Most current automotive use a warning light located in the instrument panel to provide the driver with a visual signal of the low oil pressure. It is important to note this signal, due to the importance for the life of the engine proper pressure level of the oil flow.

fig 1The warning lamp is connected in series with an oil pressure switch, as illustrated in Figure 1. The pressure switch has electrical contacts are in closed position when the oil pressure is less than 5 psi.

When the engine is not running and the ignition is on, the warning light comes on.

When the engine is started, the oil pressure compresses the diaphragm pressure switch, opening the contacts and causing the light goes off.

If the warning light stays on for more than 5 seconds after the engine has been started, switch off immediately; the oil level should be checked and if necessary put it at the right level for the engine in question. If the warning light stays on when the oil level is normal, then it might be happening any of the following causes:

  1. The pressure switch may have failed.

  2. The wiring between the pressure switch and the warning light can be grounded.

  3. The pump priming may have been lost due to failure or the suction mesh (pump filter) is saturated.

  4. The pressure regulating valve can be open and return the oil back to the crankcase. (See section: "Problems with pressure regulating valve of the oil pump").

  5. The pump may have trapped air during suction. (See section: "Loss of power to the oil pump").

fig 2In some engines, the filter is mounted in a position that allows the oil to drain back to the crankcase when the engine is stopped. This would be the case illustrated in Figure 2, where it is in a horizontal position above the oil level in the crankcase. Since the filter stacks up to 0.94 liters of oil, it takes approximately five seconds for the pump to fill it after the engine has been started, during which the warning light stays on.

To prevent the engine to run without oil pressure during that period, filters designed to those positions contain an anti-drain valve. This valve currently has the form of a gasket which allows the free flow of oil through the filter in the normal direction, but when the engine is "stopped" prevents oil discharged into the crankcase. If the warning light stays on for longer periods, after the filter has been in use in automotive for some time, replacement is recommended.

Under some conditions, the warning light may flash when the engine is running. This usually happens when the vehicle is turning or located on a relatively steep slope. If a blink under these conditions occurs, it is because the oil level is low, and the filter (mesh) of the pump is not completely submerged. The oil level should be immediately filled to the normal situation.

Summarizing, the warning light oil pressure provides vital information to the driver. If the light remains on or flashes even when the engine is running, it must be switched off immediately and investigate the cause.