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Odd HVAC behaviour


Harald Hansen
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The winter has arrived here up north, and of course the Disco heater decided to act up. I can't really make heads or tails of the symptoms, though, and hope the forum have some ideas:

Mostly just cold air comes out of the vents, but from time to time a blast of heated air comes on for a minute or two. I've put the temperature control to "HI", which I believe to mean "output heated air no matter what the temperature is". No specific temperature give me much warm air anyway. The engine temp stays rock solid throughout. But still cold air for most of the time. What could cause this?

Other things that probably does not have any bearing on this, but I include them just in case: The fan is on the way out, and sometimes needs a small knock on the casing to start. Once started it keeps going, though. The Webasto fuel heater has given up the ghost, but as far as I can tell that should not impinge on the rest of the system. The lights for the lower HVAC control panel have gone out.

I've rented a heated space for Sunday to look into this. Mucking about outside in -21C does not appeal to me...

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Thanks, Gordon! I had a look, but it seems to me that blend motors usually stop when they fail, not open and close at random? Maybe they're just having a difficult time opening, hence the long period with cold air, and an easier time of it closing, hence the short period with warm air?

I'll have a look with the Hawkeye box to see if I can run them manually and read out their position.

Any other suggestions?

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Hi

You can run a self diagnostic check on the HVAC, I'll paste the info below -

Climate control self diagnostic check

The climate control does a self check on start up, if it has no faults all remains quiet. If it has faults then you get chimes and the climate control panel flashes before returning to normal and the climate control Ecu adjusts to accommodate the faults where possible.

To check what the faults are you need to do a diagnostic check manually which results in fault codes being shown on the Climate control panel. To put it into diagnostic mode press and hold the AUTO switch and the air distribution switch, then turn the ignition switch from off to on. The audible warning sounds and the control panel display illuminates. FC is shown in the LH temperature window and the results of the check are shown as a two digit fault code in the RH temperature window. If a fault is detected, the audible warning sounds three times and the AUTO window on the display flashes on and off for about 20 seconds. If more than one fault is detected, the fault codes cycle in numerical order, at 1 Hz. The audible warning sounds as each fault code is shown. In low light conditions, to avoid false sunlight sensor fault indications, the sunlight sensor should be illuminated with a strong light source.

Fault Codes

00- no faults found

11-incar temp sensor

12- outside temp sensor

13-AC evaporator temp sensor

14-heater radiator temp sensor

21- AC sunlight sensor left

22 right

31 AC/heater air mix flap motor left

32 right

33 AC/heater air direction motor

Hope that helps.

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  • 2 weeks later...

I'm a bit embarassed to admit that the problem was air in the cooling system. After I bled it as per the workshop manual everything was fine. But I have no idea how the air came in or the coolant escaped. There are no signs of any leaks, nor any water in the oil or excessive smoke from the exhaust.

I had a useful Sunday in a rented, heated garage anyways. I managed to fix the fuel burning heater as well, and used the rest of my time to sort out small faults on the car.

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I'm a bit embarassed to admit that the problem was air in the cooling system. After I bled it as per the workshop manual everything was fine. But I have no idea how the air came in or the coolant escaped. There are no signs of any leaks, nor any water in the oil or excessive smoke from the exhaust.

I had a useful Sunday in a rented, heated garage anyways. I managed to fix the fuel burning heater as well, and used the rest of my time to sort out small faults on the car.

Good result in the end at least, and you got some extra jobs sorted as well, bonus!

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