Oil Pressure warning light

[Superjack44]

l bought my 92 CB400 Superfour as a basket case in 2009 built it up and ran it till now with no problems to speak of, however it has developed an oil pressure warning light problem (it does not work) tried the bulb ok ,so looked for a bad connection and cant find one any where outside the main harness l have tried putting power through to the sub loom in the head light and can get the bulb to light on the dash but l am blowed if l can find where the fault lays.
I have replaced the 2 wires that connect the neutral switch and oil pressure switch to the main loom to no avail
Am l correct in thinking that the power is always in the loom and is only interrupted by the engine firing and so pressurising the switch cutting the earth off and putting the light off.

[MadMotoUK]

How do you know the light does not work as it should? It should only illuminate if you have low oil pressure which indicates a problem.

[magg]

Your thinking is correct. Oil pressure switch closed provides earth to lamp, ignition ON condition. Engine running provides oil pressure, opens switch removes earth lamp off. You should get lamp ON if your earth the wire at the oil pressure switch. If not then the wiring is faulty. If lamp illuminates then switch faulty. Does neutral lamp work ok.

[Superjack44]

The neutral lamp works ok ,having said that prior to this current problem the neutral light would illuminate as the gear lever selected different gears using the clutch lever but would immediately go out on releasing the clutch lever now it illuminates only while in neutral this is how it should be l think.
l have used a multi meter to see if there is a break in the two wires from the neutral and oil switch to the dash but everything seems to be ok.
l stripped off some of the plastic protective cover of the sub loom coming from the clocks and noticed that there was a black wire 3/4 of them with a brown stripe coming from different dash lights and being joined together so that there is only one wire entering the main loom via a white 6 pin plug in the headlight shell, l have tried crossing over to different connections usually resulting in blowing a 10amp fuse (several times) but can get the oil pressure light to light up once l hit the correct wires blue with a red stripe and a black with a brown stripe wire l think l am close to where the problem lays but blowed if l can find it.
l was kind of hoping this problem had occurred before and someone knew where it lay.
Thinking back the bike has stood a few months and when l started it up to put on the lift everything seemed to be ok and was going through the bike prior to the MOT callipers were sticking so all that was cleaned up along with checking the lights tyres etc it was as l lowered the lift that l noticed the light was off so assumed failed bulb so started to strip out the clocks the light would suddenly come on and off so started to move the loom coming from behind the headlamp and this effected the light so took out the oil pressure light checking for corrosion but everything was clean by this time l could not get the light to come on no matter what l did, l tried swopping bulbs from the various holders but everything thing worked in the different holders except for the oil pressure one there are only 2 wires BL/RD and BL/BR l even soldered the BL/BR wires together where they were joined with a crimp to try and eliminate any bad joint.
The bike runs fine just the one thing not working as it should is this oil light and its doing my head in.
Thanks for the replies guys

[MadMotoUK]

So are you saying that the lamp is not illuminating with the ignition ON but the engine not running? You've not made it clear WHEN the bulb doesn't illuminate.

[CB1pod70]

The easiest way to check if there is a problem with the wiring or oil switch is to disconnect the connector by the airbox and connect the Blue/ red wire (on the main loom side) to the negative battery terminal & turn on the ignition - the light should illuminate. If it doesn't then you have a problem with the wiring or bulb.
You can check the switch by connecting the multimeter to the blue / red wire on the other part of the connecter & the positive battery terminal - current should pass when the engine isn't running but stop once oil pressure is built up.

I suspect there is probably just some corrosion on one of the connections somewhere

[Superjack44]

So are you saying that the lamp is not illuminating with the ignition ON but the engine not running? You've not made it clear WHEN the bulb doesn't illuminate.

Apologies I should have said , Ignition on but engine not running no oil light on

[Superjack44]

Well l went through the procedure as you described and connected the blue/red wire to the negative terminal result ign on oil light on so then l checked the blue/red wire to the switch via the multimeter and got no reading at all so l reconnected the small 2 pin socket to the main loom ign on still no oil light on so then l went on to earth the switch to the negative terminal ign on oil light on indicating dodgy oil pressure switch.
l replaced the oil pressure switch switched on problem cured happy days.
What threw me was the fact that when l moved the wiring loom behind the headlamp the light would come on then go off eventually staying off which caused me to believe the problem was a bad connection around the headlamp and clocks.
The only cause l can come up with now is that somehow by moving the loom l was somehow moving the bike enough to agitate the fault in the switch.
Many thanks to MadMotoUK magg and CB1pod70 without you l would still be banging my head off the wall.