Topic: Electrical issues '73 CB750

[Mark K]

Installed new wiring harness, finished connecting components, except turn signals. Installed & connected fully charged battery. Turned key to 1st position, neutral light lit dimly, then went out. Checked the new fuse block, 15A fuse blown (although it looked like a smaller fuse). Replaced the burnt out fuse with a 15A and turned key to 1st position with same result, 15A fuse blown. Bike is in neutral but did not have a neutral light.

New components - wire harness, fuse block/fuses, starter relay, positive cable from battery, turn signal flasher unit, coils, coil wires, plug caps, spark plugs, left and right hand controls, new "dash" light bulbs and harness. Also have Hondamans electric ignition, but have NOT installed it yet.
Old components - headlight, brake light, regulator, ground cable, rectifier, safety starter unit, front and rear brake light switches, no brake fluid in front brake.

I have not started the bike yet, although I did do an oil psi check a month or so ago using my truck battery, starter turned engine and got positive oil psi (after some unrelated issues).

Any suggestions on how to proceed to determine the electrical issue would be greatly appreciated. I'm not very good with electrical. I do have a multi-meter and can do simple checks. Thanks in advance.

[newday777]

Sorry for the problems.
You'll first need to inspect closely each connection that they are plugged in the correct corresponding connector.
And also inspect each wire for pinched wires grounding out causing a direct short.

[TwoTired]

First verify the fuse melts near center of glass rather than near the endcaps.  The former is an over current, the latter is a clip heating issue requiring a different approach.

Assuming over current,  rig up a lamp to connect between fuse clips in place of the fuse.

The lamp will safely limit, destructive current, while allowing you to sequentially disconnect things until the lamp goes out.  When that happens, you found the branch that has the problem, which you can scrutinize further to find the fault.

An accurate schematic of the wiring is pretty handy when troubleshooting bike wiring issues.  Treat it like road map pathway for electron flow from battery post to battery post.  Everything DC travels in a loop between those two posts.

[Don R]

 I like to print out the color wiring diagram from the old man honda web site. I've been known to glue it to a piece of cardboard and hang it where I can see it with both hands busy.
 Be sure everything is turned off, There was a male to male connector wire in the headlight bucket, I remember that jumper being confusing. Be sure it's right.
 Everything TT said. He helped me out a couple times and I like to think I'm good at wiring.

[newday777]

I like to print out the color wiring diagram from the old man honda web site. I've been known to glue it to a piece of cardboard and hang it where I can see it with both hands busy.
 

Link to the 'old man honda website '??

[newday777]

I like to print out the color wiring diagram from the old man honda web site. I've been known to glue it to a piece of cardboard and hang it where I can see it with both hands busy.
 

Link to the 'old man honda website '??

Is this it?

https://www.oldmanhonda.com/

[Mark K]

First verify the fuse melts near center of glass rather than near the endcaps.  The former is an over current, the latter is a clip heating issue requiring a different approach.


Assuming over current,  rig up a lamp to connect between fuse clips in place of the fuse.

The lamp will safely limit, destructive current, while allowing you to sequentially disconnect things until the lamp goes out.  When that happens, you found the branch that has the problem, which you can scrutinize further to find the fault.

An accurate schematic of the wiring is pretty handy when troubleshooting bike wiring issues.  Treat it like road map pathway for electron flow from battery post to battery post.  Everything DC travels in a loop between those two posts.

The melts did occur in the center of the fuse.
I've been using the color wiring diagram I found here on SOHC and the in the FSM. I downloaded the one from the old man Honda site and will reference that.

Thanks for your help.

[Mark K]

I like to print out the color wiring diagram from the old man Honda web site. I've been known to glue it to a piece of cardboard and hang it where I can see it with both hands busy.
 Be sure everything is turned off, There was a male to male connector wire in the headlight bucket, I remember that jumper being confusing. Be sure it's right.
 Everything TT said. He helped me out a couple times and I like to think I'm good at wiring.

Tried printing out wiring diagram from old man Honda but my color printer is acting up and is of no use after f&$%ing with it for the past 30 minutes, changing out ink cartridges, etc.

There was a medium blue wire coming out of the left hand control and the 'dash' lamp harness that both had male connectors. Switched one out for a female connector. I believe it's for the headlight high beam indicator, but I am no expert.

Thanks for your help.

[Mark K]

Sorry for the problems.
You'll first need to inspect closely each connection that they are plugged in the correct corresponding connector.
And also inspect each wire for pinched wires grounding out causing a direct short.

I'm going to check every connection and for pinched wires. I feel like Clark Griswold in Christmas vacation, "Russ, we checked every bulb, didn't we?"

Thanks for your help.

[Nurse Julie]

Is it an aftermarket new main wiring harness?

