Smaller Engine Cooling Fan cut in/out?

[diysv6]

I have recently replaced the coolant and radiator cap in my SV6. Sept 2004 build. Having read all the cooling threads I am wondering if there are any surprises on the horizon. My query is, when does the second engine cooling fan cut in?
My old VT Berlina had both fans running whenever the A/C was on. From memory, the VZ has never had the smaller fan running whenever I have popped the bonnet open with the motor running etc. I did a quick check of the fan and fan relay continuity - both OK. Swapped over the fan relays but no difference. Fusible link is OK also. When should the second fan cut in? What controls it? My engine temp gauge sits on the second bar most times I look at it.
Any service manual/circuits or factory specs information would be a great help.
Thanks