Temperature gauge question

Malcyb

Member
Hi,

Starting a new thread as I made the mistake of hiding my questions in the “today I...” thread. So I’ve had some kind help from some members (thanks @depronman and @A2Z especially) but a bit stumped now.

First, to go back to the original problem, that the temperature gauge never reaches 90, and only gradually wanders between 70 and 80 on any kind of journey. @depronman gave me some pointers that might help establish whether it’s the thermostat or temperature sensor that‘s at fault and I had decided that I might start by replacing the sensor. However looking around I can’t see any green ended sensor, all I can see is this, looking down from just by the coolant reservoir...

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is this the temperature sensor or am I completely in the wrong place? The connector is 1J0 973 702, a 2 pin connector, not 4 pin. I found this old thread referring to some “oddball” 2 pin sensor....


so is that what I’ve got here?

Having said all that, I’m now having doubts that it’s anything to do with the sensor since on further reading I found that it seemed to be to blame for symptoms that people had such as the temperature wildly jumping all over the place. I’ve never seen it do that, it’s just always on the low side and wanders gently around. Might this suggest this isn’t the culprit and I’d be better off looking at the thermostat?

Thanks
 
The symptoms you describe, IMO, point to a faulty thermostat. I had a similar problem on mybTDi. It was sorted by changing the thermostat. Be aware that some aftermarket ones do not fit without modifying the housing slightly. I got an (expensive) audi one which fitted pefectly. As a result I have a new ECP one here which is yours for the price of postage!
 
Hi. This has been covered before. Try searching posts by this user.
AudiLondonLion
If you buy the temp sensor make sure to buy an extra retaining clip and o ring. They do tend to disappear if dropped!
Good luck.
Steve
P. S. Think the photo is of your fuel temp sensor.
 
The temp sensor is on the back of the engine block, top right hand side, when facing the engine. You will need to take the engine cover off and initially at least, you can feel it better than you can see it. You will loose some coolant so you will need to have some of that spare. It's much easier to replace the sensor than the thermostat but if you are keen then doing both at once will mean only having to refill the coolant once and pretty much a guaranteed fix

The temperature gauge and engine (ECU) temp are not connected - there are two separate sensors in the same unit, so one can fail without the other being affected. If you have no other signs of the engine running cool, like taking forever to heat up and poor fuel consumption, then it is possible that the sensor is the only culprit. The fact that the displayed temp varies could mean that the thermostat is at fault as well. You can display engine temp using OBDII diagnostics (if you have the tools) and compare that with the dashboard temp which might also help identify if there is more than one problem.
 
What you have shown in the picture is not the temp sensor
It’s got 4 wires going to it and when you remove the plug from the end it’s green in colour as per the picture in the link you posted in the today thread
It is located in the general area under the tandem pump
It can fail in a number of ways and generally reading low is one of them
I have used the ones that euro car parts sell without problem but only when I’ve not been able to get a genuine one or the owner of the A2 did not wish to pay Audi prices

Being honest I think it’s more likely to be the thermostat but that is a lot harder to change, would will need to drain the coolant and it’s still about 70 / 30 in favour of the stat. I have often changed both at the same time to know that I will have cured the problem but changing the sensor is a logical first place to start. If it does not fix it then you have a spare known good sensor. They are notorious for failing so likely will come into use in the future anyway

Paul


Sent from my iPhone using Tapatalk
 
The item in your picture is the FUEL Temp sensor, not water temp sensor. It is No 21 in below diagram

1585987725981.png


It is behind and below the tandem pump. The thermostat is located behind the upper end of the pipe with clip No32 on it.
 
Thanks for the replies everyone. I’ll have a better poke around for the sensor and decide whether I’ve got the stomach for changing it myself. Everyone says it’s easy but I don‘t believe that yet ;)
 
The dual coolant temperature sensor is a very simple bit of electronics, housing two identical thermistors. One thermistor goes to the gauge on the cluster and the other goes to the engine management unit.
When I first investigated this sensor I expected to see one thermistor with a bit of electronic trickery so that the cluster gauge and the engine ECU both saw the same resistance from the same thermistor. That way there would be correlation between the gauge reading and the ECU's calculations. But, with two totally independent circuits, it is possible to have one circuit fail whilst the other does its job as usual. By having two totally independent thermistors and electrical circuits, erratic temperature readings on the cluster's gauge doesn't necessarily mean the ECU is receiving a dubious signal. Equally, there's the possibility that the ECU could be receiving a dubious signal and therefore injecting too much fuel, whilst the temp gauge in the cluster shows a sensible reading.

