC) #20

Wow, that’s a very nice, clean setup. Does your crossover return to the right intake of the radiator. Looks like stock radiator, or am I mistaken?