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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.
 

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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
 

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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
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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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
 

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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?
 

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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
T1a-001.jpg
 

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The switch is set at .3 bar = 4.3 psi but can range from 3.6-5 psi
 

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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
 

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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
View attachment 295928
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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Discussion Starter #11
The light has never even flickered at low RPMs, then just came on and stays on at all RPMs below 8 or 9,000. You'd think if the bearings were going it would have been gradual, like coming on at 2500 or less, but going out at highway speeds, not just going out almost entirely .all at once.
 

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The light has never even flickered at low RPMs, then just came on and stays on at all RPMs below 8 or 9,000. You'd think if the bearings were going it would have been gradual, like coming on at 2500 or less, but going out at highway speeds, not just going out almost entirely .all at once.
Maybe pull the cam cover and kick it to see if you are getting oil to the cam and rockers. That is supposed to be the weakness of these twins. If oil is pumping up there while kicking, then it probably is the switch. But I don't really know these bikes that well. Just throwing that out there.

Larry
 

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Had a BMW F650GS single do this last year after a top end rebuild. Before we freaked out we changed out the sending unit with a new one. End of issue. Before you go crazy with engine tear down change the oil pressure sensor. David Silver has OEM and repro in stock. Repro 37240-P13-013P. Cheap.
 

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I have come across a few "failing" oil pressure switches in the past, with replacements available.

It may be possible to get an electronic oil pressure sender unit or even a bourdon tube mech type, but as said space is limited under the clutch cover.

It is possible to check actual oil pressure by getting an other oil filter bolt and adapting it, so a hole and thread can be made thru the bolt to plum in an oil pressure gauge, as a good workshop test.

https://images.cmsnl.com/img/partslists/honda-cm400t-1979-z-usa-lower-crankcase_bighu0131e7z13_7634.gif

This is not your exact bike as there is no balance chain adjustment access hole in the clutch cover, so check.

Part number 2 is the oil pressure relief valve, same as yours??

The oil filter bolt has a blocked filter bypass valve.

It is possible, but really really messy and a bit dangerous, to run the engine with the clutch cover off for a very few minutes ( but keep a real good eye on oil level ) , so you can see the oil pressure relief valve working, so relief pressure is achieved. This is possible with either the horizontal relief valve, or the vertical one screwed into the bottom crank case/oil filter chamber.

To remove those chewed oil pump screws, try a small sharp centre punch and small hammer. You could always drill the screw heads off and catch the swarf.

Be careful with an impact screwdriver ( bit late for JIS or that method now ) and dont break the alloy casting.

Make the effort and make an adapted oil filter bolt and gauge, its not that difficult and not too expensive.

If you have a spare oil filter cover you could adapt that.....

I have only ever seen one other of these bikes with a drain hole by the oil pressure switch, was/is yours a black one???
 

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Discussion Starter #15
Pulled the valve cover off today, and it seems I'm getting oil to the valves so maybe it's just the switch gone bad. I'll buy another, this time a better one than a $10 after market one (I think Dimecycles has a $7.50 one right now). The oil pressure tester using the oil plug sounds like an interesting DYI project but I'd like to try it with an oil filter bolt that I buy off of ebay, ie a spare. Running the bike with the clutch cover off sounds dangerous to the bike and the mechanic (me). My pressure relief valve in on the bottom of the case and opens into the oil filter compartment. I know it will open, as I pushed up on it with a screw driver and I know the spring works. I don't know at what pressure it opens or how well it seals. Anyway, thanks for all the comments. I'll let folks know if the switch was the problem or if I have to go into the engine to figure it out. If I do that, I'll likely wait on Spring.
 

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About 65 psi, its in the manual. Was/is your bike a black one?

"The oil pressure tester using the oil plug sounds like an interesting DYI project but I'd like to try it with an oil filter bolt that I buy off of ebay, ie a spare."

Using the sump plug ( the oil drain plug/bolt ) will not work, but a modified oil filter bolt ( or oil filter cover ) can be adapted to plum in an oil pressure gauge, as said above.
 

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Discussion Starter #18
The oil pressure relief valve is black. And I understood the that it was using the oil filter bolt, not the plug which wouldn't be pressurized by the pump. Thanks again for the info.
 
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