What to do when the oil pressure light comes on
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  1. #1
    Member cfile's Avatar
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    What to do when the oil pressure light comes on

    I've got a 79 Honda cm400t. Had it out the other day and it was doing fine when the oil pressure light came on. Pulled over and checked the oil and it was full. Drove it a few miles home and the bike sounded and ran fine. Changed the oil and filter today, but the light still stays on. I suppose I'll pull the side cover when the weather permits (too chilly right now) but don't quite know what to do after that. If I revve the engine, the light goes out, and when the filter was off, oil would pour out when I turned the engine over, so the pump is working a bit. Could be the oil pressure sending unit is bad; I bought it online about a year ago. Pump could be bad, but I don't know how to tell. Any suggestions? I've split the engine case in the past and don't want to do that again unless I have to do so. Oh, I did push the pressure relief valve when changing the oil filter and it seems to work, though I have no way of know if the spring is weak or it is not sealing.

  2. #2
    Member FerrisBueller's Avatar
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    Can you replace the oil pressure switch with a Guage? That is what I would try first. Then you know if the pressure is fine, then it is you sensor.

    Larry

  3. #3
    Member cfile's Avatar
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    Quote Originally Posted by FerrisBueller View Post
    Can you replace the oil pressure switch with a Guage? That is what I would try first. Then you know if the pressure is fine, then it is you sensor.

    Larry
    The switch is under the cover and there is very little room there. Not sure they make a sending unit that would fit and have a gauge you could mount for it on the bike. Open to recommendations as I don't like the idiot lights.

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  5. #4
    Member FerrisBueller's Avatar
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    Quote Originally Posted by cfile View Post
    The switch is under the cover and there is very little room there. Not sure they make a sending unit that would fit and have a gauge you could mount for it on the bike. Open to recommendations as I don't like the idiot lights.
    Can you take a picture of it's location. Maybe a 1/8" npt elbow.

    Larry

  6. #5
    Member cfile's Avatar
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    You can't see the oil switch without the cover removed, and once it is removed, there is no way to show how tight it is. I pulled the cover today and attempted to check the switch with my air compressor. It certainly works, but I have no way of really measuring at what oil pressure it opens the contacts. Certainly less than 10 lbs. I've fiddled with the oil pressure relief valve and it appears to be workinging in that the piston will move when pushed and the spring seems fine, but again, I have no way to measure at what point it opens to let oil bypass the engine. The oil pump works in that it will pump oil to the hole where the switch goes when turning it by hand, but I have no way of knowing if it is putting out the necessary amount of oil to oil the engine properly. I can't get it out of the bike as the screws are stuck and the more I attempt to remove them, the worse the screws are destroyed. I know of no way to test the pump since you have to have the cover on to run the engine and once it's on, you can't get to the place on the engine where the witch or a sending unit would go. When I had the engine torn down about a year ago, I checked the rod bearings with plastigage and they were fine. Before I did this stuff today, I started the engine and pulled the plug for adjusting the balancer chain, and it blew lots of oil out, making me think the pressure relief valve was open, but maybe not. At this point, I'm stumped. Suppose I'll put it back together and forget about it until spring. Any ideas?

  7. #6
    Super Moderator longdistancerider's Avatar
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    Quote Originally Posted by FerrisBueller View Post
    Can you take a picture of it's location. Maybe a 1/8" npt elbow.

    Larry
    See the small hole in the case cover? That's the drain port for the oil pressure switch that's under the top hump of the cover. It's buried with no room for switch plus gauge fitting and there's no way to connect a gauge to do a running oil pressure test w/o drilling out the cover
    What to do when the oil pressure light comes on-t1a-001.jpg
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    1978 CB400T1 restored
    1972 CL350 nuts and bolts restore in progress


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  8. #7
    Member cfile's Avatar
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    Can you even buy a new oil pump for this machine?

  9. #8
    Super Moderator longdistancerider's Avatar
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    The switch is set at .3 bar = 4.3 psi but can range from 3.6-5 psi
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    1978 CB400T1 restored
    1972 CL350 nuts and bolts restore in progress


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  10. #9
    Super Moderator longdistancerider's Avatar
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    Quote Originally Posted by cfile View Post
    Can you even buy a new oil pump for this machine?
    CMSNL shows 1 left for 143 Euro's. I would suspect the switch going bad first, then too much bearing clearance unless the pump rotors are in really bad shape
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    1978 CB400T1 restored
    1972 CL350 nuts and bolts restore in progress


    https://www.hondatwins.net/forums/22...d-attempt.html and https://www.hondatwins.net/forums/1-...r-attempt.html
    Road Trip https://www.hondatwins.net/forums/11...here-i-go.html or "where's Jim now?"
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  11. #10
    Member FerrisBueller's Avatar
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    Quote Originally Posted by longdistancerider View Post
    See the small hole in the case cover? That's the drain port for the oil pressure switch that's under the top hump of the cover. It's buried with no room for switch plus gauge fitting and there's no way to connect a gauge to do a running oil pressure test w/o drilling out the cover
    Click image for larger version. 

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    Wow, talk about inaccessible. I guess there is no real way to put a mechanical Guage. Not sure what I would do at this point. As you were in the engine last year, you know its condition better than anyone else. Could bearing clearances be out far enough you lose pressure at lower rpm's? Longdistancerider is probably your best bet for info.

    Larry

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