Topic: Honda CB750 Wiring

[bryanj]

There were some markets that had white/clear lenses, could be some po fitted amber bulbs because the lense fadded

[Kelly E]

Don't feel bad about using the term "thingy". In my official Honda CB 550 repair manual it says when removing/replacing the carbs to "wriggle" them in/out.

[kyle750]

Trying to use the words "thingy" and "wriggle" in the same sentence 

In order to properly relocate the "thingy" also known as the rectifier connector you first need to unclip the front and then"wriggle" the plastic plug coupler loose and then reinstall on each side of the metal bracket located just above the rectifier and then press in the clip locking the coupler securely in place on the bracket.

[MauiK3]

Just an observation, the green wire in the upper right of the photo, is the insulation a bit melted or does it just look that way in the photo?

[kyle750]

Just an observation, the green wire in the upper right of the photo, is the insulation a bit melted or does it just look that way in the photo?

Thanks Mate.  I'm not exactly sure about the condition of the green wire (loom harness ground) but I will take a closer look in the morning. I agree it looks like it might have melted. 

Also you might notice that the previous owner installed a new red positive cable to the battery terminal but left the old, original positive wire with rubber cap still attached and dangling with no purpose (highlighed in red box in the photo). It should have been removed.   As this old wire is still connected (but no longer in use) it could cause problems if it came in contact with metal anywhere on the bike.

While testing the wiring Tony made an excellent observation that the battery is too tall.  There is only a small clearance between the top of the battery and metal seat pan. If the battery were jolted up (for example hitting a speed bump while riding)  there is a possibility it could come in contact with the seat pan 

[bryanj]

That green wire gets hot if the ground wire at engine end is a bit dirty and high resistance

[kyle750]

I see that Honda changed from the early dangling single fuse box on the early CB750s to a 3 fuse box with coupler for later CB750s and CB550s (Main 15amp, Headlight 7 amp, and Tail Light 5 amp).  After the time spent identifying the short I am quessing this was changed to make it easier for owners to troubleshoot the location of shorts if and when they occur in the electrical system.

[Deltarider]

On mine the 5A and 7A have never blown and are still the original. The 15A has blown several times.

[kyle750]

I'm fairly certain it was the tail light wiring that blew the fuse on the bike.   If it had happened on a later 3 fuse system only the 5 amp fuse would have blown.  The rest of the bike would still have power - is this correct?   

[BenelliSEI]

Correct.

[kyle750]

Thanks for confirming.  The reason I suspect it was indeed the tail light wiring is before Tony rigged up the 10 amp temporary fuse I removed all the connections from the tail light.  Once we had the temporary fuse in place I turned on the ignition and the bike turned on. The newly installed temporary fuse did not blow so therefore the problem must lie somewhere in the tail light wiring either at the rear or in the tail light wiring inside the headlight bucket.

With a single fuse system ANY wiring problem anywhere on the bike - broken wire, poor connection, loose ground, corrosion, frayed wire, or increased resistance from dirt or grime can shut down the entire electrical system.

[MauiK3]

I've heard that some chopper builders just use the rear tail light as a fuse, no actual fuses at all. That must be a real treat when something goes wrong. Minimal wiring is fine but no fuses?

[kyle750]

The older CB750 "All-or-Nothing" electrical system is very challenging

Just because the bike is starting now and all the electrical components are working now does not mean it will stay this way once on the road.  Vibration and the movement of handlebars with old original wiring inside the bars could easily act like a "kill switch" for the whole bike.

On a more postive note:  Both the headlight and starter motor are very strong    Today I went to a very large specialized electrical supply store and bought all kinds of supplies - correct smaller length 15 amp fuses, different width and color heat shrink tube, and heavy duty shrink tube pre-split (wrap-around) sleeves.  I'm going back into the electrical panel later this afternoon to try my best to clean up this mess.

[kyle750]

I just found this short video taken from the seller of the CB750 when I bought the bike so we know the electrical system was starting OK when the bike first arrived

https://youtube.com/shorts/MlcaGuUI2eM

Nothing has been done to the engine or carbs since this video was made.  Does the sound of the engine running in this video bring up any concerns or possible problems? Thanks in Advance!

Late Edit:  I have been advised to check and adjust the valves to make sure they are not too tight before riding the bike?

[BenelliSEI]

The older CB750 "All-or-Nothing" electrical system is very challenging

Just because the bike is starting now and all the electrical components are working now does not mean it will stay this way once on the road.  Vibration and the movement of handlebars with old original wiring inside the bars could easily act like a "kill switch" for the whole bike.

On a more postive note:  Both the headlight and starter motor are very strong    Today I went to a very large specialized electrical supply store and bought all kinds of supplies - correct smaller length 15 amp fuses, different width and color heat shrink tube, and heavy duty shrink tube pre-split (wrap-around) sleeves.  I'm going back into the electrical panel later this afternoon to try my best to clean up this mess.

Not as daunting as you think. Clean and tighten up every connection and move on. Most major components are well isolated from vibration so all you have to do is keep the wiring clean and protected. The harnesses on both our regular riders (cb750k1 and K3) are original and work just fine.