[Bodi]

An aftermarket harness can be a bit "off" but normally are good. If there are differences in model years, they can be trouble if another year's harness is used.
The color and Haynes etc diagrams are not super accurate. Get a Honda diagram from an owners booklet or service manual, they are online various places. If the downloaded copy is blurry and unreadable keep looking. The switch function diagrams are funky but once you get the idea it all makes sense.
73 ... not familiar but does it have one or three fuses? If three, unplugging the two sub fuses isolates their wiring.
Any load problem is going to be on the black wire since that's the main power from the keyswitch.
Using a tail light bulb in place of the main fuse is a great trick. Saves a lot of fuses and lets you fiddle with wiring while seeing if you have disconnected the problem branch.
Look around the electrical panel first. The regulator and flasher wiring may be wrong. Check that the solenoid is wired correctly.
The rectifier is not your problem I think, it connects directly to the battery so the keyswitch is not involved in its wiring at all.

[Mark K]

Is it an aftermarket new main wiring harness?

Yes, new aftermarket harness, most of the colored wires match up, although there seems to be a few extras.

[Mark K]

An aftermarket harness can be a bit "off" but normally are good. If there are differences in model years, they can be trouble if another year's harness is used.
The color and Haynes etc diagrams are not super accurate. Get a Honda diagram from an owners booklet or service manual, they are online various places. If the downloaded copy is blurry and unreadable keep looking. The switch function diagrams are funky but once you get the idea it all makes sense.
73 ... not familiar but does it have one or three fuses? If three, unplugging the two sub fuses isolates their wiring.
Any load problem is going to be on the black wire since that's the main power from the keyswitch.
Using a tail light bulb in place of the main fuse is a great trick. Saves a lot of fuses and lets you fiddle with wiring while seeing if you have disconnected the problem branch.
Look around the electrical panel first. The regulator and flasher wiring may be wrong. Check that the solenoid is wired correctly.
The rectifier is not your problem I think, it connects directly to the battery so the keyswitch is not involved in its wiring at all.

Mine ('73) has three fuses, 5A - taillight, 7A - headlight, and 15A - main.
I'm using a simple circuit tester, has a lightbulb in the handle, connected that at the 15A fuse connector, as suggested by TwoTired. I've verified the regulator is connected correctly by the stampings on the regulator leads and FSM. The flasher has two leads, does it matter how they are connected? I couldn't see the difference in the wiring diagram.
The solenoid is new and I've verified the connections are correct.

[Mark K]

The lamp will safely limit, destructive current, while allowing you to sequentially disconnect things until the lamp goes out.  When that happens, you found the branch that has the problem, which you can scrutinize further to find the fault.

I've connected a simple circuit tester (lightbulb in the handle) to the 15A Main fuse holder. With the battery connected and the ignition switch in position one, the light is lit. Unplugged safety starter unit - light stays on, reconnected. Unplugged rectifier - light stays on, reconnected. Unplugged the fuse block from the harness, turns the light off. Plugged it back in, light comes on. Unplugged ignition switch connector, light goes out. So I think I'm headed in the right direction.

[Don R]

  When I first started messing with these bikes the black being hot/switched by the key took a minute to get used to. The cars I always worked on were black grounded.

[bryanj]

The headlamp in lh switch is dark blue and goes to a double connector on the lamp along with the high beam indicator bulb. White is dip beam. Mid blue is normaly an indicator along with orange then same colours with white stripe for running lights, grey is indicator power from relay.
What other colours do you have

[TwoTired]

The lamp will safely limit, destructive current, while allowing you to sequentially disconnect things until the lamp goes out.  When that happens, you found the branch that has the problem, which you can scrutinize further to find the fault.

I've connected a simple circuit tester (lightbulb in the handle) to the 15A Main fuse holder. With the battery connected and the ignition switch in position one, the light is lit. Unplugged safety starter unit - light stays on, reconnected. Unplugged rectifier - light stays on, reconnected. Unplugged the fuse block from the harness, turns the light off. Plugged it back in, light comes on. Unplugged ignition switch connector, light goes out. So I think I'm headed in the right direction.

The key switch routes power from battery throughout the bike via the black wires.  It makes sense that the fuse issue goes away when that is disconnected.  Follow the diagram where the black wire routes. And disconnect things from that black wire.  If, for example, if all the destinations for the black wire are disconnected and fuse issue remains then somewhere the black wire is pinched, the insulation compromised, and rubbing frame components.  While it could be anywhere, near the steering neck and bars, especially if internally routed, bears focused scrutiny.  Also possibly you connected that black wire to a grounded device, particularly if the color coded wires are not exactly the same as the stock harness.   I find a methodical process gets the job done.  Quick fixes only come with experience and thorough understanding. 
Cheers,

[Mark K]

Went back and unplugged everything connected to wiring harness in the headlight housing. Went step-by-step and connected everything by the color wiring diagram. And then tried it again, same results. Disconnected the black wire from the harness going into housing with no change.

Side note - with everything connected, moving the kill switch to off, causes the test light to slightly dim.

I have a brown wire with black stripe that does not have a match in the housing. The only place I see this wire is coming from the fuse block (one of the 6 wires). I believe it's either the headlight or tail light, but have not figured out what it's supposed to connect to.

One other question, could my issue be caused by a bad ground wire from the negative battery terminal to the ground or the frame ground wire from the harness? I haven't had the chance to do anymore trouble shooting.

Thanks for all the help.

[HondaMan]

Side note - with everything connected, moving the kill switch to off, causes the test light to slightly dim.

