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Location of coolant temp sensor on AFN TDi?

Discussion in 'A4/S4 forum(B5 Chassis)' started by Nessy, Jun 27, 2013.

  1. Nessy
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    Nessy VW + Audi mad

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    Search as I might I can't find a pic or info as to where exactly the CTS is on my pre-facelift AFN, can anyone please help?
    My temp gauge is giving some very odd and low readings (matching the temperature readings I'm getting on the climate control panel); the thermostat was replaced last month with a Genuine item and the dash vents produce boiling hot air when the heat is turned up.....
    I therefore think the CTS is a prime suspect as the current one (fitted by a previous owner) is 10 yrs old, but haven't a clue where exactly it is to be found!
    Thanks in advance!
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  2. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    Its on the back of the cylinder head in a wee plastic housing.

    If you have VCDS, compare the reading from the engine control unit, with the reading in instruments. If they match, its unlikely to be the CTS.
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  3. Nessy
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    Nessy VW + Audi mad

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    Thanks, I only have VCDS Lite?
    Coming home from work tonite on motorway at 70mph with cruise engaged, suddenly looked down and saw dash temp gauge reading approx 70, quickly put CC panel into block 49c and got a reading of 69-70 degrees the rest of my journey until I pulled off the M4....slowed down and gauge went back up to 90??
    CC panel display going no higher than 87.
    Thermostat I hear you say, but all the time at 70 degrees on the CC panel I was getting roasting hot air on demand.
    In the past whenever I've had dodgy thermostats I've had lukewarm air from the vents?
    As the CTS was last changed in 2003 I was hoping that was the cause?
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  4. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    Typical thermostat failure tbh.

    High speed and fairly low load means the engines not emitting much heat and getting lots of cooling. If the stat doesnt close off when it should, the temperature will drop under those circumstances. When you slow down, the reduced airflow thru the rad, typically also combined with more varied driving raising the heat output, means it comes back up to 90.

    70c is plenty hot enough to give you roasting air, so thats a bit of a red herring. If the stat was jammed open and your coolant temp was only 40c or something then i could see the air getting cooler.

    The CTS has two sensors, one for the ECU and one for the cluster. This is why i said check with VCDS, its unlikely that both sensors will fail simulatneously, so by checking the ECU's output and comparing to the clusters output, lets you see wether its the CTS or not.

    In my experience, CTS's fail a LOT less than people seem to think. Most of the time its the thermostat. People either do the CTS first because its "easier", or change both the CTS and stat together when infact its only the stat at fault.
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  5. Nessy
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    Nessy VW + Audi mad

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    Thanks Aragorn.
    Just to report though that yesterday I changed the CTS and this appears to have sorted things, managed to get a Genuine sensor plus O ring plus clip for less than £20 delivered via Ebay.
    Before the sensor change, the CC display panel (over the last week or so of journeys to work) wouldn't go any higher than 87 or so and the display would be racing up and down between 78 and 87 constantly.
    I'm now getting a constant 90.
    Suppose time will tell if it has cured things, but fingers crossed it has.
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  6. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    90 exactly?

    Doesnt sound right.

    mine runs about 92, and fluctuates a few degrees depending what your doing with the pedal.

    If its not moving from 90 its not working properly.
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  7. Nessy
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    Nessy VW + Audi mad

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    You're spot on as usual!
    Poor choice of words on my part sorry!
    The new CTS shows a display of mostly 90, but yes it does get to 91-92 depending on what I'm doing with my right foot.
    In my ignorance I just put that down to perhaps a very sensitive new CTS,lol....
    When I first learnt about the CC display panel info several years ago I found/thought? that the temperature my car ran at was 90 bang-on and I have never ever seen it higher than that.
    For that reason I'm wondering if my CTS has been slightly playing up for several years....
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  8. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    The usual error people make is using channel 51 rather than 49. 51 displays 90 constantly once the engines been running a while, whereas 49 shows the actual temperature.
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  9. Nessy
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    Nessy VW + Audi mad

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    Yes, I've been guilty of that.......
    The new sensor has definitely done the trick though, no more erratic gauge readings!
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