yes the closest one to the engine is the low pressure side, and that is the one you charge the ac system through, if you are converting a r-12 to r-134a make sure you bleed out all the r-12, most shops will do it for no charge since r-12 is so valueble,, but make sure you get the right retro fitting for the valve, and you have to turn the ac on high and running the engine when you charge the system