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15 psi would be around normal at idle ( depending on the idle rpm of around 650-700) with up to 40 psi at 2000 rpm
loosing oil is a requirement to check for sump leaks (front or rear main seals or rocker cover gaskets )
Burning oil however is a different set of indicators that may indicate
bad valve stem seals, worn rings and increased compression blow by and a possible big end bearing problem that is allowing excessive oil onto an oil control ring causing excessive oil burn . That may account for the low oil pressure if the book states that it should be higher at the recommended rpm for idle
It could be that you need to use a 20w 30 grade oil rather than a 15 w 20 grade oil to see if that helps with the oil loss / pressure problem

Oil light switch is a normally closed unit so first off test with an ohmmeter to see if there is an open which would not complete the circuit not allow the light to work.Then check to see if there is any current to the oil switch. If there is you could jumper the wire to ground which if the light then goes on indicates that the switch is bad. If no current to the switch you will have to trace back and find out where the open in the circuit is.

There may be no oil pressure due to lack of oil or a faulty oil pump. Check the oil level and add oil if low, then restart the engine and verify that the oil pressure light goes off and that the oil gauge indicates pressure. If the problem still exists you will need to check out the oil pump.

If the gauge shows oil pressure but the light does not go off it may be that the contacts in the pressure sending unit are not opening to break the circuit and shut off the light.
If the gauge does not show pressure it may be that the variable resistor in the sender is shorted to ground.



Locate the oil pressure sending unit on the front right side of the crankcase and verify that the electrical connector is properly connected to the sending unit. If the oil pressure gauge indicates pressure but the low oil pressure light remains on remove the electrical connector from the oil pressure sending unit and using a small jumper wire place one end on the indicator lamp terminal (the closest to the latch on the sending unit) and place the other end on the crankcase. The Ohmmeter must read less than one ohm. THEN start the engine and run at a fast idle. The ohmmeter must read infinity (OL). Replace the sending unit if the above meter readings are not obtained.

If the low oil pressure light functions correctly, but the pressure gauge does not, remove the electrical connector from the oil pressure sending unit, turn the ignition switch to IGNITION and the gauge must read zero. Ground the (BN/GN) wire terminal to the crankcase. The gauge must read full scale (60 PSI). Replace the sending unit if the above gauge readings are obtained but if the gauge readings are not obtained, replace the pressure gauge.

If there is no power to the gauge diagnose the cause could be broken or disconnected leads to the gauge or an open gauge winding. Test by switching the ignition switch to IGNITION and verify that 12VDC is present at "+ terminal on the gauge, then with the ignition switch turned to OFF check the ground terminal for continuity to ground. Replace the gauge if 12VDC is present and ground terminal is grounded. If 12VDC is not present, trace wiring until disconnected or broken wire is found and repair as necessary. If ground terminal is not grounded, repeat the procedure used to trace a for 12VDC lead.

The power lead fro the oil pressure and neutral switch is an orange wire on pin 6 of a 10 pin connector.

Change the wires around. I had same prob on my 94 flhtcu. Now gauge works properly and light doesnt come on.

Where's it leaking from?

One common spot is the oil seal that's behind the transmission rear drive pulley/sprocket.

This is typical oil pressure on a Harley engine when it is hot. This is also why I suggest that you never install an oil pressure gauge on your Harley. All it's good for is scaring the fool out of you.

A Harley oilling system is a high volume low pressure system. Since a Harley engine is constructed using roller bearings in the bottom end instead of babbit shell type bearings, it will not hold oil pressure like the average *** bike. Take a drinking starw and cut it into sections about 1/2 long. Line the sections up in a row and notice that each section touch the table in one spot only. If an oil hole were there, it would be blocked and oil pressure would build up. But as soon as the roller moved, the hole will open and oil will flow with no resistance. This is why a Harley engine won't build up a lot of oil pressure, the oil flows freely into the engine.

Good Luck
Steve

The only thing you can do is look for a loose wire or replace the sending unit, pressure switch.

If your engine is indeed losing oil pressure, the hydraulic tappets would be making enough noise that you would know that something was wrong. If your engine sounds like it normally does, you've go oil pressure. Just something lying to you. Gauges will do that sometime.

Good Luck
Steve

oil pressure guage has its own wireing as does the air temp(light) and no computer work is needed