Climate control stuck in one setting - advice please

[Tamarillo]

A4 1998 2.4, 160,000km. RHD in NZ. Car well serviced and in excel nick.
Climate control stuck on giving slightly cold air only ever.
If left on auto will just slightly cool air (outside temp 20-24c). On manual I can set any temp I like but it does nothing, cannot heat air ever.
Fan comes on and off randomly it seems, even on manual it won't do what the bar graph says on its display, but will come on sometimes, mostly on start-up.
Diagnosed (inexpertly) as needing new thermostat. If this was simple I'd do it but it seems it's a big job on the V6 as it is almost a full cam belt job, and it is not in need of that for another 40,000km.
And I am not confident of this diagnosis.
Water temp comes up to 80 and pretty much stays there, drops to 75 and will rise to 85 at standstill. There is hot water in heater pipes leading through bulkhead so can't see why stuffed thermostat will stop it heating.
When turned off there is hissing noise from behind bulkhead - passenger compartment side that is, though hardly noticeable inside car, more noticed outside car with engine lid open.
Coolant level good and not using any, no leaks at all.
Checked fuses 15 and 25 and all else is good electrically. Dash displays all work as does climate control displays.

I'm not a mechanic though handy and patient with tools so if it is something I can get too will happily pull it out, others its off to Audi mechanic.

Any experience of this good people?

[ricekikr]

Thermostat for cold/erratic water temp.

Bleed coolant in the hose to the heater for heater to warm up. Could also fix erratic water temp (air trapped)

Hissing could just be the refrigerant. Happens when compressor turns off.

[Tamarillo]

Thermostat for cold/erratic water temp.

Bleed coolant in the hose to the heater for heater to warm up. Could also fix erratic water temp (air trapped)

Hissing could just be the refrigerant. Happens when compressor turns off.

Thank you, exactly what I need. Will do a flush and bleed. Can't see how that explains the Nono cooperative climate control electrics but one thing at a time. Will do this then see what I have.

[ricekikr]

Thank you, exactly what I need. Will do a flush and bleed. Can't see how that explains the Nono cooperative climate control electrics but one thing at a time. Will do this then see what I have.

I misread what you posted. The cabin heat could very well be a climate control issue.

But the bouncing water temp could still simply be in need of a bleed. But most likely thermostat/waterpump.

[Tamarillo]

I misread what you posted. The cabin heat could very well be a climate control issue.

But the bouncing water temp could still simply be in need of a bleed. But most likely thermostat/waterpump.

Thanks, think I will do a flush and bleed as I can, then let Audi doctor sort it out. Ouch! And yes his workshop is called audidoctor. He's hungarian and bloody good.

[Tamarillo]

Update, where is heater solenoid?
Audi dr found a fault code that was preventing climate air working, cleared and now all good, it blows, heats and gets .... Cool.
Aircon guys found gas is good, pump good, suggest heater valves stuck open.

I think there is a solenoid somewhere? If so please can someone point me in right direction to find it?

1998 A4 2.4 Quattro. Thanks for help.

[walky_talky20]

There is no heater valve. Other brands use those, but not Audi/VW. Our cars run [ostensibly] hot coolant through the core at all times. If they had a valve, I'm sure the cores would get clogged even more often!

I think I would perform basic settings on the HVAC (relearn all blend door positions/limits) and perform an output test to make sure everything responds properly.

[Tamarillo]

There is no heater valve. Other brands use those, but not Audi/VW. Our cars run [ostensibly] hot coolant through the core at all times. If they had a valve, I'm sure the cores would get clogged even more often!

I think I would perform basic settings on the HVAC (relearn all blend door positions/limits) and perform an output test to make sure everything responds properly.

Thanks so much, will tell them tomorrow! I assume workshop did the Hvac thing when he plugged in and found code and bleared but will check.