Where is the oil sending unit on 1984 gm sierra classic?

The oil pressure sending unit on a 1984 GMC Sierra Classic is located on the driver's side (left side) of the engine block. It's usually near the oil filter, but not always directly on it. It's a small, usually one-wire, sensor that screws into the engine block. You'll likely need to consult a repair manual or a diagram specific to your engine (e.g., a small-block Chevy or a big-block Chevy) to pinpoint its exact location because the precise placement can vary slightly depending on the engine.