Touareg: 2008 R5 TDI
Join Date: Dec 2016
Second Vehicle: Still looking
Thanked 36 Times in 34 Posts
Your standard radiator might be barely able to keep the coolant temperature at acceptable level (because of being partially clogged, restricted or just simply filled with gunk between the fins), and if you start the A/C, the extra stress and heat of the A/C radiator might just push it over the tipping point.
Or there's something seriously wrong with the drive of the A/C compressor that generates so much extra resistance or heat, that the cooling system can't keep up with it. Even though that should presumably make some serious noise then.
Did you or anyone read the fault codes?