Oil Pressure Warning Light

The oil pressure warning light (fig. 6-26) is used in place of a gauge on many vehicles. The warning light, although not as accurate, is valuable because of its high visibility in the event of a low oil pressure condition. Because the engine can fail or be damaged in less than a minute of operation without oil pressure, the warning light is used as a backup for a gauge to attract instant attention to a malfunction.

The warning light receives battery power through the ignition switch. The circuit to ground is completed through the oil pressure-sending unit that screws into the engine and is exposed to one of the oil galleries. The sending unit consists of a pressure-sensitive diaphragm that operates a set of contact points. The contact points are calibrated to turn on the warning light anytime oil pressure drops below approximately 15 psi in most vehicles.

When oil pressure is low, the spring in the sending unit holds a pair of contacts closed. This action completes the circuit and the indicator light glows.

When oil pressure is normal, oil pressure acts on a diaphragm in the sending unit. Diaphragm deflection opens the contact points to break the circuit. This action causes the warning light to go out, informing the operator of good pressure.