Coolant temperature sensor TDI90

A2Steve

A2OC Donor
Wales
I put Peppa in to have a slide bolt released 2 weeks ago and seeing as there was snow on the ground I had them to change the thermostat and coolant temperature sender at the same time.

I sourced the parts from @CreweAudi and they worked great for about 10 days, but on the way to work Friday the temp guage didnt move until about 20 minutes into my commute and even then barely hit 60 degrees. The fans were warm long before this so assume the thermostat is ok.

I'm guessing that the coolant temp sender is already naffed. Surprised seeing as I used genuine Audi parts.

My question is is changing the temperature sender a complete coolant drain job or can it be done without losing all the coolant?

CrewAudi have said they can't send me a replacement unless I send the 2 week old one back for testing. I've also offered to buy another new one and then have the faulty one refunded but they don't think they can do that either (not the best service to be honeat)

Took me a while to find the sender because I expected it to be around the front by the thermostat. Thanks to SteveB for helping me find it.

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I think you can change it without draining all the coolant Steve if you are quick. Its then simply a top up with antifreeze. The plastic circlips can be a bit fiddly.
 
I think you can change it without draining all the coolant Steve if you are quick. Its then simply a top up with antifreeze. The plastic circlips can be a bit fiddly.

It looks to be the same sort of connector as a headlight so is that all that holds it in?
 
Steve, I suggest that you read the temp using Vagcom as one of the measuring blocks shows the temp that is being sent to the ECU, there are two circuits in sender unit. one feeds the dashboard gauge and the other the ECU
The two should be within a few deg of each other. It is very rare that both sides of the sender unit fail at the same time
IF you do need to change it then no need to drain the cooling system down as long as the replacement is to hand, pull out the C clip, pull out the sender unit and immediately push the new one in and replace the C clip, if you still have the old original sender then you could put this back in whilst you return the potentially faulty sender to Crewe Audi.
I have done a few and only lost about a cup of coolant, do it when the engine is cold and do the plastic bag over the coolant filler trick such that any loss of coolant causes a vacuum, this does help to minimise coolant lose
Cheers,
Paul
 
Steve, I suggest that you read the temp using Vagcom as one of the measuring blocks shows the temp that is being sent to the ECU, there are two circuits in sender unit. one feeds the dashboard gauge and the other the ECU
The two should be within a few deg of each other. It is very rare that both sides of the sender unit fail at the same time
IF you do need to change it then no need to drain the cooling system down as long as the replacement is to hand, pull out the C clip, pull out the sender unit and immediately push the new one in and replace the C clip, if you still have the old original sender then you could put this back in whilst you return the potentially faulty sender to Crewe Audi.
I have done a few and only lost about a cup of coolant, do it when the engine is cold and do the plastic bag over the coolant filler trick such that any loss of coolant causes a vacuum, this does help to minimise coolant lose
Cheers,
Paul

Hi Paul

I haven't actually got vcds. I've only got a code reader as that is sufficient for the breaking I do. A laptop is on the 'to-do' list.

Thanks for the advice, I stupidly threw out the dud unit because I didn't think I'd have an issue with the new one.

I'll just get a new unit if it comes to it.
 
Are you sure that this is not the consequence of having a correctly functioning thermostat? The coolant temperature sensor is probably at the coolest part of the circuit. In cold weather it will take a long time to get to the "normal" indicated temperature (not the same as the engine temperature). This will especially be the case if the heater is on. You should try this in a 1.2Tdi in cold weather! All perfectly normal (possibly).

RAB
 
Are you sure that this is not the consequence of having a correctly functioning thermostat? The coolant temperature sensor is probably at the coolest part of the circuit. In cold weather it will take a long time to get to the "normal" indicated temperature (not the same as the engine temperature). This will especially be the case if the heater is on. You should try this in a 1.2Tdi in cold weather! All perfectly normal (possibly).

RAB

Hi Rab, the thermostat was changed at the same time using a new Audi item. The car is warming up i.e the fans are blowing hot but the temperature guage isn't moving at all, even after an hours use.
 
Yes, that is a bit too long! It is, in fact, two sensors in one. It would be good to check both with VCDS.

RAB
 
Hi Steve,

I wonder if it might be an air lock or even that the thermostat is actually stuck full open (even though it's new).

As you've a TDi90 you've not got the Webasto additional heater, but you DO have an electric additional heater.
The Webasto heats the coolant in the cooling circuit, so it assists in getting the engine up to temperature.

The electric additional heater only heats the heat exchanger (I believe) and not the cooling circuit.
So, you may well feel cabin heat only because of the electirc heater, and there might be underlying issues elsewhere.

I think I'd start by checking the actual temperature of the top and bottom radiator hoses and make sure they both feel like they're under pressure from water within.
Next would be to use a diagnostic tool (VCDS or similar) to examine the data blocks that look at both electrical circuits of the new temperature sensor - the sensor will give a good indication of actual water temperature.

Perhaps removing the expansion tank cap (when everything is cold) and letting the car idle for 15 to 30 miutes will clear any air lock.

Failing the above, I'm struggling a little.

Cheers
Jeff
 
Hi Jeff

It run fine after the sender and thermostat were fitted 2 weeks ago and worked fine for 10 days. It just now seems that something has failed.
 
Hi Steve,

Yes - I read the orgiinal post.

I'd not agree straight away with an failure of a newly fiited part - although i've had it occur several times even with genuine OE stock.
Air locks can occur after several days of normal running and obviously electrical connections can become faulty too.

Best to give it all a good check over because decent cabin heat in a TDi90 doesn't mean the coolant circuit is working properly.

Cheers
Jeff
 
Steve,

One other thing - hopefully the new sensor was supplied with a new O-ring.
It's important to fit a new O-ring with the new sensor, as some non OE sensor's have slightly different O ring sizes, and a worn O-ring can leak coolant as well as draw air.
The new O-ring is usually quite tight, and therefore it takes a lot more time (a relative term) to fit a new sensor with a new O-ring than it would a new sensor with a used O-ring.
I usually loose about 3/4ltr to 1ltr of fluid when changing this sensor on a TDI.

Hope this helps a little.

Cheers
Jeff
 
That's actually a good point Jeff. Why the sensor doesn't come with the rubber sealant ring is odd considering they are pence to buy separately.
 
Well Jeff, you may have been bang on the money with the airlock. Drove it in to work this morning and after 45 mins the temp gauge hadn't moved. Checked pipe when I parked up and they were hot and under pressure.

Run the car 12 mins with the cap off and the temp gauge warmed up to 90 as it should.

So could it have very well been an airlock?

Will find out on the commute home.
 
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