Where is the oil sending unit on a 90 blazer 4.3?

The oil pressure sending unit on a 1990 Chevy Blazer with a 4.3L V6 engine is located on the driver's side (left side) of the engine block, near the oil filter. It's a relatively small unit with one or two wires connected to it. It's often a single wire unit, but some variations might exist depending on whether the gauge is also an indicator light. You'll likely need to look closely around the oil filter to find it.