Low Oil light on at idle after warm up

So this is nothing new, it seems to have slowly developed over a few years. So oil light turns on or flickers after a good long ride. I am guessing the oil gets warmed up and gets a little thinner. I sometimes give it a little throttle sitting at the stop light to idle higher to avoid the red oil idiot light. I think it's trying to tell me I'm an idiot for not changing the oil more frequently. I did change oil and filter recently, and the oil level is good.

Maybe I should just buy another pressure sensor(I hear they are fairly cheap), or look for a real oil pressure gauge to get a decent measure. I have no burning desire to tear the engine apart. I have learned the oil-pump is a separate assembly that I think could be replaced without removing the engine, just taking side panel off? It has a oil pickup with screen, but I think that requires engine removal?

So mostly just fishing for suggestions on the oil red light of doom and what if anything should be done.

Oh, I have been taking temperature measures with hand held IR scanners. When it's all warmed up and running hot it get's up to 300F or so measured between plug and exhaust. Sometimes it varies from side to side, but that's hopefully attributed to my carbs not being tuned perfectly. I've read it might be useful to get a reading on oil temp, and you could get a gauge and put in the fill hole.

Oil pressure sending units can go bad over time, but so do oil pumps wear and potentially drop in pressure output. Jim will be along later and he knows these engines as well as anyone, so he can tell you if the sending unit is the potential culprit or if it's more likely something else.

How many miles on the engine, and what oil are you running?

A new sensor would be a good idea for starters.

jkv45 said:

How many miles on the engine, and what oil are you running? A new sensor would be a good idea for starters.

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Hi jkv45 in WI! 81 CM400T with 24K miles. I just changed oil and filter, using a 15W-40 Diesel Oil, posted picture of it in my carb thread - https://www.vintagehondatwins.com/f...1-Idle-passage-plugged-cannot-clear-81-CM400T. Owned this for about 11 years, put under 5K miles on it, probably just used whatever was going in my cars up until recently. But I have been getting help at Vintage Honda Twins forum. I had top end apart, but it was doing the oil light thing at warm idle before that.

Idle speed can affect the oil light, too low. The light triggers at @3psi so if it's coming on then there is a pressure problem. I've tried the Honda GN4 oil, once, and I had the light popping up within 200 miles running in Texas heat, well over 100 ambient. Switched back to Castrol 4T 10-40w and never had the problem reappear, this was before the rebuild and on the 1st road trip.
I know lots of people like running Rotella and that's fine for them, I will stick to MC specifically designed oil.
Besides the possible oil pressure switch and the oil the other cause could be just plain wear on the rod and main bearings. I would address this by switching oil 1st, replacing the pressure unit 2nd and if that hasn't cleared it up then it's time to get intimately familiar with the inside of the engine.

kbongos said:

Hi jkv45 in WI! 81 CM400T with 24K miles. I just changed oil and filter, using a 15W-40 Diesel Oil, posted picture of it in my carb thread - https://www.vintagehondatwins.com/f...1-Idle-passage-plugged-cannot-clear-81-CM400T. Owned this for about 11 years, put under 5K miles on it, probably just used whatever was going in my cars up until recently. But I have been getting help at Vintage Honda Twins forum. I had top end apart, but it was doing the oil light thing at warm idle before that.

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Well, that oil should be thick enough to give adequate pressure at idle. What is the idle speed set at?

If internal parts are worn significantly, it can reduce oil pressure at idle. A worn oil pump could also.

It could be just the sending unit...

jkv45 said:

Well, that oil should be thick enough to give adequate pressure at idle. What is the idle speed set at?
If internal parts are worn significantly, it can reduce oil pressure at idle. A worn oil pump could also.
It could be just the sending unit...

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I try for around 1200RPM, but I'm always adjusting it at stop lights, it's become a habit. I think my carbs are not synced or setup all that great.
Some kind-of analog reading would be nice, maybe I will try and find some adapters, add some flex tubing, analog gauge. It works just fine for short trips, and so not that big a deal, just fishing for ideas on how much to worry about it, what to look for. Thanks for the suggestions everyone. I suppose someday I will find time to take the lower half of the engine apart and have a good look inside. For now I consider it just a bit of a nuisance.

kbongos said:

For now I consider it just a bit of a nuisance.

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Fine for now, but if it is a sign of worn rod bearings and you continue to ride it with low oil pressure it may well eventually result in a rod bearing failure and become a much more expensive problem.

