Oil Pressure Warning Light
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.
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.
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.
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.