refill trap in drain for heat pump condensate

Bad smell in laundry room. We have a clear plastic condensate drain (D) from air handler (for heat pump) in attic. In the laundry room, D is visible in the wall opening that holds the water pipes for washing machine. We know that D goes into black drain piping (A) and does a right turn (can see it in basement). In the laundry room, ~12" to the right of D/A is an access cap (B). We are guessing B attaches to a pipe C that goes to A. We also guess there is an S or U or P bend in A (between D and C), to create a water trap. We are guessing the trap is empty, therefore the smell. So the question: if we open the cap B and pour water down C, will we fill the trap? If yes, how much water should we pour in?