2006 Jetta TDI cooling fans turning on when Aux heater turned on

Hi everyone, the temperature has dipped here in Toronto this past week so the need for using the Aux Heater is a must.

I have never had any issues with the aux heater in previous years and have now started to notice something strange.

When I start the car in the morning, idle is normal but when the temperature dial is set to max and the econ(a/c) button is depressed, the engine rev's up to 1100rpm and the cooling fans turn on as if the a/c is on.

The fans will cycle on and off until I turn the aux heater off.
Heat coming out of the vents is not as warm as it used to be when the aux heater is on. I'm suspecting this due to the cooling fans running.

Once the engine reaches temp while dirving, heat is amazing.

This has never happened to the car before so just wondering what it could possibly be.

Any help as to what would it could be is greatly appreciated!

Thanks,
Anthony

I'd replace your thermostat as a matter of course. It appears that coolant is being directed to the radiator and the temp sensor correctly turning on the fans. This should not happen on a cool engine nor should the heater be able to warm the engine to the point where a good thermostat should open.

If the heater happened to be at fault and was somehow overheating the system, then you'd have warm air in the cabin immediately. Lack of such tells me that you're dealing with a cold block and a warm radiator.

My guess is that the cold coolant in the block overwhelms the warm coolant in the radiator when the engine is starting. You may easily verify this by merely feeling around the block before starting. I'm guessing stone cold. I'll also guess a nice toasty radiator.

"Aux heater"? You're talking about the electric heater inside the cabin, right, or are you talking about a coolant heater?

There are many functions that can be tested with VCDS on your car's AC system.
There are also many faults that can be identified.

It will probably let you find out what is wrong.

I would also try the basic settings tests to see if that would show the problem.

I assumed a Frost Heater or similar. If correct, then I stand by what I said. If you're speaking about the inbuilt electric heater installed on TDI's to defrost windows before the engine makes heat, then we have another situation.

Assuming I was wrong, then you may well have just run into one of those Catch 22 situations. Warm and damp enough for the A/C to cycle on which will cause the fans to run, but just cool enough for you to feel the effects of your A/C as it easily overpowers the heater.

actually its the internal cabin heater that i turn on in the mornings.
engine, coolant and rad are all cold as the car sits outdoors overnight.

any other thoughts?

If it was warm enough wouldnt it still turn the a/c on to assist defrosting?
if it did it would still cycle the fans. I'm not sure if the system operates like that just throwing out ideas

Good point about the AC.

The AC compressor is always turning on this car (unlike the earlier ones); but your idea seems correct - the controls will increase the compressor displacement if it is above about 35 F, to aid in defrosting. This will raise AC pressure and turn on the fans; when it is cool, the fans may reduce pressure in the AC system to the point where they turn off again.

If this is the scenario (and it sounds plausible) then the fans would only turn on and barely get above minimum speed, then turn off again.

When my A/C fans turn on, with the above scenario, they're turning so slowly that I don't even hear them.
OP: are the fans turning at a high rate of speed?

Yes, the fans are turning on and revving to high speed. That's how I noticed they are on as you can hear them.

It seems to me the most likely cause is a high pressure sensor that is reading higher than actual pressure.

But 20 minutes on a VCDS, with a set of gauges would tell you if that was the problem, and probably identify any other problem you had with the system.

did you figure this out? if so what was it?