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01 1.8 Coolant temperature sensor - Captain Zetec

Does anyone know whether the Mk3 mondeo 1.8 petrol has two different coolant temperature sensors (one for the guage and one for the ECU) like the Mk2? I know there is one built into the thermostat housing, but not sure whether this is for guage, ECU or both.

The car can be quite difficult to start and lumpy when cold, but fine when warmed up. Leads, plugs, battery etc look OK. Temperature guage reads OK.

01 1.8 Coolant temperature sensor - shadyarea
When i had my mk3 2.0 petrol my cooling fans kept running, i went to local ford dealer and asked what sensor informed the ecu and temp gauge and was told the head unit one.I replaced it and all was fine. Last winter i had no heat in car and temp gauge hardly moved, so i replaced the thermostat which was pricey at £85 but needed doing as it had failed in open position.I understand that the sensor in the thermostast housing
informs the ecu which in turn open's and closes the thermostat as it is electronicly controlled which is why its expensive to replace compared to a normal thermostat.
01 1.8 Coolant temperature sensor - Captain Zetec

Haynes manual shows the temperature sensor on the left hand end of the engine (left viewed from the drivers seat).

What confused me is that , as you say, the thermostat "assembly" also has a temperature sensor, so I wondered if that one drives the gauge?

I'm not convinced the thermostat works electronically - the actual thermostat itself looks very similar to the cheap (~£10) conventional ones used in the Mk2, albeit molded into an assembly.

I was absolutely horrified by the price of the MK3 thermostat assembly! Why the parts for the Mk3 are so much more expensive than the parts for the MK2 I have no idea. I'm really getting fed up with this car, much less reliable than the MK2, and much more expensive to repair.

I think mine has had all of the common faults - rear sub frame, seizing rear calipers, EGR valve, thermostat stuck open, hopefully not the coil pack too.... :o(

Thanks for your help.
01 1.8 Coolant temperature sensor - shadyarea
I have found this thread explains how the thermostat works 4th answer down

01 1.8 Coolant temperature sensor - Captain Zetec

Looks like Haynes might have it wrong then. In the instructions for changing the thermostat they tell you to disconnect the connector from the temperature sensor on the thermostat housing. They appear to make no mention of the fact that it's electronically controlled, and if I remember right I think they state that its a regular thermostat albeit one that cannot be separated from its housing.

I also assumed it was a temperature sensor, because it says something like "50 ohm 98C" inside it.

I suppose since the temperature guage is reading OK then the ECU must be getting the right values from the temperature sensor at the left side of the engine near the throttle housing, so I can rule out that theory. (I presume the ECU drives the temperature guage?)

From what I've read I think the coil pack and the swirl plate actuator will be the next suspects.

I really wish motor manufacturers wouldn't make things so complicated. Nothing wrong with the old type thermostat IMO. This does help to explain the high price for the assembly, but it is still a shock when you're expecting to spend about £10 on a 'stat!!!

01 1.8 Coolant temperature sensor - shadyarea
You mentioned the swirl plate actuator, do you have a rattle sound at idle and a fast idle like a timing chain, i had this around 113k on mine and changed the inlet manifold which was £285 ford wanted over £500 to supply and fit so i did it myself.

This is a common problem the spindles that the swirl plates are attached to wear and then start to rattle not a service part so whole thing has to be changed.I know of one engine that had this problem and the spindle did break and got injested causing valve,guide and piston damage.Apart from manifold and thermostat mine never failed to start and never missed a beat.
01 1.8 Coolant temperature sensor - Captain Zetec

When cold it turns over but won't "catch" sometimes. It can also be rather hesitant and sometimes stutters when cold, particularly when in a high gear at low revs. It's tolerable if you drive in lower gears and keep the revs up, but has a habit of almost stalling when pulling out of junctions.

I read another thread having similar problems and that turned out to be a bad coil pack. I've already replaced the plugs (with the expensive triple platinum NGK ones) and the leads meter out OK with no sign of tracking. I can't understand why it is OK when it warms up though, unless there is some kind of heat related electronics problem. I beleive the coil pack on the other thread was cracked.

The only other thing I can think of would be the manifold absolute pressure and temperature sensor, though a local mechanic told me he's never heard of one failing.

01 1.8 Coolant temperature sensor - shadyarea
Have you tried removing the throttle position sensor and giving a clean and check wires going into it.If it only does when cold could be a mixture issue going back to your op
about cylinder head sensor, might be telling ecu engine is warm which in turn is leaning mixture back.Also have you checked that none of the breather hoses are split going into manifold.
01 1.8 Coolant temperature sensor - Captain Zetec

Now I know that there only appears to be one temperature sensor for both gauge and ECU I'm not convinced there is a problem with the temperature sensor - the guage reads OK. Presumably it is reporting the same temperature data the ECU is using to determine cold start conditions.

Haven't checked all of the hoses yet.
01 1.8 Coolant temperature sensor - shadyarea
If you press in and hold the mileometer reset button and start engine you can access the onboard diagnostics, check to see if any codes show and search for the coolant temperature read out.When my thermostat was gone my gauge after a while would read half and stay there but digital reading was only about 58c when thermostat was fixed gauge read half, digital cycled between 85 and 100c.Screwloose posted on a focus thread about a cooling and gauge issue a few months back, that the ecu only tells you what it wants you know.If i remember when i replaced the head sensor it was only about £8 worth a shout.
01 1.8 Coolant temperature sensor - Captain Zetec

Interesting - didn't know about the onboard diagnostics. Thought that sort of thing could only be obtained using a code reader. I'll check this out when I get a spare moment.

So, with regard to the temperature guage, are you saying it doesn't give a true reading - just an approximation updated when temperature changes radically?

01 1.8 Coolant temperature sensor - shadyarea
The best way to get a accurate reading is to go into the digital read out the rev counter is the best example might say 750rpm on clock the digital will flicker between 700-720.

I cant find the instructions for the mondeo but the focus is the same. read 3/4 down page for self diagnostic mode.

01 1.8 Coolant temperature sensor - Captain Zetec

Thanks. Very useful to know. Wish I'd know this when the EGR failed - could have saved myself £30 + vat having the code read!!!

01 1.8 Coolant temperature sensor - ITman
Dashboard readout does not read all the engine codes. You need a proper code test to read them all.

Have had a similar undercooling problem with my 03 TDCi mondeo. Bought a third party thermostat which still did not fully cure the undercooling, bit the bullet and bought a Ford one and everything is now fine.

Luckily mine is a thermostat only change which only cost me £40

Thermostat is made by Motorcraft 80 - 102 degrees


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