A closer look at the oil pressure switch and light - Honda CX500, GL500, CX650, GL650
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Photos are coming.
The oil pressure switch is quite a simple design; it acts as a conduit for the ground connection for the oil pressure warning light. Inside the switch is a spring-loaded contact, which is normally closed - meaning that when the engine is off or the oil pressure is very low, the circuit is complete and the oil pressure warning light comes on. Once the oil pressure builds above the warning threshold (overcoming the spring strength) the oil pressure pushes the contacts apart which breaks the circuit, meaning that the oil pressure warning light no longer has a connection to ground and thus goes out.
The oil pressure switch is located on the front engine cover, on the right hand side of the engine (left if you are looking at it from the front).
The oil pressure switch has a trigger point of somewhere below 10psi of oil pressure, making it more of a "stop now" than a "hey, your oil pressure is starting to dip a bit" warning.
Troubleshooting
This is a very basic circuit, so there's not a lot to test. The oil pressure light should come on when you turn the key to "on" and it should go out within a few seconds of the motor starting.
If the engine oil pressure light comes on while you're riding or doesn't go out once the engine is started, don't ride the bike. Starving your bearings of oil can lead to an engine seizing - possibly while you're on it, travelling at speed. Not worth the risk.
Firstly, some models of CX ('79 US Customs, possibly others) have a "feature" where a burned out brake light will trigger the oil pressure switch as a means of warning you that your brake light is out - verify that it's not that first!
Light stays on after engine starts
Either you have no oil pressure, your wiring is faulty and you have a connection to ground before the oil pressure switch or your oil pressure switch is broken. The first two are probably more likely.
Check your oil level. If that's OK, you might have something along the lines of a faulty oil pump, a blocked oil pump screen, a stuck oil pump relief valve, a broken oil pump chain, a missing oil jet, a missing O-ring or something similar. Those are harder to check, so let's rule out electrical things next.
Check the wiring between the oil pressure warning light and the oil pressure switch. Sometimes the wiring gets frayed by the cooling fan. Undo the wire on the end of the oil pressure switch; if the light remains on and the wire isn't attached to the switch, the wire is finding it's way to ground somewhere else. Trace the wire up and see if you can find where that is.
If the oil pressure light goes out as soon as you unplug it from the switch, the next step is to take the front engine cover off and check that the oil pump, oil pump screen, relief valve and oil pump chain are all working OK, all O-rings are present and that the little oil jet near the clutch is present and oriented correctly.
Light never comes on
If it never comes on, is your bulb blown?
If it's OK, check that it's getting power - use a multimeter and check that 12V is coming in to the bulb with the ignition set to "on". If it isn't receiving power, find out why. If it's receiving 12V just fine, check the cabling - the most likely problem here is an unplugged cable (probably at the switch body or under the tank where there is a connector) or a cable severed by the mechanical cooling fan. If that's OK, check that there's continuity between the terminal on the end of the switch which the wire connects to and the frame of the bike; if there's no continuity, your switch is not returning to the electrically closed position at rest and needs to be replaced. In this situation, you should be able to manually ground the end of the wire going into the pressure switch to the engine or frame and the light should come on, where it won't if connected to the pressure switch.
Light flickers on and off
Treat as the worst case scenario of the engine only just barely having oil pressure, and investigate from there. It's probably just a loose connection, but best to be sure.