Is it making any odd knocking sounds at any time that would suggest excessive bottom-end or rod bearing wear?

jkv45 said:

Is it making any odd knocking sounds at any time that would suggest excessive bottom-end or rod bearing wear?

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No odd knocking sounds. It does seem to idle a little rougher when in this hot state, and can occasionally stall, feels like it will miss a spark if it stalls. So I will rev it a little or turn up the idle speed if it's hot and flickering that oil light and I'm stuck waiting on a intersection light.

ancientdad said:

Fine for now, but if it is a sign of worn rod bearings and you continue to ride it with low oil pressure it may well eventually result in a rod bearing failure and become a much more expensive problem.

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Yes it is something to be concerned about, and the word nuisance is not a good choice. I'm going to try pulling the sensor out, it is right there, hopefully turns out easy enough. Then shop for a pressure gauge and fittings to get a better reading on high and low pressure. So preferably something I could leave in place while riding for a while. I suppose one could try and test the sensor off the bike by subjecting it to some controlled pressure(I assume air would work) and see when it switches on/off. I should probably study what I can get to when taking off the clutch side cover. I'm thinking my clutch might need some new pads at some point soon. Clutch works ok now, just seems to not have much play, and I have been a little rough with it in the past.

The 3 engines I've encountered with spun left rod bearings apparently made no obnoxious noises prior to failure, per owner conversation. Failure noted by the starter would barely, if at all, turn the engine over. Doing by hand the crank would stop just short of a 360 turn, reverse direction did the same. All 3 needed new cranks and left rods minimum to fix, everyone chose replacement engines.

LongDistanceRider said:

The 3 engines I've encountered with spun left rod bearings apparently made no obnoxious noises prior to failure, per owner conversation. Failure noted by the starter would barely, if at all, turn the engine over. Doing by hand the crank would stop just short of a 360 turn, reverse direction did the same. All 3 needed new cranks and left rods minimum to fix, everyone chose replacement engines.

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Thanks for the words of encouragement, encouragement to take engine apart sooner than later to check my bearings I did have that top off and some how managed to get it back together. So really I've done half the job already, learned how to get those exhaust pipes off in the middle(much easier after you do it a few times). Would it be reasonable to leave the cylinder head on for inspection of the lower?

I did get the pressure switch out, easy peasy. Until you try and test it. It has like a spring loaded rod out the end of it. Resistance measure always showed open pressing rod in, or putting pressure on, or both with the oil switch out and testing to the switch body. I measured to see if the spring loaded rod would contact something down in case, and it does appear that it will. Measure shows continuity between rod and switch contact. So I assume the rod grounds out at low pressure to somewhere inside, and with proper pressure rod is pressed in to remove contact.


Isn't that a little odd? I would think most pressure switches are more self contained. I tried rigging up my bicycle air pump to see the rod get pressed in, but then it blew off across the room and I gave up, happy enough to roughly know how it works. I'll assume I could replace it with a simpler type that grounds to the case. Thinking an electronic analog sensor type along with a gauge I could mount on my dashboard might be nice. I measured 0.38"(10.0mm) diameter on threads, .23"(6.0mm) for 5 threads, 0.68"(15.6mm) length(not counting rod stick out) of what sticks in case or space washer consumes.

That's the neutral safety switch you've been checking. The oil pressure switch is under the right engine case cover, cover has to come off for access.
The open the bottom end you remove the clutch and shift rod form the right side, rotor and stator from the left side. Flip the engine upside down and remove all the bolts, tap along the seam split to pop the lower case loose and remove. If you have to remove the crank then the top has to be opened up to remove the camshaft so the chain is free.

LongDistanceRider said:

That's the neutral safety switch you've been checking. The oil pressure switch is under the right engine case cover, cover has to come off for access.
The open the bottom end you remove the clutch and shift rod form the right side, rotor and stator from the left side. Flip the engine upside down and remove all the bolts, tap along the seam split to pop the lower case loose and remove. If you have to remove the crank then the top has to be opened up to remove the camshaft so the chain is free.

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Thanks Jim. I see it now, the oil pressure switch is the next switch over on that same harness set of wires tucked behind and on top of that right cover. I'll have to take that cover off, not quite as easy peasy but not bad. Give me a chance to look over the oil pump, clutch, balancer and whatever else is behind there. You mentioned your bearing failure cases being on the left side, do you think that is because oil pump is on the right side? I remember reading, possibly in LDR overhaul doc about the rubber balancer parts getting old and decomposing, maybe sticking up oil passages. I'm thinking I'll just take my chances with no long hauls this summer and this fall tear engine apart in the kitchen or living room, well maybe the basement since I am lucky enough to have one of those.


