Saxo - temperature light on coolant ok issue

[PaulC5]

I know you do not want to replace the blue temp sensor but this might be putting the high temp warning on if it is faulty. It will certainly cost a lot less than all the other things you are doing. You could also try just unplugging it according to ozvtr and see what happens.

[ozvtr]

The "bottom" hose goes around the back of the engine to the water pump. A fairly long way, and a possibility of a blockage here.

I have had water flowing from a hose from the top hose outlet through the top hose into the radiator (with the new cap on the expansion tank) and then through the bottom hose up to the water pump housing when the water pump was being replaced.

I have had radiators that have water flush through them fine, but fail to perform at highway speeds. These were the old style copper and brass style of the 70's and 80's. I must admit my 2002 Mitsubishi Lancer and the C3's aluminium radiators have not given me any trouble.

I replaced the rad mainly due to the previous owner who was told it would be their next point of work by his "rather lame and lazy garage" - (in my opinion they did not want the work on such an old car) The outside looked fine and I was not certain that head gasket repair juice had not been attempted at some point despite the fact that the rest of the system did look clean. Plus it was not that expensive and I felt with the new head skim and trying to make a reliable car for my lad it was silly not to. Surely if the water flow through the top hose through the rad and bottom hose this is a good enough check for a blockage here.

I have repaired 3 C3's with TU3JP (KFV) engines in them. I am currently running "head gasket repair juice" in one, because of (as emperor Palpatine might say) my lack of vision! I simply replaced the head gasket in the first one that I did and nothing else. I have since learned (the hard way) that, if overheated, the TU3 cylinder heads turn into bananas! I have also found the OEM head gaskets are @#$%!! But that's another story. So my last two have had trips to the machine shop and I do not regret it.
In the first engine I did, some (expletive) dropped what looks like a nut down the intake, and it has damaged the No4 piston crown and the squish band of the cylinder head! It was not worth taking it to a machine shop. So I just moved on. It began to have "hard starts" in the mornings and when the car had been left standing for a while. Long story short, coolant was leaking into cylinders 1 and 2, causing the hard starts. Added "head gasket repair juice", problem solved! Dodgy as hell? You bet, but as I said, the engine is not worth performing any sort of repairs. I have 2 other known good engines to go in there at any time.

Again, does the radiator fan come on when the temp light comes on?

no it doesn't but that is because the rad is not hot enough to trigger the sensor 65 deg tops on the top hose area (unless the sensor is the same as the green red light one? but even this only gets to about 85 tops) It did work before the engine was taken apart so I have seen it work but never since I have reassembled - the wiring all checks out - I have yet to try to see if the motor powers but I am confident it will and I did not yank or stretch the cables so have no reason to suspect that the cables are damaged but appreciate it might be but but the temperature of the unit tells me that the rad fan sensor would be silly to even consider coming on as it is not hot enough to warrant it despite the temp light coming on and the head being around 100 to 105 in places. The thermostat housing still registers not hot enough to open!

That's not right. I am assuming this is a fuel injected engine? If it's carburetted I can't help (sorry). If it is fuel injected, the engine ECU runs the radiator fan. There is only one coolant temperature probe that the engine ECU uses...the green one. The engine ECU uses this probe to determine when to go into closed loop mode, what the air/fuel mixture should be, when to turn the temp light on, when to turn the radiator fan on and so on. It doesn't matter what the actual coolant temperature is, If the engine ECU "thinks" the temp is "whatever" it will perform according to what it "thinks" the temp is! The radiator fan should be on way before the temp light comes on. If the temp light comes on and the radiator fan is not running then you have a problem! Again, if the engine ECU "thinks" there is an over temp then the fan SHOULD be on! Irrespective of the ACTUAL coolant temp.
Measuring the temp from the outside might not yield accurate results. I will try and get resistance values for the green sensor at various coolant temps.
A quick test of the indicator circuit is to unplug the coolant temp plug. The coolant temp light should come on and the radiator fan should come on full blast! This might take a few seconds, be patient. Again if you're radiator fan doesn't come on there is a problem in the fan circuit!

[ozvtr]

I did a few tests.

