oil pressure gauge
is one of the most important instruments in a car. It
acts as an indicator of the
's overall well-being and as an early-warning
notice of any problems so you can investigate the cause
before an expensive breakdown occurs.
How oil pressure gauges work
The gauge monitors the oil
in one of the main oilways near to the
. To do this it has a tapping in the
, into which is
(for electric gauges) or an oil pipe take-off (for mechanical
The sensor for the oil warning light is screwed into the union or T-piece at
this point, whether or not the car is fitted with an oil pressure gauge.
Mechanical gauges work by literally forcing the needle around the dial under
pressure from the engine. Electric gauges, on the other hand, work by using the
sensor screwed into the engine block to produce a variable
affects the amount of
passing through the
containing the gauge
Oil is sent to the gauge from the tapping in the engine's oilway by a
small-bore (3mm) pipe, usually copper or plastic. The pipe is routed away from
anything that could damage it, because if the pipe is punctured the engine's
oil would leak out.
The pipe enters the passenger compartment via a grommetted hole in the
bulkhead, and joins the stub on the back of the gauge via a knurled
The gauge contains a flexible coiled tube called a bulb, the open end of
which is rigidly mounted to the gauge's outer casing. The other end of the bulb
is closed and connected by a lightweight
to the bottom end of the
needle, which is itself mounted on a pivot.
Oil is fed into the bulb from the supply pipe at very nearly the same
pressure as it left the engine. The bulb tries to straighten under the pressure
and in doing so moves the needle around the calibrated gauge
. The greater
the pressure, the more the needle moves.
Electric current is supplied to the gauge from a fused power supply in
practice the current is taken from one of the many wires or printed tracks
The current passes through a wire-wound
mounted around or within the
needle's pivot and produces a
that moves the needle across the
calibrated scale of the gauge. How far across the scale the needle goes - what
reading it gives - depends on how much current flows through the gauge. This in
turn depends on the resistance of the gauge's return wire which is earthed to
the engine block through the sensor.
The resistance of the sensor depends on the oil pressure. Oil enters the end
of the sensor which is screwed into the engine block and pushes against a
. The diaphragm moves a wiper inside the sensor which runs up or down
a blade of known resistance this blade is connected to the return wire from the
gauge. The more the diaphragm moves under pressure, the further down the
resistance blade the wiper moves. So the resistance of the sensor varies with
oil pressure and moves the needle of the gauge accordingly.
All gauges are illuminated so that they can be read at night.
gauges are lit from one of the panel lights which also illuminate the rest of
the instruments, while separate gauges have a small (0.5 to 3
mounted in a holder at their rear. All these lights are usually connected into
the side/panel light circuit.
Pros and cons
Electric gauges are easier to integrate into the modern one-piece
instrument panels used in most modern cars, they are less bulky and
it's easier to route and connect a wire than a pipe.
Mechanical gauges are not as common as electric ones, although they are
still available in accessory shops. Because the oil arriving at the back of the
gauge is at engine pressure, you have the problem of messy leaks if the pipe
comes undone. However, some people find a mechanical gauge more
Oil warning lights
Many cars do not have an oil pressure gauge fitted as standard, but
because some indication of low oil pressure (especially zero reading) is so
important, they will at least have a red or orange warning light that
shines if the oil pressure drops below a certain level.
Often this warning light will be connected to an oil-level sensor as
well, and in some cars the light doubles as an engine temperature warning
Lubrication: Oil Pressure Switch
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