cb750 oil pressure idiot light

[Slemdog]

I have a cream puff 76' cb750 that has 21,000 miles on it (dad was the original owner) and am having a idiot light issue.  I don't ride the bike much, but recently the oil pressure light came on and won't go off.  The bike has oil in it and I changed the oil and installed a new oil pressure sensor in it too.  The bike starts and runs okay, but i don't want to roach the motor, so I haven't been riding it.  What are my options?  I can't believe the oil pump has gone bad.  Is there any cheap tool/gauge that I could screw into the case in place of the oil pressure sensor to double check pressure without the electricals of the bike?  Please help!  Thanks.

[TOOLS1]

Welcome to the forum. You could install a mechanical oil pressure gauge. They can be installed by either replacing the big nut on the right side of the case behind the cylinder with one all ready drilled, and tapped, for it, or just remove yours, and drill, and tap it, for one. However I would first check, for a short in the wire from the idiot light to the sending unit. This wire is normally hot, and the sending unit is the grounding mechanism. If the wire is shorted it will make the light stay on. Try removing the wire from the sending unit, and turning the switch on. If the light comes on the wire is shorted. If the light is off then it is the sending unit, or oil pressure.
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[Daddyman]

Here is what it looks like
Here is where ya buy one

http://www.ebay.com/itm/HONDA-CB750-OIL-PRESSURE-GAUGE-chopper-bobber-cafe-cb-750-/290632279576?pt=Motorcycles_Parts_Accessories&vxp=mtr&hash=item43ab08a618

[Piute]

   Yeah Dog that why they have these manual gages as you called the other Idiot ,

   COOL  That was next on my list and was going for about the same price in Summit I Think1 was s.s. and other blk housing that I was , this could be the easiest improvement on my bike so far  Thanks Guys

[Ceruzziracing]

Not to threadjack, but where can you fit the mechanical gauge on the DOHC? Was considering running a steel braided line and remote mounting one off the upper triple tree.

[TOOLS1]

Just tee off the oil light sending unit, or the high pressure line going to the head.
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[Ceruzziracing]

Yeah, i figured id have to use the one off the idiot light sender but its tucked in the middle of engine, would be hard to see on the 79. I'm going to play dumb on the second part of your answer, what high pressure line?

[TOOLS1]

It comes out of the case just to the right of the sending unit, and goes to the head. You also could tap into the oil galley, by drill, and taping a hole to the right of the line, by the right side cover.
TOOLS

[sgtslag]

T off of the Idiot light sender unit, on top of the transmission, behind the vertical cylinder assembly, with an automotive oil pressure gauge -- not one of the stubbies.  That way you can mount it wherever you need to.  If you go with a mechanical gauge, be careful not to bend, or kink, the (Bourdain) tubing between the sender, and the gauge, as that will ruin it.

[Lucky 1]

It is very common to hear of the oil pressure light being erratic after an oil change.
Not uncommon.

By the way.. The oil pressure sending unit on the Honda CB750 (1978) does not work until 57 lbs. oil pressure.

That is why just turning over the engine with the starter will not get a result.
The engine has to start to get a result. If the idle is set very low that can cause fluctuations also.

But if you loosen the plug on the oil gallery on the CB750 and turn it over then you will find out right away if you have pressure.

[TOOLS1]

You mentioned, you installed a new sensor. Did, you use Teflon tape on the threads? If, so the new sensor might not be getting a ground.
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[Lucky 1]

Good point TOOLS1 !!!

[Slemdog]

I finally took your advice and removed the terminal from the sending unit, turned the bike on and started and the check oil light is still on.  According to your information, I have a short in the line.  Where do I start?  This has always been a garage kept bike and free of any rats or mice that might have chewed through a wire.  Thanks again

[seestheday]

Ceruzz, have you done this mod on your DOHC?  I really want to do it, and am wondering about other people's experiences.

[sgtslag]

If you are referring to adding a mechanical/electrical oil pressure gauge, it is easy to do on the DOHC by using a T-adapter on the idiot light sender connection, or just replace the sender for the idiot light with the sender for the gauge.  Do NOT use Teflon Tape.  Instead, use hydraulic thread sealant (has to cure for 72 hours? impervious to temperature, and pressure, as well as oil).  I tried using Teflon/Plummer's Tape, on my electric sending unit for my oil temperature gauge, installed in the oil cooler adapter, at the oil filter attach point.  It leaked after a few hours, no matter how many layers I used.  The Thread Sealant is still holding, thousands of miles, dozens of hours of use, later.

