DC coupling on the input/output will always give higher gain because AC coupling involves inserting a capacitor, which adds impedance and augments the signal. AC coupling is sometimes necessary though, for such purposes as eliminating DC offset on the base of a transistor, or eliminating a DC offset in a single supply opamp circuit.

It allows AC current to go through, but blocks DC current.

The expansion coupling is located just behind the passenger side rear wheel well. The rear evaporator is located on top of this coupling.

The orfice can be found either in the ac condinser or at a coupling sometimes at the radator but it is the ac line on the high side I do believe.

To Block DC voltages and couple AC signals through to the next circuit.