[kyle750]

Not as daunting as you think. Clean and tighten up every connection and move on. Most major components are well isolated from vibration so all you have to do is keep the wiring clean and protected. The harnesses on both our regular riders (cb750k1 and K3) are original and work just fine.

Thanks Mate.  Very good to know.  Amazing that 55 year old wiring still works.  I can't even imagine swapping in a new harness wiring loom and rewiring all the wires running through the handlebars. 

[BenelliSEI]

Just listened to your “Start Up” video. Bike sounds generally healthy but when it comes down off the choke I’d suggest it could use a carb sync. You have a few four cylinder/ four carb bikes? Invest in a good carb sync tool (Morgan Carb Tune works for me) and learn to use it. Your bikes will idle smoother and feel nicer.

[kyle750]

There were 2 electrical components we did not check 

1.  Passing light switch that briefly activates the high beam in the headlight
2.  Horn button - took a while to locate this (because I am a slow witted blockhead) but it is just the PA switch pressed in 

Wiring the CB750 has gone from overwhelming and daunting to "This is Fun" -  Thank You Gentlemen!

Late Edit:  Actually 3 - we also did not check the JDM overspeed warning indicator light (blinking red meatball)

Late Late Edit:  At lunch my German friend Tony said the combination PA/Horn is sometimes called a "Dead Man's Switch"

Photo of the existing horn rewiring by the previous owner

[Hinksy]

Kyle , I rewired my F2 last year and in the end I just got a new harness . I started , when the bike was stripped , by stripping and cleaning all of the components . Then rewrapping the hatness before storing it to reinstall when the motor and frame were finished . This took 2 yrs and by the time I was ready to fit it , all of the switch blocks were either crumbling or showing signs of old age and corrosion ( greenish plugs ) . So I got all new connectors and bullet plug connectors to redo the wiring . During this I noticed how stiff and brittle the old wiring was and because of that I would have to trim a hell of a lot . So I got a new harness and it was really simple to install , except for the headlight bucket , that tested me . As you did , I found out everything earths at there point of contact . I was scratching my head and pulling hair for ages untill it clicked that the wiring diagram shows an earth but the actual thing just grounds out . I also changed stuff from an american import to aussie spec , which I found quite simple once I knew that the american wiring has the headlight cutout through the starter and we dont for those years . I had to buy a new throttle side switch block from germany and it all worked out luckily . Good thing with the F2 is the wiring isnt through the bars and most of the switchblocks terminate into plugs , but there was still a #$%*ton of others and not much space . I also got the magic smoke 3 times/fuses on startup due to a sticky starter solenoide . Dont know how it fixed itself but I was just checking and testing things and after I did the solenoide , things worked . Oh the fun I had and now you can enjoy too , have fun

[kyle750]

Does anyone have a JDM overspeed warning indicator light that works?  Tony said it would be very easy to check. 

I know where the overspeed relay switch is located under the tank and looks like a toasted marshmallow.  I am quessing that it works very similar to the turn signal flasher relay that flash the turn signals on and off.  The turn signal relay also has an audible beeper signal when switched on.  Does the overspeed warning relay switch have a smiliar audible beep when it is activated?

[Deltarider]

From my travels in SE Asia I remember having seen cars (commercial vans) that had a big light on top, the size we have them on towtrucks. You'd see them flash intermittently. If the flashing stopped and the light stayed on, then only the police could reset the light. This after writing a ticket ofcourse. I was told they had the same arrangement with an audible signal, where the horn sounded and again only the police could reset it. This was all 40 years ago to disciplin traffic. The first country come to my mind, is Malaysia but other countries may have had similar techniques.
The term deadman's switch is probably from german language. In Dutch we have the same: dodemansknop. That term is reserved for the killswitch, both on bikes as well as on powerboats where it is law to have your wrist connected to the switch by a cord. How a dead man can operate the killswitch  on a bike is yet to be seen.

[kyle750]

Red Oil Warning Light.  If the light does not come on then it is assumed that oil level is OK.  Perhaps this is a dangerous assumption.  Checking that there is sufficient oil in the oil tank reservoir is simple but how do you check the oil warning light to make sure it is working correctly?  Thanks in Advance!

[Deltarider]

Red Oil Warning Light.  If the light does not come on then it is assumed that oil level is OK.  Perhaps this is a dangerous assumption.  How do you check the oil warning light to make sure it is working correctly?  Thanks in Advance!

Your check is every time you start the engine. Bringing the IGN key switch to ON, brings the oil lamp on. When you start it should go extinct. It is a pressure switch, not to indicate oil level. Some more modern bikes have a warning for that too.

[kyle750]

Your check is every time you start the engine. Bringing the IGN key switch to ON, brings the oil lamp on. When you start it should go extinct. It is a pressure switch, not to indicate oil level. Some more modern bikes have a warning for that too.

Thanks Delta.  I got it.  Yes Red Oil light comes on at ignition and goes extinct at startup. Thank You!

[kyle750]

At ignition with turn signal and high beam headlight switched on.  Only for testing as I would not start the bike with the headlight switched on