The mechanical gauge is what I would use, in retrospect.  The electrical requires a good electrical contact, with the block, and it requires a good, full voltage, connection to the battery, to work (electrical connections can get flaky, over time).  The mechanical only requires the Bourdain Tube be kept safe from bends, cuts, kinks, and it only needs power for the internal light -- it works even with the power removed (more of an issue with temperature senders, though).  Mechanical gauges are a few dollars less expensive than electrical, and they can be more accurate (if they have a 270-degree needle sweep, as opposed to the 90-degree sweep of the electrical, same pressure/temperature range; Bourdain Tube types are typically accurate to within 3%!), as well.  Cheers!

[masiste]

I am installing a mechanical oil pressure in my 82 DOHC CB750F It is easy. Like it was said, remove the idiot light presure sender. You can use a "T" connector. I do not care about that idiot light, useless specially if using a gauge. ....
( will use that light space in the cluster for something else. I thought about a brake lamp parrot light. If I connect the remaining circuit the oil sending unit used, to the brake tail lamp, then the idiot light  will nolonger be an idiot light but a smart light. How many of us have ridden w/o a brake -stop lamp working, only to learn later on after a cop pulls us over, hopes, or someone rearending us because of no brake lamp. This lamp would light ed on the cluster only if the tail lamp brake lamp works. If it fails, the cluster lamp would not work, that is a Smart Light !)

                                      ...so all you do is go to "McMasterCarr" ( excellent place for fittings. adaptors etc, and they sell via internet, shipping to your home.) and get an adaptor. Make sure you remember the threads on the oil idiot light sender are "British Standard Tapered" or BST. All vehicles except US vehicles allover the world use BST threads.  So the proper size is 1/8th and the thread is BST.

 Do not use NPT ( National Pipe Tapered) on the BST or you will cross thread your block.( might work going in and staying in, but if you ever take that adaptor off, or if it leaks and you need it off, forget it, you will be in trouble.) So you will need an adaptor that will be male BST ( into engine ) and female NPT ( the gauge and all fittings used mostly in US.)

(1) The adaptor 1/8th /male 1/8  BST female 1/8 NPT. (2) 1/8th Elbow. Female end 1/8  NPT and male end 1/8 NPT.  The male  1/8 NPT connects to the female 1/8 NPT in the adaptor. The female end of the elbow will go to a small extension pipe ( called a"nipple" in piping jargon, at least that is what McMaster Carr calls it ) which will be threaded on both ends. Both ends are 1/8 NPT. Hence your (3erd) part is  a 1/8th NPT Nipple threaded on both sides  and 5.5 in in length ( clears the distance from sender unit to visible side display).

 Your (4) item is another elbow. This one is female on both ends, 1/8th NPT. One end connects to the end of the "nipple" or conneting pipe at the end of the 5.5 in. The other female endof the second 1/8th NPT elbow connects directly to your mechanical gauge.

   Your gauge should display 0 to 100 lbs. The engine runs at about 78 lbs of pressure, so it is good practice to have more capacity within the gauge, so if you go over 78 lbs, an 80lb gauge will not let you know. You need to be able to see it is going over ( hopefully not) if it were the case .

   Jegs has good inexpensive gauges. 1/5 in diameter. Get a "fluid filled" gauge. The fluid ( oil or glycerine) helps the gauge needles stay put despite vibration and shudder from engine  work, and it also keeps the face of the gauge protected from direct elements.

  Will post some photos as soon as I get some.

[masiste]

 Gauge should be 1.5 inches, not 1/5 in as I eroneously posted. Typo error.

[clintwillis]

you made that extremely detailed damn.

sounds like a better idea than even having the light.  and it looks sweet.

[masiste]

 Yes indeed.

   Got all my parts together.  Ordered parts last Saturday from Mac Master Carr and surprisingly parts arrived Monday noon !!! ( I live about 20 miles from that warehouse.)  Yes, close by, but still, great service. They couldhad processed the order Monday, ship Tuesday to receiveit by Thursday or Friday. Not this time

   Will take some photos to post parts before installation.

   About the "idiot" lamp, I think it is just that, an "idiot" lamp. Every time I had the chance to check oil levels in any vehicle, finding out oil is needed , all those times I found out oil level was low not because an idiot light went off, just because I a a carefull driver/rider smart enough to check fluid levels periodically. I think a engine could blow up or seize and the idiot light might still be "napping".

   Seriously that bulb in the cluster could be used in a better way, away with "idiot" stuff ,let's turn that bulb into a "smart" light !    

[clintwillis]

true i have talked to so many people who don't even do routine maintenance and just ride waiting to die.

i have a 79 i want to install the mechanical gauge on ... if you could be so helpful to provide links to parts you ordered or the invoice i could search them myself, lemme know.