2. The bypass system (fig. 6-25) diverts only a
small quantity of oil each time it is circulated
and returns it directly to the oil pan after it is
filtered. This type of system does not filter the
oil before it is sent to the engine. The oil from
the main oil gallery enters the filter and flows
through the filter element. It then passes into the
collector in the center of the filter. The filtered
oil then flows out a restricted outlet preventing
the loss of pressure. The oil then returns directly
to the oil pan.
Oil galleries are small passages through the
cylinder block and head for lubricating oil. They are
cast or machined passages that allow oil to flow to the
engine bearing and other moving parts.
The main oil galleries are large passages through
the center of the block They feed oil to the crankshaft
bearings, camshaft bearings, and lifters. The main oil
galleries also feed oil to smaller passages running up to
the cylinder heads.
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
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.
Oil Pressure Gauge
The oil pressure gauge is mounted on the
instrument panel of a vehicle. Marked off on a dial in
pounds per square inch (psi), the gauge indicates how
regularly and evenly the oil is being delivered to all vital
parts of the engine and warns of any stoppages in this
delivery. Pressure gauges may be electrical or
In the mechanical type, the gauge on the instrument
panel is connected to an oil line tapped into an oil
gallery leading from the pump. The pressure of the oil
in the system acts on a diaphragm within the gauge,
causing the needle to register on the dial.
In the electrical type, oil pressure operates a
rheostat connected to the engine that signals electrically
Figure 6-26.Oil pressure warning light.