Now my neutral switch here wasn't working. I just ignored it and had to hold the clutch cable in to start as there is a clutch switch in there that is in parallel. I put this switch back in yesterday to ride it, and I saw the neutral light flicker some but that's about it. I put it back in this morning after reading your reply. Now my neutral and indicator light is working great and I no longer have to hold the clutch in. Now I had to take it out yet again just to prove I have not completely lost my mental bearings. And yes, no measure of contact as you push the spring loaded rod in and the switch case, which confirms that the contact is between the rod and some metal ground in the lower engine when it is neutral. I verified this another way where I measured with it in the bike and only got continuity when screwed fairly tight - this was probably what the problem was with neutral not working. If I loosened it a little the contact goes open. Tried with washer out, and sure enough it shows contact even when not screwed in tight. Do you all think I am correct about this? Do you all think I should try to summarize and post this neutral switch info to try and be helpful? It appears sticky posts might be a useful target on the forum. I suppose I should try a few googles and look over what is already available. Do that now, of course it leads a number of posts by LDR on the old forum, I get a kick out of how many times I bump into your posts. Anyway, one of the posts (Neutral switch replacement) you mention how that washer is a critical thickness and if missing can cause the switch to hang up or lock the shift drum. Wow, who'd have thought it could do that!

The cause of the left rod bearing failure has to do with the oil passage drillings and oil supply. If you look at the oiling circuit in the FSM you'll see that the right rod is fed from the right main bearing. The left rod is fed from the center main bearing. The left main bearing feeds nothing.
The center main bearing is also feeding oil pressure/volume to the transmission, both balancers as well as the left rod. This is a design weakness IMO. Also the reason I don't like the 450 engine which uses the center main to feed the head/cam along with everything else listed.
Take the inherent design weakness and couple it with low oil level where the oil pressure drops off at times because it's sucking air and you've got the recipe for failure.
Now the one thing I do like about the 450 engine is the cam/rocker oiling being pressure fed, that's why I did the modification to my engine.

I'm sure you sprayed some sort of cleaner into the neutral switch while working on it. I've found that the failure of those switches for the most part is actually old oil build up preventing a good contact internally. A good cleaning seems to have fixed everyone I touched and replied to on the old forum.

LongDistanceRider said:

The cause of the left rod bearing failure has to do with the oil passage drillings and oil supply. If you look at the oiling circuit in the FSM you'll see that the right rod is fed from the right main bearing. The left rod is fed from the center main bearing. The left main bearing feeds nothing.
The center main bearing is also feeding oil pressure/volume to the transmission, both balancers as well as the left rod. This is a design weakness IMO. Also the reason I don't like the 450 engine which uses the center main to feed the head/cam along with everything else listed.
Take the inherent design weakness and couple it with low oil level where the oil pressure drops off at times because it's sucking air and you've got the recipe for failure.
Now the one thing I do like about the 450 engine is the cam/rocker oiling being pressure fed, that's why I did the modification to my engine.

I'm sure you sprayed some sort of cleaner into the neutral switch while working on it. I've found that the failure of those switches for the most part is actually old oil build up preventing a good contact internally. A good cleaning seems to have fixed everyone I touched and replied to on the old forum.

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I did spray a little in there and work plunger rod, and I could see how they could fail by the spring loaded rod getting hung up. But I still contend that the switch does not contain both contacts, that the wire connection is just directly connect to the rod and the other contact is the engine itself. Maybe a small detail, but it would be important if you test it off the bike and expect to see a contact change.

I took bike out today for 10 mile 2 way trip, and it is fairly consistent developing a blink only when hot(after 10 miles) and at idle(1200rpm). And in this state it will stall out on me, while it will not stall at idle before going hot. I measure the temp at top of cylinders around 300F and if I point IR temp gauge into oil hole I got 200F.

I was thinking about the clutch and realized that it probably does not have what I called pads like in a car, where they wear like a brake pad. I assume it still has to be a friction thing with wear, and whatever is shed goes in the oil. I googled and some say organic-fibrous-paper or cork. I suppose this is supposed to come out in the wash(the filter) and not clog up your arteries(oil passages). If oil passages blocked I suppose the best cleaning is taking it all apart and diligently cleaning these passage ways.

Larger pieces of debris are caught in the pickup screen and those that make it thru there get caught up in the paper filter.
This is what the pick screen looks like when the balancer rubber have disintegrated.