EdGasket

Coolant tenp sensor resistance range?

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Can someone please tell me what the resistance should be for a Fiesta Mk6 coolant temperature sensor (CTS) please at cold and hot temperatures?

The guage never gets past midway but the fans dont come on and the engine overheats. If I bypass the CTS with a 1K ohm resistor then the high speen fan comes on so the circuit is working and I'm thinking the CTS resistance is not going low enough to trigger a high temp.

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If the CTS has failed, it will also affect performance and fuel consumption, as this provides an input to the ECU to control the mixture.  If the ECU thinks the engine is cold then it will be running rich.

Do you have any fault codes ?  You shuld be able to put the display in test mode, then cycle through until you get to DTC, and see what there is.

There is a spec for CTS resistance (2k3 @ 80deg C) so if you can measure with a hot engine,  It has a -ve temperature coefficient.

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Hi Thanks for the figure at 80deg C.

There are no fault codes. The gauge never goes above mid-way even with the coolant boiling out of the expansion tank. I am thinking the sensor must be faulty as it never triggers the fans to come on or the gauge to rise above mid-way. If I connect a lower resistor then the fans do come on so the fan circuits are live and working ; just waiting to be turned on.

Does anyone have the resistance or voltage figure for when the low-speed and high-speed fans should kick in please? If you have a Hayne's manual, it might say in there under the cooling section specifications.

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Dug out the Fusion Haynes.....

No figures for temperatures, but spec for CTS is different !

20deg C =6k1

80deg C= 620

These figures support your diagnosis that CTS is faulty.  Haynes warns that when CTS removed, the coolant will escape!   If  you do substitute CTS, and it works, please post resistance of new one at 20deg C !

When you scroll through the readings on the display in Test Mode, it should display engine temperature, so if that is wrong with the old CTS, but is correct with the new one, it' should then be OK.

Never noticed my fan come on, apart from when A/C is on, either in Fusion, or Fiesta.

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Thanks.

I think if you were sitting in a jam or slow moving traffic, your fan would need to come on to avoid overheating. Mine boils up just sitting idling in the driveway after about 20 minutes. The radiator gets very hot but with no airflow it can't dissipate heat.

I've got a 5K ohm variable resistor on order. I can substitute that in the sensor socket and find out what resistance is required to fire up the fans slow and fast. Then I can test any new sensor I get will provide that resistance BEFORE fitting it.

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That's a very good idea!

Do you have a screw in CTS into an alloy housing, or a clip in CTS into a plastic housing ?  You might be asked this.

I try to keep away from sitting in traffic, mainly by avoiding M3/M27 !!

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Well I bought an aftermarket temp sensor but the fugures don't look good as it reads much too high:

20 degrees C    43K ohm

65 degrees C    11.5K ohm

82 degrees C    6.8K ohm

100 degrees C   3.3K ohm

which is never going to turn my fans on; I need something nearer 1K ohm at high temp. I'll have to return the sensor and hopefully get a refund.

I'll measure what the ECU actually needs when my variable resistor arrives

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Andrew, thanks for that update - when you get your pot and connected, with the dash in test mode as you adjust it you should get a change in the temperature displayed in degrees.

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I found this chart on the internet for a Ford CTS and the values are close to what my aftermarket sensor reads although my aftermarket is still on the high side:

https://www.google.com/search?channel=trow&q=mazda+temp+sensor+resistance&tbm=isch&source=univ&client=firefox-b-d&sa=X&ved=2ahUKEwi0msb7y4DkAhVVilwKHVNHDTwQsAR6BAgEEAE&biw=1214&bih=943#imgrc=dv7nvSbK6rdcPM:

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OK with the 5k variable resistor (if anyone tries this I would recommend a 10K variable resistor as with 5K the ecu things the temp is up to normal and its a bit hard to start on the lean mixture):

Fans come on slow at 1.65K ohm

Fans come on fast at 1.2K ohm  (and temp gauge goes to nearly red)

So the fan circuits work but neither the temp sensor on the car or my aftermarket one go down low enough in resistance. The aftermarket one is PN: FOR4337456 which is the correct one. I am now a bit stumped as even that Ford chart (see above) shows the temp sensor at 2K ohm when boiling point is reached so even a sensor conforming to spec isn't going to turn my fans on. I'll maybe see if a breakers has an original one otherwise I could wire up some kind of switch but as the temp gauge with the current sensor never goes above normal, how would I know when it nees to be turned on?

Does anyone know if you can remap the ecu to the temp sensor with Forscan?

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Andrew, I have no experience of Forscan, so cannot help you there at all.

Thermostat opens at 88deg C according to Haynes, so would that correspond to normal temperature ?  I have an old "Turbo Guage" in the garage somewhere, I'll try and plug in to my Fiesta, also 1.6 petrol, and see what the actual temperature is, and perhaps try and get the fan to come on.

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I've got a plan. I have some 20K thermistors on order. These reduce to 1.6K at 95 degrees so in parallel with the existing temp sensor that should turn on the fans well below boiling point. I will connect one in parallel with the existing coolant sensor via a switch (so as not to compromise cold-starting) and strap it to the cylinder head. That should be job done I hope.

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4 hours ago, EdGasket said:

I've got a plan. I have some 20K thermistors on order. These reduce to 1.6K at 95 degrees so in parallel with the existing temp sensor that should turn on the fans well below boiling point. I will connect one in parallel with the existing coolant sensor via a switch (so as not to compromise cold-starting) and strap it to the cylinder head.

It sounds like your engine needs a lower than normal resistance to work. Maybe a wire or connector between the ECU and sensor has gone high resistance. If so, it is likely to be unstable, and I would repeat the pot test occasionally to confirm, though with experience, you could probably detect serious drift from the gauge.

Or I would not be entirely surprised to find that Ford had had a supply problem with the normal sensor value, and substituted another value, plus modding the software, without telling anyone.

As far as I know, although Forscan provides a means to read and write the software in modules, it provides no means to make minor modifications. It relies on having an external source for the software.

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