The Great Headlight Mystery turned Wire Harness Rebuild

[RuncleUckus]

TLDR: Issue with headlight caused me to say fuck it and redo the main wiring harness. I will update this daily and add pictures that helped me until the wiring project is done.

Item list will be at the bottom.

Photo labeled HONDA_CB750A I used for reference when doing the tear down

Photo labeled DOHCWire was used for ignition and starter

Photo labeled Chopper_Wiring_Diagram was used for light circuit.

If you go with an LED headlight, you can use the Hi/Lo switch as aux lighting or anything else you'd like.

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New to the forum and I am an amateur when it comes to electrical work. Just picked up a 1980 CB 750C. Had an issue with the headlight. Spliced in right after the ignition and jumped the pigtail going back in to the wires on the h4 headlight plug. Worked fine for a couple week but now it has completely lost power. I basically rebuilt the harness for the front (headlight, turn signals, gauge cluster, etc.) After it lost power.. Fixed wires, changed bad connectors, checking switches/fuses and everything looks fine but still no power. Before I rip into the main harness chasing wires, how difficult would it be to just make a new one? I need to redo the tail lights also anyway so I figured why not? If I rebuilt the harness, I'm wanting to simplify it as much as possible. Anything that isn't necessary goes. Can anyone shed some light on any ideas of connections or grounds that usually fail or of a diagram for a bare bones harness? Thanks


Day1: I built the harness off the bike to make sure it worked. Got a good circuit worked out that runs head and tail lights with switches and a fail safe for the brake lights. (They will still work even if there is an issue in the circuit) Running wire and install is tomorrow.

Day2: Some fabrications have to be made for mounting the switches, tail lights and new key ignition. I need to make a bracket for mounting temporary tail lights. Still need to get a wire loom. But the lighting circuit still works on the bike. Had an issue with the starter solenoid (broken connector) but I believe I got it working properly. (Check Day3) Today was mostly planning mounting points and labeling wires.

Day3: I ended up not liking the original mounting brackets I designed so today was mainly just redesigning the brackets. Finished both the ignition/switch bracket and temporary tail light mount. installing them tomorrow morning will update with pictures

Day4: got rained out and since I didn't have a covered place to work, the bike got tarped and I refined the brackets today

Day5: everything is wired in and working except my tail lights. I have a feeling its because of the cheap amazon lights. Gonna get a new tail light and hopefully that fixes it


Item List

4 gauge THHN strained wire
(Battery and starter wires)

14 gauge strained wire
(Lighting, ignition, etc)

2 Waterproof Switches
(One for main light switch. One for hi beam)

Miscellaneous connectors/ pigtail connectors

Universal Key Ignition with start function
(If you don't have the start function you will need a momentary push button)

10 amp inline fuse

30 amp inline fuse

H4 plug
(To keep original headlight, might change it out for a LED bar)

Tail lights
(I used some cheap amazon LEDs until I figure out a better solution)

 

[Loose Chain]

I'm wanting to simplify it as much as possible. Anything that isn't necessary goes.

They don't get much simpler already. Honda didn't run any electrical that wasn't necessary. Either get yourself a wiring diagram for your model of bike or buy yourself a new wiring harness. Do a search on this site and you will probably find yourself a diagram.

 

[RuncleUckus]

They don't get much simpler already. Honda didn't run any electrical that wasn't necessary. Either get yourself a wiring diagram for your model of bike or buy yourself a new wiring harness. Do a search on this site and you will probably find yourself a diagram.

You wouldn't happen to know what gauge wire I would need would you?

 

[Loose Chain]

It would all depend on what you are planning on doing. Why not undo the shortcut you made and fix the problem? Find out what the original problem was and fix that.

 

[RuncleUckus]

It would all depend on what you are planning on doing. Why not undo the shortcut you made and fix the problem? Find out what the original problem was and fix that.

I did, I cut out the splice and repaired all wires, checked and rechecked all fuses,, Still nothing. So since its seeming like I'm going to have to tear into the main harness, I want to go ahead and remove pretty much everything, entire gauge cluster (I'm getting new gauges and will wire them separately when the time comes). My plan for now is to build a harness with only whats needed to make it road legal. Here thats head light, tail/ brake lights and horn.

 

[RuncleUckus]

It would all depend on what you are planning on doing. Why not undo the shortcut you made and fix the problem? Find out what the original problem was and fix that.

I did find this diagram and other than a horn, its what I need

 

[mrtwowheel]

I may have a clue to your problem. I clean switches and I have never been beat by a switch. I even disassemble and clean those little rear brake light switches. I was almost beat by the hi/low switch.

Power for the headlight goes to the starter button, then it goes to the hi/low switch, then to the headlight. I had the hi/low switch disassembled and cleaned. Power was not coming from out of the hi/low switch. So, I put on my glasses and saw that the contacts inside the hi/low switch were not lining up with each other. After many tries to get the contacts to line up I realized there must be more to assembling that switch than I thought. One contact would have to be ramped up over the other contact for this switch to work. After many more tries of assembling this switch I thought about using a feeler gauge as a ramp to assemble this switch. I used a .005" feeler gauge inserted between those two contacts while pushing the parts into position. Finally the two contacts were in the right positions and working. Never had this type of problem assembling any switch.

