Topic: Oil Pressure Switch

[pschuyler]

When I bought my 1973 CB750K3, the previous owner had disconnected the blue/red wire leading to the oil pressure indicator light.  I plugged it back in, and started it up for the first time last night, and the oil light never went off.  I decided not to run the engine anymore until I can figure out if the pressure is bad or just the switch.

If I pull the switch out is there anyway to bench test to see if it works, by blowing air into it and using a multimeter?

I just changed the oil, is there anything else to check?  I do not have an oil pressure gauge.

Pete

[mattcb350f]

From an old thread..

Switch is closed when there's no pressure, completing the circuit to ground.  Test by disconnecting the lead at the switch and touching it to a ground point, light should come on.  If not, your problem lies elsewhere.

mystic_1

The oil pressure opens the switch to turn the light off. The reason the light is on is either there is not enough oil pressure to open the switch, the switch is faulty and grounding internaly, or there is a ground elswere in the circuit.


I've seen quite a few switches go bad myself, I bet that's all it is. I once had a switch rupture and spew oil everywhere too.

 Matt.

[Borkunit]

I had an ongoing intermittent oil light problem.  Turned out to be that the wire going to the switch was rubbing on a sharp edge on the engine case.  It had worn through the insulation and was completing a path to ground, thus illuminating the oil lamp. Follow the wire all the way back to the harness and make sure it's intact.  On my bike (750 K4) the wire goes from the sensor and almost immediately into the engine case.  I believe there is an inspection cover that you can remove and continue following the wire back.  If I recall correctly, it was somewhere under this cover that it was shorting out.  Good luck!

[pschuyler]

So if I disconnect the wire but leave the switch in, start the motor, I should see a continuity meter tone until the switch turns off?  This way I think I can rule out a grounding or wire issue?  Any thoughts to my reasoning?

[bryanj]

If the engine has run for over 5 minutes without exploding the oil pressure is ok ish so suspect the switch or wiring.

if you disconnect the switch and light stays on its wiring if it goes out its switch

[jumpjg]

 quote: "So if I disconnect the wire but leave the switch in, start the motor, I should see a continuity meter tone until the switch turns off? "

Sounds like you already did something very similar. The wire was disconnected - I assume the light never came on. You hooked up the wire & the light doesn't go out, indicating the switch doesn't open. It sounds like you have a bad switch or no oil pressure. You could probably remove the switch, put air pressure on it (30 psi should be more than adequate) and check to see if it opens up (switch turns off). Might be more prudent to do that first rather than running the engine without oil pressure.

My money is on a bad switch.

Joe in St Louis

[Borkunit]

If you want to use the continuity setting of your meter to test, do this:

Testing the wiring - Engine off, disconnect the wire from the switch.  Test continuity between the end of the wire and the engine case.  If there is continuity, then the wire is shorting to ground somewhere.

Testing the switch - Engine running, test continuity from the terminal on the switch to the engine case.  If there is continuity, then the switch is bad or the oil pump is not producing enough pressure to open the switch.

Don't blame me if your motor explodes while running, though!   

I'd test the wiring, and if that checks out OK, then replace the switch.  If the light still comes on, then your problems are more significant.

[mystic_1]

Nowadays I can verify the operation of my oil pressure switch by looking at my oil pressure gauge. 

mystic_1

[pschuyler]

  Thanks to all, my neighbor happens to have a 1978 CB750, and I borrowed his oil pressure switch, and it fixed the problem, saved me alot of headache.

Pete

[mystic_1]

Excellent.

Did you borrow it, or "borrow" it 

"Switch?  What switch?"

mystic_1

[mark]

I was gonna say something like...


hope ya left a note!

 

[andrew750]

Hey, wait a minute!  I used to have an oil pressure switch in BOTH of my engines!!!

[mystic_1]

Heh, busted!

 

mystic_1