Oil pressure is one of the most critical measurements on any engine because, without it, your engine goes from powerhouse to paper-weight. A number of companies sell aftermarket oil pressure gauges that allow you to check the pressure while driving, but there's no sense in installing a gauge if all you want to do is check the pressure for diagnostic purposes. Rather than spending hours and perhaps hundreds of dollars on installing a gauge, you can check oil pressure using the vehicle's stock electrical oil pressure sending unit.
Locate the oil pressure sending unit on your engine. These things can hide in some pretty out-of-the way places, so break out your repair manual and use it to locate the sensor. You'll usually find it somewhere on the engine block.
Follow the wiring harness coming out of your pressure sending unit to the harness plug, and unplug it. Start the car; if the sensor is working, then you'll see a check engine light illuminate. Note that some cars may not start if you unplug the oil pressure sensor. If the car starts, then go ahead and shut it down.
Connect the black needle probe from your digital multimeter to the darker of the two sensor wires, and the red one to the lighter of the two. Set your multimeter to read resistance in ohms (Greek symbol Omega). Refer to your repair manual for the correct specs. An oil pressure sensor works by increasing resistance in ohms with pressure, so the sensor should read zero or close to it with the engine off.
Secure the probes to the sensor wires with electrical tape if you need to; reaching down into a running engine is never a good idea. Start the car and monitor the multimeter. Compare the rpm readings and the sensor's resistance in ohms to you reference material; a specific ohm reading will correspond to a given pressure. Generally speaking, one ohm equates to around one psi of oil pressure.
Remove the multimeter and plug the oil sender back into the harness.
