The first type of oil sending unit used was a very simple design. It was literally a port into the engine block with a tube leading away from it. The tube filled with oil when the engine was running, and the oil would actually push on the needle of the oil pressure gauge inside the passenger compartment. This type of sending unit was very dangerous because of something in the line failed, hot oil would spurt into the passenger compartment or engine bay with serious consequences. For obvious reasons, this type of oil pressure sending unit is not used in automobiles today, and have been replaced with the "idiot light" and the electrical gauge style.
The mechanism known as an idiot light is a very simple design. Electrical signals coming from the oil pressure sending unit ignite a single bulb in the dash board if the oil pressure drops dangerously low. In theory, the idea is a good one. However, most idiot lights do not come on until the oil pressure drops to below five pounds. By that time, the engine may already be in serious trouble. The average stock automobile engine runs at about forty pounds when idling, and sixty or more when under a load. The other reason that idiot lights are not the preferred method of oil pressure observation is because if the light goes out over time, the sending unit might register a proper electrical current, but the driver will never know it because he or she never sees the warning beacon in the auto's dashboard. The first generation Volkswagen Beetles are one type of automobile outfitted with an idiot light.
The most common and safe method to track oil pressure used today is the electrical oil gauge and sending unit. As the oil pressure changes in the engine, a transducer inside the sending unit alters the amount of electricity being sent to the gauge in the dashboard, which affects where the needle sits. Therefore, it has the ability to accurately measure constant oil pressure safely, instead of simply announcing when the engine is in critical danger. If your vehicle has an actual needle gauge for monitoring oil pressure and not just a light that comes on, then it contains this type of system. On most applications, the oil pressure switch screws into the side of the engine block and taps into one of the oil galleys.
