Oil Pressure Switch Operation:
relay unit for the indicator light or gauge on instrument panel. It
contains a diaphragm that responds to changes in oil pressure. When the
oil level is over a pre-set limit, the diaphragm opens the switch,
turning off the light. When level is below the pre-set limit, the dash
light comes on. In later model cars, if the oil pressure drops, the
switch turns off the fuel pump, to stop the engine and limit damage to
internal parts. Oil pressure switches can be grouped into two
categories, units for electrical gauges and for mechanical gauges.
Oil pressure can be checked at the sending unit passage with an
externally mounted mechanical oil pressure gauge. To get accurate
results from the test, make sure you follow the manufacturer's
recommendations and compare your findings to specifications.
Testing the Oil Pressure Gauge
Testing Oil Pressure
- Test the gauge by grounding the wire that leads to the sending unit on the block.
- When the wire is disconnected from the sending unit with the key
switch on, the needle on the oil pressure gauge should be either all
the way high or all the way low. Grounding the wire should make the
needle move to the opposite side of the gauge from where it was before
the wire was grounded. If the vehicle is equipped with an oil pressure
light, touching the wire to ground and then removing the wire from
ground should turn the light on and off.
- If either happens, the gauge and wiring are good and the sending unit is at fault.
Pressure can be tested by temporarily installing an oil pressure
gauge in place of the sending unit. Before testing, look up the oil
pressure specifications for your vehicle in the appropriate service
- Bring the engine up to operating temperature.
- Now shut the engine off.
- Locate the oil pressure sending unit; usually it is on the lower side of the engine block.
- Disconnect the wire from the sending unit and use an open ended wrench to remove the sender. Using pliers can damage the unit.
An oil pressure sending unit is removed with a special socket. Courtesy of Chrysler Corporation.
- Tighten the oil pressure test gauge into the hole in the block where the sender was removed.
Measuring oil pressure. Reprinted with permission.
- Check the engine's oil level and fill, if required.
Be extremely careful when working near a running
engine. Always wear safety goggles or glasses with side shields when
working around moving machinery and be sure that your clothing is not
- Now turn the engine on and check the oil pressure on the gauge.
- Watch as the engine warms to note any excessive drops due to temperature.
- Make sure the engine speed is set to specifications for testing pressure.
- If necessary, us a tachometer and adjust the engine idle speed.
- Record the measured oil pressure, then turn off the engine.
- Compare the test results with the manufacturer's specifications.
- If the oil pressure is within specifications, this shows that the oil pressure sending unit may not be working correctly.
- In many cases, replacing the oil sending unit will correct the problem.
- After the test is complete, reinstall the oil pressure sending unit, start the engine, and confirm there are no leaks.