At 30'C (ambient and engine cold) sensor reads 5.3KOhms. Yep, welcome to a nice cool Australian morning! At, say, 20'C I would guess 6-7K Ohms.
Thermostat opens and top hose gets hot at 600Ohms. That's probably around 85'C coolant temp.
Radiator fan comes on around 380Ohms. Not sure, around 100'C?
With the car moving and a good cooling system, the coolant temp should be around 90'c +/- about 5'C.

This is for the green temp sender.

This is a Negative Temperature Coefficient (NTC) thermister. The higher the temp the lower the resistance. But it's logarithmic, not linear (as you can see).

So If you monitor the resistance of the sender and the thermostat opens around 600 Ohms, you'll know that there is nothing wrong with the thermostat and nothing wrong with the temp sender.
I would then suspect the radiator fan circuit.

Now as far as the light coming on while the car is moving forward, I have no clues at this time. I would concentrate on one problem at a time and get the radiator fan working first.
Is it possible that you have two faults? Sounds like it to me.

[alviseven]

Thats great to have the resistant readings and yes the car is not a carburettor car.

Pretty certain that at the moment the temp light goes out when the green sensor is disconnected because I have considered running it like this!

If the temp light comes on and the radiator fan is not running then you have a problem! Again, if the engine ECU "thinks" there is an over temp then the fan SHOULD be on! Irrespective of the ACTUAL coolant temp.

So following on from your ECU notes - wondered if this is actually earth related? Here is the joined up thinking I have made from you comments.
I assume the ECU is the black pancake type box on the OSF wheel arch (UK) - cambelt end. Concurrently with this engine work I have pushed on with the rot repair in this top arch panel which was not visible till after we removed said pancake box - never disconnected or dragged tight but moved up out of the way and wiggled around to get both the water pump in and the cambelt changed - we did that is two jobs.
I have only just finished this work on the topside this week so it has not yet been run with the ECU fully remounted and screwed down. Could this be part of the issue as the return via an earth would not be either working or as good?

Wonder now if I start the car and do the green plug off test if the fan will cut in as the ECU was properly mounted when the car was first bought and we were testing the car after finding the thermostat jammed solid and hoped it might then be ok.

I have since learned (the hard way) that, if overheated, the TU3 cylinder heads turn into bananas!

Me too with the Honda D16 engine in my Rovers. Did the first one after a steel rule check and hoped but not to be. The Rover 214 engines used to keep the chap I used in business as he has several every day to machine! With this TU head being aluminium I could not risk it so stripped the valves and got it skimmed and machinist confirmed it was identifying as a banana and he reckoned to have taken almost 1mm off to get flat.
Head gasket sets I have had the other - Rover Honda engine I only use the OEM expensive ones but this time I was doing a cheap job to check the car would work so it got ebays finest!

So I think another engine test with a multi meter and the ECU fixed back down in position is due - have to be next weekend now as I am off in the Sunshine here but only about 12 degree on a Fiat 500 club trip out and about.

Many thanks

[PaulC5]

I know ozvtr is saying the blue temp sensor does not do anything but in this topic ozvtr says different and the op replaced the blue sensor and the false high temperature warnings went viewtopic.php?t=87195

[ozvtr]

I know ozvtr is saying the blue temp sensor does not do anything but in this topic ozvtr says different and the op replaced the blue sensor and the false high temperature warnings went viewtopic.php?t=87195

I have never said it does nothing. I said you can disconnect it and it wont matter (to the operation of the engine).
The "blue" sensor shorts the "green" sensor to ground when the No1 cylinder head temperature gets to 120'C. The theory is; if there is no coolant in the thermostat housing the green sensor will only be measuring air, not coolant. Theoretically this air will be "cool" but the cylinder head will just get hotter and hotter as there is now no coolant in it. So the blue sensor is erroneously labeled as the 'coolant level sensor'. It should be labeled as 'cylinder head over temp sensor'.
The blue sensor is normally open circuit, meaning if you disconnect the blue electrical connector (under normal circumstances) nothing would happen.
Yes, if the blue sensor were to randomly short out, or short at too low a temp, it would cause the over temp light to come on. I never denied that. And is it still a possible suspect...yes.

HOWEVER, the big problem is the fact that the radiator fan does not come on when the over temp light comes on. As I said before, solve one problem at a time, the fan not working is a big problem!

