Quote:
Originally Posted by dmfelger
I would appreciate an explanation how the active and passive DEF tanks work. Which one gets filled when needed? Do either of them need to be drained prior to re-fill? Just curious.
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Active tank is small and heated when needed to prevent freezing. Passive tank is larger and passively heated by adjacent components. When the active tank level is low, DEF is transfer pumped from the passive tank to the active tank.
Neither needs to be drained prior to fill. That was an early BMW protocol, perhaps until it was realized how soluble urea is in water.
My intention is to not fill the active tank, to fill the passive tank only.