Topic: Oil/idiot light wont go out. What to check and how to fix?

[Gorms]

Just got the engine running recently ('77 550k) and finally got the lights (replaced with LEDs) working.  Flashers and neutral light work fine but the oil light stays on, even after the bike is started.  Haven't ridden it but had it idling in the garage.  What could be the problem?  If the oil sensor (?) under the left side trans cover fails, will it fail to on (a fail safe maybe)?  Oil level is fine, valve rockers and covered in oil, and the engine runs fine.  Suggestions?

[KingCustomCycles.com]

Pop the cap when running and look for swirling oil from the engine return.  If I remember correctly, the sending unit completes the ground to turn the light on.  I suspect that wire got pinched when you installed the chain cover or the clutch cover and the wire is completing the ground independant of the sending unit.  Worth a look.    

[Gorms]

Sorry but explain how to check for swirling?  Its a 550 without an oil tank or oil cooler.

Actually the oil sensor wire was pinched between the case and the trans cover and it almost severed it.  I cut and resoldered it and made sure it wasnt pinched.  I'll check again but I dont think this is the problem.

[KingCustomCycles.com]

Sorry, didn't check your machine size or type, all the old Hondas are wired the same.  You can check the sending unit with a ohmmeter.

[Bodi]

Sounds like the switch has failed. Using a LED should not matter - the circuit is dead simple. 12V power is at the oil light whenever the ignition is on. The light grounds through the NC oil switch, with pressure the switch opens and the light goes out.
Try taking the wire off the switch (on the oil pump, under left cover as you thought). If the light stays on, wiring trouble. If it goes out, switch trouble. Possibly the switch is "leaky", allowing a few mA of current to pass when it should be fully open. That wouldn't light the original filament lamp but would light a LED pretty brightly.

[Gorms]

Fortunately I think my wiring is fine.  I have another oil unit from another older, more worn engine that I can swap out.  How can I test if my sending unit is functioning?

[tlbranth]

Put it in another engine - like a car engine and see what happens. But I don't think it's worth messing with. It's most likely a bad switch. But if everythings working, you're doing  your engine in by running it. Do like Bodi suggested, take the wire off the switch. If the light goes out, buy a switch.

[JohnG]

Set your multimeter to Ohms.  Put one lead on the post of the sending unit, the other on the body. You should have continuity  (near 0 ohms).  Then take your air nozzle and put air into the hole in the bottom.  The continuity should cease  (near infinite ohms).  

The sending unit is just an On-Off switch that either grounds out your 12v bulb or it doesn't.  The cut off is real low - like 3 psi.

[Gorms]

I'll try thanks.