That is just a simple switch with a ball bearing, spring and one tiny brown insulator, but it does have that assembly quirk.

 

[RuncleUckus]

I may have a clue to your problem. I clean switches and I have never been beat by a switch. I even disassemble and clean those little rear brake light switches. I was almost beat by the hi/low switch.

Power for the headlight goes to the starter button, then it goes to the hi/low switch, then to the headlight. I had the hi/low switch disassembled and cleaned. Power was not coming from out of the hi/low switch. So, I put on my glasses and saw that the contacts inside the hi/low switch were not lining up with each other. After many tries to get the contacts to line up I realized there must be more to assembling that switch than I thought. One contact would have to be ramped up over the other contact for this switch to work. After many more tries of assembling this switch I thought about using a feeler gauge as a ramp to assemble this switch. I used a .005" feeler gauge inserted between those two contacts while pushing the parts into position. Finally the two contacts were in the right positions and working. Never had this type of problem assembling any switch.

That is just a simple switch with a ball bearing, spring and one tiny brown insulator, but it does have that assembly quirk.

I checked all the switches again today,

I may have a clue to your problem. I clean switches and I have never been beat by a switch. I even disassemble and clean those little rear brake light switches. I was almost beat by the hi/low switch.

Power for the headlight goes to the starter button, then it goes to the hi/low switch, then to the headlight. I had the hi/low switch disassembled and cleaned. Power was not coming from out of the hi/low switch. So, I put on my glasses and saw that the contacts inside the hi/low switch were not lining up with each other. After many tries to get the contacts to line up I realized there must be more to assembling that switch than I thought. One contact would have to be ramped up over the other contact for this switch to work. After many more tries of assembling this switch I thought about using a feeler gauge as a ramp to assemble this switch. I used a .005" feeler gauge inserted between those two contacts while pushing the parts into position. Finally the two contacts were in the right positions and working. Never had this type of problem assembling any switch.

That is just a simple switch with a ball bearing, spring and one tiny brown insulator, but it does have that assembly quirk.

I rechecked the switches today and now I'm not even getting power out of the main harness to the front assembly at all. I was hoping the issue wasn't in the main harness but it looks like it is so I'm gonna just make a bare bones harness. I should be able to get all the parts and wire for about a hundred bucks. I'll keep progress updated on this thread

 

[mrtwowheel]

$100 ?????????

 

[Loose Chain]

I checked all the switches again today,

I rechecked the switches today and now I'm not even getting power out of the main harness to the front assembly at all. I was hoping the issue wasn't in the main harness but it looks like it is so I'm gonna just make a bare bones harness. I should be able to get all the parts and wire for about a hundred bucks. I'll keep progress updated on this thread

I don't understand why you want to reinvent the wheel instead of finding the problem. If you are talented enough to make a new harness you should have no trouble finding and fixing the problem.

 

[mrtwowheel]

Do you use a multimeter to check wires for continuity and voltage? If not, buy one and read the little book over and over. You gotta have one and know how to use it to work on a motorcycle.

 

[RuncleUckus]

$100 ?????????

Yea. Wire, connectors, key ignition, switches, h4 plugs, start button, and 2 inline fuses one for 10 amp. Second for 30amp. Came out to be I think 103 USD

 

[RuncleUckus]

I don't understand why you want to reinvent the wheel instead of finding the problem. If you are talented enough to make a new harness you should have no trouble finding and fixing the problem.

Because I'm removing everything that isn't needed to make it road legal. With in my state is head/tail lights and horn

 

[RuncleUckus]

Do you use a multimeter to check wires for continuity and voltage? If not, buy one and read the little book over and over. You gotta have one and know how to use it to work on a motorcycle.

I've checked it with a multimeter. The issues is within the main harness and since I already had plans to redo the wiring it just got pushed forward a bit

 

[mrtwowheel]

I really have no sympathy for these types of operations, like the guys who remove the headlight and say,"There was a rat's nest of wires in there, so I ripped out the wiring harness".

 

[Loose Chain]

Because I'm removing everything that isn't needed to make it road legal. With in my state is head/tail lights and horn

Wow. Good luck.

 

[RuncleUckus]

Wow. Good luck.

Surprisingly easy. I wired everything up off the bike with temporary connections to make sure everything worked. And tomorrow after work I'm making a bracket for new ignition, lights etc. And mounting it to the bike. Even added in some led tail lights too.

 

[RuncleUckus]

I really have no sympathy for these types of operations, like the guys who remove the headlight and say,"There was a rat's nest of wires in there, so I ripped out the wiring harness".

Right? Fuck me for trying to build a project bike. Why would you want to grow your knowledge and learn new skills? Just stay ignorant and unhelpful right? By the way, total time it took to build without going and getting components was 2 hours.

 

[Loose Chain]

total time it took to build without going and getting components was 2 hours.

Did it work?

 

[RuncleUckus]

Did it work?

Yes. All lighting on my bike is now working. (Atleast what I wired in, no oil or neutral light, blacklighting for the gauges, etc.) Just have to fabricate some mounts for the switches, the new key and my temporary tail lights tomorrow. I was having a little issue with the starter/ignition circuit because of some worn out connectors on the starter solenoid but I think I have it figured out. Only thing left is running wires from the spark units to the coils and hooking up the starter.