Topic: Oil Pressure Sender switch

[chumma7]

Hi,
I've been lurking on this site and using the search option for sometime now while rebuilding a 1971 cb500 for a friend. Well I finally have run into something that's worthy enough to warrant a first post. After months of wrenching the bike is running like a balanced top but there is one last thing to sort out before I can say the bike is back to mechanically perfect.
The original oil pressure sender wasn't working as the red oil light would refuse to turn off. I checked the oil pump and it's  well within spec and is working fine as there is much oil flow into the upper cylinder tappet area when viewed at idle with the valve covers off.
I swapped the sender with a used one from another cb500. It worked perfectly for about 5 minutes... The red light would light when the bike was off and ignition was in the on position. Then gradually turn off when the engine was started and warmed.
After those 5 minutes of perfect operation the oil light now functions like this:
The light is now off when the bike is off and ignition switch is in the on position.
When the bike is started and warmed, the light turns on and stays on till the bike is shut off.
Seems like the oil light function has inverted. Anybody experience this? Do i have a grounding issue with the sender? Could this used sender also be faulty? I could not check whether it was functional as the donor bike was parted out.
Thanks in advance,
C

[Kevin400F]

Maybe the vibration / heat of the running engine causes the oil light wiring to catch a ground somewhere between the sender and the warning light.

When the light comes on, can you wiggle the wiring and get the light to flicker?

When pull the wiring off the sender and check its function with an Ohmmeter.  Engine off, the terminal on the sender should have continuity to ground.  When the engine starts and you get oil pressure, the circuit should open up.  If you still have ground (hence the light), you have another bad sender.

Kevin

[chumma7]

Thanks for the reply Kevin,
I went at it again after your post and it's working perfectly now. I'm a happy man.
I did a combination of things. First of all, as you said, the wire was frayed in areas, grounding it to the outer cover. I shrink wrapped those areas and routed the wire where it wouldn't be touching the frame or cover.
I also noticed I had overtightened the sender. It started to function as I slightly loosened it with the engine on. I then swapped the new oil sender with the original one that was on the bike and it now works as well. 
Thanks for the help,
C

Maybe the vibration / heat of the running engine causes the oil light wiring to catch a ground somewhere between the sender and the warning light.

When the light comes on, can you wiggle the wiring and get the light to flicker?

When pull the wiring off the sender and check its function with an Ohmmeter.  Engine off, the terminal on the sender should have continuity to ground.  When the engine starts and you get oil pressure, the circuit should open up.  If you still have ground (hence the light), you have another bad sender.

Kevin