That is normal: when the RUN switch is ON it is powering the coils, when OFF it disconnects power to the coils. So, the light will be brighter (more load) when the switch is ON.

I have a brown wire with black stripe that does not have a match in the housing. The only place I see this wire is coming from the fuse block (one of the 6 wires). I believe it's either the headlight or tail light, but have not figured out what it's supposed to connect to.

This wire connects to the middle fuse in the fuseblock, and powers the lights. This should be involved with the START button: when the button is released this circuit should be live, when the START button is pressed it should go out. Thus, the lights all go out when the starter is running. The K3 was the first 750 to have this feature, but in the K4/5 bikes this harness changed a bunch (no 'safety module' in most K4/5 versions, just a diode). Sometimes (like in a bike I reconstructed from boxes) the new wiring harness might not work quite right in this circuit. Disconnecting the Brown/Black wire should give a clue to what is being powered by it, at least?

One other question, could my issue be caused by a bad ground wire from the negative battery terminal to the ground or the frame ground wire from the harness? I haven't had the chance to do anymore trouble shooting.

The battery ground cable should be captured between the upper left rear engine boss and the frame. The inside of the frame boss is bare at that site (if still OEM). There is another Green ground wire from the harness at the coils: in the K3 factory harness this was connected to the front coil mount, trapped against the frame by the 6mm bolt that goes thru the frame to mount the coils. The frame must be bare at that site.

[Nurse Julie]

Have you got these 3 wires in the correct order 🤔🤔🤔🤔:-
Top.... Green
Middle..... White
Bottom....... Black.

[Mark K]

Hondaman,

"Disconnecting the Brown/Black wire should give a clue to what is being powered by it, at least?"

My problem with the brown/black is I don't know where it's supposed to connect to.

"The battery ground cable should be captured between the upper left rear engine boss and the frame. The inside of the frame boss is bare at that site (if still OEM). There is another Green ground wire from the harness at the coils: in the K3 factory harness this was connected to the front coil mount, trapped against the frame by the 6mm bolt that goes thru the frame to mount the coils. The frame must be bare at that site."

The battery ground is located just as you describe. I did sand the paint off, but will remove and double check. The aftermarket harness I have, has the frame ground attached between the seat latch and the frame. I'll also double check that one.

[HondaMan]

Hondaman,

"Disconnecting the Brown/Black wire should give a clue to what is being powered by it, at least?"

My problem with the brown/black is I don't know where it's supposed to connect to.

"The battery ground cable should be captured between the upper left rear engine boss and the frame. The inside of the frame boss is bare at that site (if still OEM). There is another Green ground wire from the harness at the coils: in the K3 factory harness this was connected to the front coil mount, trapped against the frame by the 6mm bolt that goes thru the frame to mount the coils. The frame must be bare at that site."

The battery ground is located just as you describe. I did sand the paint off, but will remove and double check. The aftermarket harness I have, has the frame ground attached between the seat latch and the frame. I'll also double check that one.

You can ground at either place: Honda moved it around now and then. It just needs to be nice and clean where grounded, and maybe add a drop of oil to preclude future crusts. It will squeeze out where it shouldn't be when you tighten things up.

[Mark K]

Have you got these 3 wires in the correct order 🤔🤔🤔🤔:-
Top.... Green
Middle..... White
Bottom....... Black.

Yep, those are connected in the correct way.

[HondaMan]

"Disconnecting the Brown/Black wire should give a clue to what is being powered by it, at least?"

My problem with the brown/black is I don't know where it's supposed to connect to.

In the aftermarket switches, I have seen different colors coming from the RH switch that are supposed to power this one. If you can find those wires from the switch, look for one that isn't connected to anything (probably in the headlight bucket). It might be Brown, or Black/Yellow, or even just Yellow: it should have 12 volts on it when the key is turned ON, but loses the 12 volts when the START button is pressed. If this wire isn't there at all, then the RH switch might not be the exactly-correct one: many of the vendors have this confused. If that is the case: you can connect the Brown/Black on to an available Black (in a pinch), which will ensure the taillight is always lit regardless of whether you have the Headlight switch ON or not.

This particular situation arises in the K3 because it was the first 750 to have the "lights off while START pressed" function, which was accomplished by adding the 3-fuse block under the left side cover, and running the lights from the middle fuse (7 amp), which was fed power from the OFF-ON headlight switch. Trouble now is: the handlebar switch vendors are fully confused by the fact that in 9/72 when the K3 production began, the RH headlight switch has the OFF-ON switch there for the lights, while somewhere around 3/73 this was discontinued for the remainder of the K3 production. Then it simply lost the RH OFF-ON switch while retaining the same shape for the housing (no slot, though, for the switch lever, so they LOOK like something is missing). Thus, when we try to order a RH switch for a 750K3, the switch that is supplied might meet both the K3 and K4 spec (no OFF-ON switch), or not (has an OFF-ON switch). This really hoses up the wiring scheme...

[Mark K]

Hondaman,

Thanks for all of the information. I will try th things you suggested next chance I get to work on it. I really, really appreciate everyone's great advice with my electrical issue.