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MG Midget and Sprite Technical - best location for a electronic coolant sensor

guys, what is in your opinion the best location for a electronic coolant sensor. As I run EFI an accurate measurement is crucial. At the moment it is located in the hose to the heater, I fear this does not give an accurate temp...

Thx. Arnold
a.o. arnold

On my V8 EFI it plugs in to the water way in the inlet manifold on the engine side of the thermostat. So it is measuring the temperature in the top of the engine, not in the radiator. How you do that on a small 4-cylinder engine I have no idea.
Mike Howlett

On my toyota work truck with efi, the coolant temp is in the thermostat housing, below the stat.

dont know if thats any help.

I'd measure the temp in the secondary loop (before the heater, if fitted)... since the flow in this loop doesn't depend on thermostat, ie. it's reasonably contstant, and the water in this line warms up with the engine head / top block (as opposed to primary system where's there's little flow until the stat is open.

This is also what I do in the Kseries for the Emerald.

The temp signal is mainly used to enrich the injection when the engine is cold; similarly it also adds a bit of advance. Typically 50% more fuel and 5 deg adv at 0C, reducing to 0 for both around 50-60C. It also provides for addtitional start-up enrchment, starting at 110% reducing to 20% at 50C.

Rather than describe, here's a pic from the Emerald.



Anthony Cutler

What is wrong with utilising the standard temp sensor position - in the head, not limited by the thermostat?

Guy Oneandahalf Sprites

Hi Guy

Arnold might want this for the gauge sensor. The ECU sensor is separate from this, and electrical...

Anthony Cutler

Hi Anthony
Yes, I appreciate that they are different. But the 1500 uses an electrical gauge and I wondered if the same output could trigger both the ECU and supply the gauge. Or maybe the gauge sensor could be relocated as it is less critical. It might make more sense fitted to the top hose anyway.

Guy Oneandahalf Sprites

Hi Guy

That would make sense, but generally ECUs power the sensors off their internal power supply - typically 8v. And the ECU temp is accurately calibrated for a particular p/n that would not be supplied with original electric gauge.

BTW - I agree your suggestion of positioning in block is fine - hence did not comment earlier.

Anthony Cutler

Hmm, perhaps my suggestion was a more complicated solution than is necessary! Another bright idea bites the dust!
Guy Oneandahalf Sprites

Since i am in the planing and gathering fase of my EFI A-series (motorcycle ITB's are on their way) i was thinking about this to.

My midget has the early vertical flow rad which has a threaded hole where the temp sender fits

So i figured i would put the ECU temp sender in the head below the thermostat.
And the gauge in the rad.

My gauge is now in the head and i can see how fast my gauge rises so i do not think it will give a false reading due to the thermostat being closed.
And moving the gauge to the rad might give a slightly delayed reading but if you know this you can just lay the (mental) panic temp lower.

Anny flaws in my logic that will cause the setup to male function?
Onno Könemann

The only thing that could but doulbtful be an issue, would be the actual signal itself as the computer brain interpurts what the voltage amount means

what do i mean... will the efi sensor voltage signal be higher or lower based on its location then in the orginal applicaton...if it is, or does, can the signal (aka voltage) be re-mapped

Im just not suure how big an issue something like this would be... My guess on a large V8 5-10 degrees off from orginal application probably not much, but Id think the smaller the engine the bigger the differance 5-10 degrees temp differance from orginal application to the midget application would make

I hope that made sence...Im not sure it will to anyone else

basicly... if the normal water temp at the efi sensor on the orginal application be 160 and thats what the cumputer expects to see, but on your application if the water temp at the efi sensor be say 180, thus sending a differant voltage signal to the cumputer brain, there by throwing off the amount of fuel allowed into the fuel deliver system

still even if this was a valid concern (and Im not sure it is) Id think just Re-mapping the signal voltage, or a sensor with a closer match temp signals to your application, or if not to out of spec, then maybe just a simple A/F ratio adjustment is all that would be needed

From my own personal experiance... a small change in voltage at the mass air flow senor does have a huge effect on the A/F ratio the computer adjusts for...even a small hole in the tube going to the TB after the Mass air flow sensor can really screw things up... so this is why i raise the issue of the sensor signal to the cumputer

DOH, This is probably why you possed the question of senor location to begin with

Prop....Location location location

No prop not why i asked the question.
Since i will be using an aftermarket ECU (not specificly designed for anny engine but for all)there is no "normal" place for the sensor.

So you will always hev to see the meassured temp as a number and not a conclusive temperature.
You see what is normal running temp and then set the start-up enrichment from there.
Onno Könemann

Your the MAN !!!

I would assume that the issue is not about the temperature figure as such - as it will need to be calibrated to what is "normal" anyway as Onno says. But the sensor position needs to be such that it provides a quick response to changing temperatures close to the combustion chambers. A more remote sensor elsewhere in the cooling system is necessarily going to be partially "buffered" to changes.
Guy Oneandahalf Sprites

If someone wanted to make a riser plate to put the sensor under the t-stat housing, is there clearance? And would this cause problems with coolant flow?
Trevor Jessie

Late mini's have a thermostat housing sandwitchplate for a sensor fitting
worked there so thanks for reminding me ;)
Onno Könemann


Thank you so much for that link, as i was going to have to have a spacer plate made because my rad hoses where to big and do not enough clearance at the thermostate housing , so that will work perfectly

But with that built in sensor connection into that adapter,,, can i put a senssor switch into that sensor hole to switch on and off the elecrtric fan based on the water temps for that location. or would that be a bad idea...

agian Thanks for that link



Prop, you need the electric fan sensor to be in the radiator.
Trevor Jessie


In the radiator is good but which side?. My understanding is that the OE fitment is typically in the outlet side of the radiator as this gives an indication of when the coolant isn't being cooled sufficiently by the airflow so the fan needs to be turned on.
David Billington

Makes sense to me David.
Trevor Jessie

How fast does water actually circulate through the system?
Another way of looking at it is that if the water coming from the engine is excessively overheated, then the fan needs to come on to improve cooling. If the sensor is after the rad and you wait until it is reading too high, then the engine has had longer to continue its overheating. And you continue to return too-hot water to your overheated engine for longer.
My rad fan sensor is in the top hose 'cos its easier to fit there. Most times, the fan doesn't come on anyway.
Guy Oneandahalf Sprites

I like the idea of using the orginal sensor location. At tjhe moment I have the feeling the sensor is no always in the coolant, reading are not stable as I would expect them to be...
a.o. arnold

This thread was discussed between 21/11/2010 and 24/11/2010

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