[ozvtr]

Pretty certain that at the moment the temp light goes out when the green sensor is disconnected because I have considered running it like this!

Please check this. It will determine how to fool the engine ECU into turning on the radiator fan.

So following on from your ECU notes - wondered if this is actually earth related?

While the metal shell of the ECU will be connected to earth through the electrical connector, the ECU wont rely on the shell being physically connected to earth (via the car chassis). In fact in the C3, the ECU sits in the battery box and the shell is isolated from earth. Earth the shell or don't earth the shell it wont matter. HOWEVER there must be one or more good earths supplied to the ECU from the wiring harness!
The big advantage of having the ECU connected to the car chassis is conducing heat away from the ECU and into the car chassis.

[Stickyfinger]

There is a LOT of effort going into "replace the bl@@dy temp sensors as they are cheep, most likely the cause and easy to do".................

[alviseven]

alviseven wrote: Today, 08:30
Pretty certain that at the moment the temp light goes out when the green sensor is disconnected because I have considered running it like this!
Please check this. It will determine how to fool the engine ECU into turning on the radiator fan.

Right had ten minutes before tea and was determined to get something done this weekend.

Started car up and all went well temp light on briefly etc and then off. ECU bolted down etc

unplugged the green sensor plug and nothing happens - no light comes on no fan starts to work.

car by this time a little warm so tried plugging in the old probably ok sensor but not screwed into the head - nothing happens nothing leave for a few minutes and even turn engine off and then on again so ecu is looking for it etc

Unplug this no head attached sensor no light on no rad fan kick on - so we have a fan circuit problem

Curiously when I then plug the sensor lead back onto the head by now a little warm but not toasty by a long way the tick over suddenly jumps to about 1500 revs and stays at this level until I unplug the sensor again when it drops back down to a normal 1000 or so rpm. Tried this with the spare sensor after this and the same thing happened even with the unit off the car.
Not had this before but tells me that something definitely electrically iffy here!
Cannot surely have put something back together wrong - no fuses broken in the engine mounted fuse box - sorry I was being lazy in best clothes and door jam is tight at the moment so was not crawling round inside the car - not sure where the box is right now anyway but have haynes manual so will locate next time.
Assume that the rise in revs is like a cold start choke effect on the engine even though it is getting warm and quite capable of running without

Just thought you would like an update and tell me I told you so!
many thanks

[PaulC5]

Plugging in the old sensor but not screwed into the head you could heat it up and see what happens, possibly with a hair dryer or gently with a blow lamp but you need to get it to over 96°C (if like our C3) so the fan switches on if it is working. Be careful not to damage the wiring/connector, you have enough to fix already.

[ozvtr]

I have been very wrong. Sorry to all concerned. I didn't think I had Saxo circuit diagrams, but I do.

I will need your RPO number to check my circuit diagram. It's on the placard that has the tyre pressures on it. ABS or no ABS?

Virtually everything is run directly, and does not go through the engine ECU!

If I have the correct circuit diagram, there is only the one green engine coolant temp sensor. And it only tells the engine ECU the coolant temperature for engine management purposes. Nothing else. The engine ECU does not run the over temp light OR the radiator fan.

The over temp light is driven directly from the blue connector under the thermostat housing. So it still works as outlined before (the whole 120'C thing) but it doesn't short out the green sensor. So if the light is coming on prematurely, then the blue sensor is faulty. Replace the sensor or just disconnect the plug. You know the light works so there is no point in testing that.

The radiator fan is run separately from the engine ECU. There is a 3 pin blue thermo switch that appears to be installed on the radiator. On the cars' near side.
If you short contacts 1 and 3 of the wiring loom plug, the fan should come on at ANY TIME. Its connected directly to the battery!
I don't know how you might test the thermo switch on the radiator. If the cooling system is working correctly, it could between 20 to 30 minutes for the radiator fan to come on (when idling). If you don't have aircon, the radiator fan rarely comes on!
You could remove the thermo switch and put it in boiling water and see if it shorts out, but I don't have the specs on this switch.

The engine only has the upstream oxy sensor and no catalytic converter. Is that right?

I have no idea why the engine revs go up when the temp sensor is connected. However, it only seems to be doing that since the engine ECU was secured to the cars' chassis? Is that right? Try isolating the ECU from the chassis?

