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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Hi Peter, thank you for the suggestion of a wire/connector gone high resistance which is a good idea. I'm not sure I can easily test that though as the wire from the temp sensor goes straight back to that huge ECU plug I would imagine and I don't fancy messing with that while everything else works in case one of the connecting tags gets damaged. It is bolted on with shear bolts that have to be drilled out so would be a mission. If I get any other problems I might try it but for now going with an extra thermistor or two to switch in as necessary. Will update if/when I have it working.

I think I recall reading something about Ford updating the temp settings as a lot of people have overheating issues but can't be certain. I guess a main dealer ecu flash is going to cost some ££'s.

If anyone else fancies taking a resistance reading of their temp sensor when the fans come on it would be appreciated. It is not easy to get at with a hot engine though; probably best to have some extra wires poking into the socket as you attach it to the sensor and measure off those.

So with Forscan, can you 'dump' the ecu settings in entirety, edit and re-write the ecu? Might be an interesting project. Has anyone tried that?

 

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Andrew, have you sorted this to a satisfactory conclusion yet ?

Finally got around to plugging in my OBD device, to read water temperature.

Fan cuts in at indicated 105 deg C which is after about 5 minutes idling following a run. Using a laser thermometer pointing at "top hose" the thermometer says about 93 deg C.  Fan only runs for a minute or two going off before I expected it !

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Hi Paul,

Yes, today I fitted two switchable 20K thermistors in parallel with the temp sensor. I only need to switch one on and that makes the slow fan come on at just above normal temp on the gauge. With both switched in I can get the fans to run fast if I want; perfect. I ended up stuffing the thermistors in an unused bolt hole in the cylinder head and packing it with exhaust jointing compound (sodium silicate) to make a good thermal bond; works well and the coolant is no longer boiling in the cylinder head.

If your fan is coming on at 93 degrees then fine; 105 seems high as it is above boiling and although the system is pressurised, 105 is a bit near danger point isn't it?

I would not be happy for my fans to come on at 105 degrees; seems too hot (although might be correct for this car; I dunno). My Mercedes never gets above 80 degrees (and coming up to 200K miles with no engine problems ever) and thats how I like it; no stress on the engine or head gasket.

 

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Andrew,

I think 105 is correct - and will cross check with our Fusion which has a similar powertrain.  Although the dash gauge is only an indicator, they normally sit mid range at about 100 deg

Thermostat does not begin to open until 88deg, so is probably not fully open till about 98 deg.  The coolant is pressurised, so that raises the boiling point, and with an antifreeze mixture is probably about 105 deg anyway at STP.  I was hoping the pressure cap on the expansion tank would tell me what the pressure it lifts at - max system pressure for Fiesta system is 1.4bar, which corrresponds to about 128 deg.  Not sure what test pressure would be (not enough on an ecoboost possibly ?) (I'm sure older systems had a radiator pressure cap that was normally labelled with lift pressure, to raise boiling point and hot engines are more efficient (obviously up to a point!))

Both Hillier's "Motor Vehicle Technology" has quite a comprehensive chapter on cooling, and the Bosch Automotive Handbook (which is far too technical!) also has a chapter.

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Yes I think you are right that they are expected to run at around 105 degrees which is when the fans are supposed to come on. However that is a bit high imho, and you are relying on the accuracy of the coolant temp sensor to stop you from boiling up; at 105 degrees there is not much headroom to allow for the tolerance/ageing of the various components meaning  you could boil up before the fans even come on as was happening with me.

My dad used to take the thermostat out of cars as he said it caused more problems than anything else. I realise they are more reliable nowadays and cars are a lot more complex but guess I have inherited a desire to keep my engines on the cool side. I reckon a bit less efficiency is worth the extra cost compared to the cost of replacing a head gasket due to over-stressed engine.

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2 hours ago, EdGasket said:

at 105 degrees there is not much headroom to allow for the tolerance/ageing of the various components

Modern cars use plastic mouldings in the coolant system, and plastic ages very quickly at over 100C. The manufacturers care little about this, as long as it outlasts the warranty it is all good profit for their franchised dealers in the repairs!

The 1l Ecoboost runs very hot, and has a reputation for coolant loss due to burst plastic parts, which totally wrecks the engine before you can even stop. Result is a bill for several £1000.

My 1.8TDCI stays at 80C in normal driving, only going over 90C if pulling a heavy load in low gear up a steep hill. Just over a year ago I noticed steam in the headlights after returning from a shopping trip. It was a burst rubber coolant hose. I repaired it (about £5), topped up the system, and was very soon back on the road, no overheating, no damage, no fuss!

I would always try to use a stat that stays below 100C because when a burst happens appreciably above 100C, a lot of water flashes into steam, and the cooling system will usually cease working immediately.

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