Hello all,

I'm new to the forum and looking for a (hopefully) quick answer for an issue I've recently experienced with my 1999 A4 2.8Q....

Recently I had driven for about 20 minutes when the coolant temp gauge on the dash went to hot and of course gave the warning noise and began to flash. As a precaution, I pulled off the road to let the car cool down. I also checked the reservoir and the coolant level appeared to be fine. However, after 30-40 minutes of waiting the gauge still had not gone down appreciably.

I managed to get home with no issues and let the car sit overnight. But now even before starting, the gauge jumps right back up past the center point, instead of all the way to the left (cold) where it should normally begin. Of course, after starting it only takes about 2 minutes before the gauge goes all the way back up to hot and the warnings begin again.

I am hesitant to drive it now, even to a repair shop. Is that OK? Any ideas? Faulty ECT sensor or even a thermostat issue? Any assistance or advice here would be greatly appreciated. THANKS!

 

If it shows half range after after sitting overnight, it cannot be the t-stat.
My first GUESS would be sensor. Not sure if a '99 has a 4-pin which is the coolant temp sensor(for the ECM) and sender for the gauge in one or if it has 2 separate sensors.
I believe either way, they will be on the coolant pipes at the rear of the engine. Unplug it (or them) and then see if the gauge shows cold. Don't try to start the car, it'll flood. With an ECT sensor disconnected it will read that as -40.
If it reads cold, replace the sensor/sender.
If I had access to a diagram, I'd tell you exactly where the sensor is and what color the wires are but I don't, sorry.
If the coolant level is good with no signs of actually overheating, you should be fine driving to a garage.

 

Hi, thanks. I just drove the car around the neighborhood for a few minutes to see what it would do. The gauge read above center (towards hot) again when I turned the ignition, as I previously mentioned, but then it actually moved back to the normal center position after about a minute of driving. Unfortunately, before I could get too excited it did not stay there for long! The needle crept back up towards hot again. This time I was able to return home before it reached the warning level.

There appears to be a neat video on how to remove/replace the CT sensor on the Audi/VW 2.8 engine here: .

So, I'm going to try that first, thanks again!

 

Almost has to be the sensor. These go bad, I've had to change mine, and keep a spare in the garage. Just be sure to change it with the coolant cold, not hot or pressurized. There is also a retaining clip known to break easily, so you are advised to get one of those too.

 

It's the sensor - change it and be done.

 

There's two coolant temp sensors on my 2001 A4 B5 2.4 (APS engine & right hand drive)
The one between the engine and the firewall. Drivers side.
And the one on the radiator hose between the radiator and the alternator area. Drivers side.

Both are Green plastic.
Ones got a broken clip, so make sure to get sensor with clip if you ever buy one.

Which sensor sends a signal to what? Each has 4 pins, so 2 sensors in each unit.

On my heater controls hack I find both 49 and 51 are coolant temps.
…
49 & 51 & the dash gauge are all consistent until the temp gets to about 70°C.
Then 41 & the dash gauge stay consistent up till about 80°C,
but 51 keeps going up till it gets to 90°C and has neverr got higher than that.
,
About that time (51 @ 90°C) 49 & the dashgauge stop being equal and waiver from each other.

My electric fan comes on as soon has the engine starts, but does vary depending on temp.
I do think the cars not heating up as quick as it should. 10 mins around town and its still cold. Hardly ever gets to 90°C, but does sometimes.

I do think the thermostat is stuck open (maybe?)
And I had the radiator partly covered to get it to heat up quicker and one day it blew most of the coolant out after it got a bit over 93°C on the dashboard.

The thermostat housings leaking (onto the alternator area) after a drive most of the time.
I'm expecting its the plastic version and have ordered a 2nd hand Aluminium version 078121121j.
(the plastic ones 078121121k) NZ$65 inc p&p from Latavia. €13 for the part)

So does anybody know what each of the 4 temp sender signals goes to?

 

Normally the outside temp here where I live in NewZealand is between -2°C & 14°C.
This week we have been getting record temps of 30°C+. 32-34
So after changing the oil today (Man, you could not find a easyier car to change the oil on.....) I took it out for a drive.
Temps got up quickly, got to 90°C on the dash, peaked at 93°C on setting 49.
I notice that setting 51 starts at about 70°C too, peaked at 90°C.

While each different temp is close, they are definatly not connected.

I think I will just swap out both temp sender units and see what happens.
Cost? About NZ$70.
Just gotta figure out which sort they are. Theres two options, square green plug, or oval green plug.
They are not a easy clip to get undone.
The front ones connected to the radiators bottom hose too.