Weird HVAC issue

mannyo · #1 29th August 2015, 07:54 AM

Here is a strange one, had it a few times over the last couple of months.

Sometimes the HVAC develops a mind of its own, take yesterday for example.

Driving up the M3 towards London with no problems, then suddenly the system increases fan speed to guesstimate 3/4 and 15 seconds later drops back to normal. Repeats over and eventually stops after 10mins.

Whilst the fan speed is at 3/4s and you press the HVAC off button, the HVAC switches off but the heater fan continues to run at 3/4 speed almost like the fan is getting power from another source since the HVAC off should have stopped the fan.

I can get the system to restore normal behaviour if I hit the off button whilst the fan is at normal speed and then switch it back on again.

No faults in VCDS, so I am guessing relay or something. Not sure how its wired, but could the solar roof relay cause this. Last night it was almost dark when this happened, so not sure.

snapdragon · #2 29th August 2015, 09:29 AM

There is a blower motor control unit 4E0820521
It might be worth looking into that or looking for a reasonably-priced replacement and just swapping it.
The blower motor plugs into it - it is about a foot away looking at the cable.
I seem to recall seeing it when I changed the pollen filter as I remember those copper coils.

mannyo · #3 1st September 2015, 04:15 PM

Good call, its the only thing I can think of causing any issue. Did it again last night on the way from Herts back to Dorset.

mannyo · #4 1st September 2015, 08:28 PM

Found one on ebay, looks complicated to replace it though as it appears to be located beneath the heater fan.

mannyo · #5 5th September 2015, 02:29 PM

Had a look at the location of the control unit, you can see it through the flaps after removing the cover and pollen filter. Looks reasonably easy to change, only headache is going to be the flap motor which I may have to remove to get the top off.

The flap motor is also jammed and has been since I got the car, so maybe a chance to replace that as well if I can find the part number.

ScottD3 · #6 4th October 2015, 09:01 PM

Mannyo,

When you do this, I guess you'll get pics and do a nice little post?

be interesting to see.

Good luck.

mannyo · #7 7th October 2015, 06:35 PM

Well I did a VCDS scan and J126 Blower controller has thrown a few fault codes lately so it does appear to be the likely suspect. I can get a third party one from Germany for around £100 plus delivery, or perhaps risk a used one from A8parts.

J126 is the electronic resistor circuit.

01844 - Control Module for Fresh Air Blower (J126)
004 - No Signal/Communication - Intermittent
Freeze Frame:
Fault Status: 00100100
Fault Priority: 3
Fault Frequency: 29
Reset counter: 112
Mileage: 228992 km
Time Indication: 0
Date: 2015.06.26
Time: 17:18:32

Looks a right ballache to change however.

mannyo · #8 3rd November 2015, 07:00 AM

I've still not got this sorted, problem seems to be getting worse though so I need to as soon as I can get the blower controller.

Symptoms,
Fan runs perfectly fine 90% of the time but the other 10% the fan runs at about 60-70% output and you have no control over it at all. Even turning off the HVAC completely the heater fan will continue to run until the engine is switched off.

At least the heat/cooling still works, its just annoying because you end up with a short period of overheating or cooling the interior.

Mechcanico Lee · #9 3rd November 2015, 08:45 AM

Like already said sounds like the fan control throwing a wobbler , some of the bmws do this, x5s and the likes, fan just goes up and down on its own , they can still work with ignition off and often flatten battery over night .

Sure i have done one of these on the passat looks just like the one in the picture SnapD POSTED