If the cluster-side thermistor has failed completely (i.e. it's either gone totally open-circuit or has shorted across its terminals), then you'll find a fault code in the instrument cluster. Similarly, if the ECU-side thermistor has failed completely, then you'll find a fault code in the ECU. However, a vast majority of thermistors fail gradually, causing their calibration to be lost. As such, a true 90 degrees celcius is perhaps reported to the ECU as being a mere 25 degrees celcius. This scenario will not cause a fault code to be logged because the ECU has no way of knowing that its 25-degree reading is actually wholly inaccurate.
Using VCDS, the only way to test a dual temperature sensor is to manually compare the temperature readings of the ECU-side thermistor and the cluster-side thermistor by checking the appropriate measuring blocks. Fault codes only appear in the case of total sensor failure or when the thermistor's reading is totally implausible. The troublesome nature of coolant temperature sensor diagnosis has resulted in Colour DIS having a specific feature for comparing the readings from the two thermistors. Using Colour DIS, you can quickly check whether the two thermistors are in agreement.

My belief is that, if you've bought an A2 without any history of the dual temp sensor having been changed, you should change it. You could change the thermostat and find that the engine now gets to full temperature and that the gauge in the instrument cluster points to 90, only to not realise that the ECU-side thermistor has failed and that your engine is constantly over-injecting fuel. Of course, if you have electronic diagnostic gear, you can test it if you'd rather.

Based on everything that's been written, I'm also of the opinion that your thermostat is the culprit, but I'd change both. If you're draining the coolant, it makes sense, and the cost of the temperature sensor adds little to the total bill.

This photo from the TDI120 thread clearly shows the location of the green temperature sensor. It's the only green thing in the entire photo. Of course, it'll have a black electrical connector over it, meaning you won't see anywhere near as much green when you go looking for it under the bonnet. However, this photo will help you to locate it.

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Cheers,

Tom
 
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Not sure if it makes you feel better but it’s a common problem and one that I’m still battling with now. Mine HAD been running fine and getting up to temperature, however in January it then developed a coolant leak from the area around the flange junction where the sensor fits in. I changed the flange, sensor and thermostat to refresh the parts and stop the leak but since then the b***dy thing refuses to get up to temperature unless I take it for a long blast down the motorway.

Two weeks of self isolation has meant that I haven’t had to think about it but today I’m finally able to leave the house and go to work, so I’ll see how it runs. I was going to refit the old thermostat to see what difference it makes, but as it’s such a pig to do I won’t touch it until I’ve got the right tool to remove the lower bolt.

I have tested the old thermostat and it does work. It just nicely demonstrates to me the old saying of “if it ain’t broke, don’t try to fix it”.

I’ll follow your thread with interest to see how you get on!
 
Thanks again. TBH in the grand scheme of things I’m not hugely worried about this issue. On it’s own I’m not that bothered about the temp readout being a bit low, the only thing that slightly irks me is that I think economy in the low 50s should be better for a 90 TDI. In part, I was looking for a project I could cut my teeth on, and build a bit of confidence, to while away the days of self isolation.
 
Thanks again. TBH in the grand scheme of things I’m not hugely worried about this issue. On it’s own I’m not that bothered about the temp readout being a bit low, the only thing that slightly irks me is that I think economy in the low 50s should be better for a 90 TDI. In part, I was looking for a project I could cut my teeth on, and build a bit of confidence, to while away the days of self isolation.

Low 50's is not so far out if you do mostly short journeys.
 
Mine is the same, albeit a 75 TDi. Economy is generally around 50-52, which I consider poor given the whole concept of the car. I’m sure the warmer weather will help raise the running temperature and the economy, but the issue will return in the winter and I’d like to get it sorted.
 
Low 50's is not so far out if you do mostly short journeys.

I know, but I'm only seeing 53 or so even on longer motorway journeys driving fairly sensibly, and that's just what DIS shows, not calculated. I've seen plenty of people report figures in the 60s for 90 TDI so I'd quite like to see better.
 
I know, but I'm only seeing 53 or so even on longer motorway journeys driving fairly sensibly, and that's just what DIS shows, not calculated. I've seen plenty of people report figures in the 60s for 90 TDI so I'd quite like to see better.

Yeah, agreed, on all short journeys mine does very low 50's but more nomal 70m round trips to work it's nearer 57, my TDi 75 is always better though.
 
Not sure if it makes you feel better but it’s a common problem and one that I’m still battling with now. Mine HAD been running fine and getting up to temperature, however..... I changed the flange, sensor and thermostat to refresh the parts and stop the leak but since then the b***dy thing refuses to get up to temperature unless I take it for a long blast down the motorway.

Similar issue with my 75 after changing the temperature sensor. Took many attempts at bleeding before the engine seemed to warm up as quickly as before the change. Take a look at the threads on bleed points in the cooling system.
 
Aha, now we might be getting somewhere, that sounds like a logical explanation. Is there a diagram or photo to help identify the bleed point(s)?
 
Google is your friend.


If you have a Webasto auxiliary heater (as I have) there is also a bleed point near that.
 
Slightly confused - are you saying I need to drain the cooling system after replacing the sensor, or just saying it needs bleeding to get air out like my central heating?
 
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