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dustyxbla

Oil pressure light. Help!

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. Okay, so I own a DOHC 79 Cb750 K/limited edition. I spent the past 2 months replacing the entire charging system because it was all bad. Last night I received my regulator and wahla!/it was fixed. So I got 4 quarts of Pennzoil 10w-40 and did and oil and filter change, excited to take it for a ride. After the boil change my Oil/Stop light came in and won't shut off. I'm demoralized. I have no idea what went wrong but the light stays on even when the motor is running (5-10 second tests). What could possibly have happened and how do I fix it? This is the last straw

1979 CB750 Limited Edition

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Is the motor making noise like it isn't getting oil? I have had to burp oil filters before as the air pocket won't let oil flow.

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I haven't noticed any odd sounds. I took the filter and cover BACK off today and inspected them and put them back on. I haven't heard of burping though.

1979 CB750 Limited Edition

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Knock the sending unit wire loose?

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The sending unit is behind the cyls on top of the transmission case, give it a few taps and see if it goes out. You can also tap into the high pressure oil feed line that goes up into the head and plumb in a mechanical pressure gauge from summit or autozone. or remove an inspection cover from the calve cover and see if there's oil shooting around up there. Or you can pay me to do any of those things.

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Update: I tapped on the sensor, maybe not hard enough? I tried "burping" the filter and lost about a quart of oil lol it's DEFINITELY pumping oil to the filter. Also, because I'm an idiot, I rode it for like 2-3 miles yesterday after the change. Would I not have blown or seized my motor in that amount of time if my oil wasn't under pressure? I bought a new sensor on eBay today, should be here next Wednesday.

1979 CB750 Limited Edition

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Pull the wire off the oil sensor and start it for a second or two. Light should be off...id that happens and since you havnt blown your motor you got a bad sensor.

If the light is still on you messed up the wire....pinched it...cut.....something to cause it to ground out.

Edited by Tonik

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I pulled the wire off, didn't start the motor, but turned the key to on and the light was still on, but it was delayed to turn on by a second or two. Grounded the wire to the sensor to the main ground and it still stayed light.

1979 CB750 Limited Edition

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I'm not sure how to test the sensor because i dont know how it works. Does the sensor kill a circuit to ground when activated, or what?

1979 CB750 Limited Edition

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Sensor interrupts a normally closed circuit iirc.

Well I double checked my fuses, they're intact. I guess I'll beat on the sensor tomorrow and wait for a replacement.

1979 CB750 Limited Edition

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Check the wire from the sending unit to the light. You're looking for that wire touching ground somewhere.

When the bike is off, the sending unit grounds the wire, when the bike is running oil pressure pushes up the thing in the sending unit  and you no longer have a ground and it turns the light off.

 

Don't use Teflon tape to put the sender back in.

Edited by Gump

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Check the wire from the sending unit to the light. You're looking for that wire touching ground somewhere.

When the bike is off, the sending unit grounds the wire, when the bike is running oil pressure pushes up the thing in the sending unit and you no longer have a ground and it turns the light off.

Don't use Teflon tape to put the sender back in.

Thanks, Gump. I'll check as much of the wire as I can tomorrow. the timing of this issue is what perplexes me. Especially because the oil system appears functional.

1979 CB750 Limited Edition

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It's an old bike. Things don't always work as they should after 35 years. Pull the sensor and work the little plunger, maybe give it a shot of wd or penetrating oil.

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You da man, crazyskullcrusher. The switch is going to be a super PITA to pull due to location and size. I'll report back once I get some time put into it

1979 CB750 Limited Edition

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You da man, crazyskullcrusher. The switch is going to be a super PITA to pull due to location and size. I'll report back once I get some time put into it

1979 CB750 Limited Edition

 

It's not the sending unit, it is the wiring.  Light was still on when you unhooked the wire...you eliminated the sending unit as part of the problem. The wire is shorting to ground somewhere. It got pinched or exposed somewhere along the line.

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 I spent the past 2 months replacing the entire charging system because it was all bad.

 

The oil pressure wire goes through where the alternator/stator is on that bike. You f'd up the wire in that area when you did the above. Look there first.

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a09dd33b5a13a65ddbd3c689061c5ef7.jpg

Is this not the oil pressure sensor? The wiring for this sensor never goes through the stator/alternator cover.

1979 CB750 Limited Edition

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https://vimeo.com/125804836

This is how the light responds when the lead to the sensor/switch is unattached from the switch. As you can see it is delayed for a second. When attached it comes on immediately. I'm not sure that I think that the wire is grounded. If it was grounded wouldn't it just come on immediately?

1979 CB750 Limited Edition

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Different things happen in the electrical system when a bike has the key on vs when it is running. For example most bikes the headlight won't come on.  I have re-read your posts and I see you have not checked it with the wire off and the bike running.

 

I understand you don't want to run it, I would not be too keen on doing that either.

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Different things happen in the electrical system when a bike has the key on vs when it is running. For example most bikes the headlight won't come on. I have re-read your posts and I see you have not checked it with the wire off and the bike running.

I understand you don't want to run it, I would not be too keen on doing that either.

Well I took it for a 2-3 mile ride with a passenger with the stop light on ( I know, I know) and she survived. Engine sounds healthy and I know the pump is working. That doesn't mean I have pressure, but I feel confident I can run the motor long enough for tests. You want me to give you the resistance between the unplugged sensor wire and ground with the bike running?

1979 CB750 Limited Edition

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No, I just want to know what the light does with the engine running and the wire unplugged.

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I pulled the wire off, didn't start the motor, but turned the key to on and the light was still on, but it was delayed to turn on by a second or two. Grounded the wire to the sensor to the main ground and it still stayed light.

1979 CB750 Limited Edition

Derp. Skipped over this somehow. Chase wiring back from the light. Pull the headlight out, download a wiring schematic from Google images and chase it back. Wire from the light should go into a connector and back out of the headlight bucket and eventually down to the sending unit. If this only started happening after you did the oil change I would walk yourself back though everything you did while changing the oil. Did you lift the bike up somehow? Chances are you pinched the wire and split the insulation enough to ground it out. To isolate the wire as the problem run a jumper from the sending unit to the hot dog connecter in the headlight bucket and see if that fixes it.

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