[ozvtr]

These are the cooling system options by model.
Any of these levels look familiar? There are all sorts of variants including single and double radiator fans. With relays and without.
The RPO number might whittle the number of options down. I'm just guessing without the RPO number.
"Level 1" is the simplest model.

[alviseven]

Right another 30 mins in the garage before tea and getting back to paperwork.

Took your advice and found the thermocouple in the rad and connected pins 1 and 3 and of course the rad fan fired off straight away so was glad to see it work - always have thought the rad never got hot enough to fire it so ...

Started car up and decided to be brave and turned the heater off! Drove out of the garage in vain search for this RPO number - not got any numbers on the ticket in the OS drivers door where the tyre pressures are but do have a chassis plate inside the rear tailgate and could give you engine number and chassis number anyway but ...

Have moved the car about and got the doors open and have found the fuse box inside my UK passenger side below the dash with a OBD port which looks weird but think that is due to its colours but that's another question for another day.

Car warmed up quite nicely with no water loss as expected and with my laser thermometer kept a strict eye on the temp of the thermostat housing and the temp rose but at about 78 there is a stutter in the engine and check the dash and the temp light is on - un plug the blue plug and the light goes out - confirming that this is the correct control plug (and probably others who said change this one as its cheap)

There is a LOT of effort going into "replace the bl@@dy temp sensors as they are cheap

sorry to those who suggested this solution but I did originally diagnosed that the temp sender unit was giving a dodgy reading but could not find a blue unit which matched the description of sender unit on ebay only a green one hence I bought that one and left the other alone - never thought to unplug it when hot and see which one operated which light!

However will now redouble efforts to find replacement and fit.

The news gets better because having removed the temp feed light so it was not on I kept watching the temp on the cambelt end of the head and the thermostat housing as it gradually rose through the 80 deg range. Top rad hose remains relatively cool but at just over 90 deg the top hose starts to warm up and suddenly now we are in proper temp range and ignoring the temp light - kept monitoring the temp so I was not worrying but was waiting for the fan to cut in
Even got a bit cocky and rev the engine to gain the heat range and then the fan cut in about 95 deg on the thermostat and crucially cut off when the temp had come back down.

So the engine set up is more than able to control its own destiny but the temp light sender must be foobarred!

For info my car is a 1998 one so far too late not to be cat fitted (UK post 92) and it does have a lamda O2 sensor in the exhaust and what looks like a cat about gearbox level under the floor
It does not appear to have ABS (it is a basic 1.1 special edition) as it has not got a block inline on the brake pipes or any ABS suspicious wiring under the bonnet but have not looked at the discs or the rear drums but surely rear drums give lack of ABS away?

So may thanks all of you for pushing me to this point - got one pleased young lad upstairs revising for exams

Next jobs
Ordered a new sunroof seal
Rebleed and replace the brake fluid - easy but not sure how old it might be
Sort a small exhaust hole somewhere (unless my ears deceive me?)
Sort the airbag fault which I gather is the connections under the seat getting grubby but have not investigated yet.

[PaulC5]

Good to see the engine fan is running as it should since if not then that would have been a worry as ozvtr rightly pointed out. I would also like to give ozvtr credit for all the effort he has put in to this topic, it must have taken a lot of time.

Assuming the clutch is hydraulic, when you replace the brake fluid also do the clutch fluid since it will share the same reservoir but beware that the outlet to the clutch will be higher up so the fluid level has to be kept high. Pumping the clutch pedal does not work that well and I have found it easiest to suck the fluid through the slave cylinder bleed screw for which I have a Mityvac pump but a large syringe would do.

Cars with rear drums do have abs, our 2010 C3 had drums and abs but I cannot remember if the 2003 C3 had it.

[ozvtr]

I'm sorry for the cock up, but I'm glad you got it solved it in the end.
Apparently I stumbled across the correct circuit diagram. So there is no need to go any further with that.

I'm not lucky enough to be able to fire the parts cannon at a problem...it never works (for me). So, I have to do things the hard way! Which involves figuring out how a thing works, then come up with a theory of what might have gone wrong, test the theory, THEN replace the part. Long and convoluted I know, but that's my lot in life.