Temperature Gauge rise then fall

asthpsw

Full Member
Apr 23, 2004
524
1
Southampton
Am I right that the Coolant Temperature Sensor that the gauge reads from is in the Cylinder Head ? (why I ask because is there a seperate Temperature sensor for the ECU ?)

Anyway, in these cold mornings I let the Engine warm up by driving along & when it gets to 90DegC I start using the demister on Hot................but I find that the gauge drops back to 60DegC...................Is this because I'm dragging all the Hot Water through the Heater coils & thus taking the heat out of the engine or is it simply that the thermostat has opened and all the cold water that was in the Radiator has now made it's way into the cylinder block ? (this water being cooled even more that it was before I started moving due to all the cold air passing through the radiator due to the motion of the vehicle)

Paul
 

Muttley

Catch that diesel!
Mar 17, 2006
4,987
31
North Kent
The coolant temperature sensor that is in the pipework next to the cylinder head actually contains two sensors, one for the dash gauge (that works off 12V) and one for the ECU (that works off 5V).

The coolant always flows through the heater matrix, even when the thermostat is shut. You can use the cabin heater as an emergency radiator if the thermostat sticks. Selecting the cabin air to hot just moves a flap so that more of the incoming air goes through the heater matrix. If it's very cold outside, then the extra cooling might affect the coolant, but I'd be surprised to see it drop to 60 unless it's well below freezing outside.

If you're seeing the dash gauge rise then fall back to 60, I'd suspect the thermostat is either opening too soon or too wide, or maybe the CTS is on the way out.
 

asthpsw

Full Member
Apr 23, 2004
524
1
Southampton
Thanks for that.......what Coolant Temp Sensor am I looking at when I use the diagnostics on the Climate Control display Channel 51 ?...............because there is no comparison between that and the dial on the dashboard.............only once have I seen them agree (both at 90DegC).......but most of the time the temp on the climate control diagonstics is 18-20 degC...............other times its been static at 37DegC. I hope this is not the sensor that you are refering to that sends info to the ECU ?

Paul
 

Muttley

Catch that diesel!
Mar 17, 2006
4,987
31
North Kent
The 12V sensor only feeds the dashboard gauge. It's for compatibility with cars that have no ECU and only 12V instruments.

The 5V sensor feeds the ECU and the Climatronic gets its feed from there.

Which Climatronic diagnostic code are you viewing for coolant temperature? It should be 51. If that is showing consistently low temperatures then I would expect you are overfuelling. You certainly need to look into it a bit further.
 

asthpsw

Full Member
Apr 23, 2004
524
1
Southampton
Yes I'm looking at code 51

With engine running I Press recirc, then Up arrow.
1c appears on display...............up arrow for Temperature until 51c then recirc again...........this is when I get the reading ?...............unless I'm carrying out this procedure incorrectly?

Paul
 

asthpsw

Full Member
Apr 23, 2004
524
1
Southampton
A couple of things (I can't get to my usual Car Service Centre to get this looked at...Christmas Holiday etc etc).....so :

This morning..............When I first started the car I used the Climatic Diagnostics as detailed above.......Code 51 Read 10 DegC although ambient Temperature was 4DegC. (& probably lower than that before the sun came up)............so in these circumstances could the ECU under fuel ? (because I do get lots of white smoke on start up occasionally & it takes a few seconds for the revs to build up to a stable 900RPM.

Secondly while I was out & about with the Diagnostics still running (still showing 10 DegC) I had need to but the cabin heater on with Air Con engaged (just to slightly demist the screen).............& then immediately the Code on 51 suddenly jumped to 60 DegC then started climing withing a minute to 90 DegC........................(& as outlined in my first post the Temp Gauge dropped slightly maybe 80DegC....I assumed because either the Thermostat opened or I was taking cold air through the Heater matrix cooling it slightly).
So by using the Aircon at the sametime it seems that Code 51 started reading more like it should do ? Anyone know why ?............as I said in the previous post "am I using this correctly".

Reason I ask all this is altimately I don't want to be Under or overfueling like Muttley rightly says but before garage opens next Tuesday I want to be sure of my facts.
Thanks,
Paul
 
Nimbus hosting - Based solely in the UK.