Topic: 78 cb750 wiring help on NH seacoast

[sethmcalister]

Is there anybody near the Portsmouth, NH area that could give me a hand re-wiring my 78 cb750? I'll supply the beer.

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[sethmcalister]

Damn autocorrect. Thx! I'm not sure where to start, to be honest. My bike had an old windjammer fairing on the front. Whomever installed it definitely made the wiring their own. I started re wiring and it was extremely messy so picked up a new harness from vintagecb750.com. But, it still wasn't quite plug and play.

My blinkers are working (led), my brake light works, but nothing else seems to work.

Some of the wires still had to be spliced, like the headlight plug. I have the wiring schematics, but wiring is definitely not my thing.

I don't know where to start though really.

I know that I have an orange/white and light blue/white coming from the left control that don't plug in to anything, but I don't see those colors on the schematics.

I don't know why the brake light works, but the taillight isn't. No horn. No headlight. No neutral light.

The front stop switch is hooked in but I don't know where it mounts and it actually looks like it's kind of broken.

You let Mr know what you need, description/pictures and I'll give em to ya.

Thx Cal

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[sethmcalister]

The blinkers are not, but those are working.  The headlight is an H4 from DCC.  The horn came from vintagecb750.  Everything else is stock.

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[madmtnmotors]

picked up a new harness from vintagecb750.com. But, it still wasn't quite plug and play.

Some of the wires still had to be spliced, like the headlight plug.

Not sure why the headlight plug would have to be "spliced" into a replacement harness, unless the headlight plug did not have connectors?

From what I understand there are a couple of peculiarities to the aftermarket harnesses. Here are a couple detailed write ups:

http://forums.sohc4.net/index.php?topic=110411.0

http://forums.sohc4.net/index.php?topic=110284.0

[sethmcalister]

The headlight plug was taken off the headlight from the fairing but it needed to be cut so I needed to splice it into the new harness. I had found a couple write ups about peculiarities, but do you think I could find them again? I'll read over what you've posted now.

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[sethmcalister]

Alright. Made some progress. One of my fuses was blown so picked up a new one. Following the post links from MadMtn resulted in my jumping the single leftover brown in the headlight bucket into one of the black four way connectors (there was no single black).  My taillight works now, however, now when I flip a blinker it just stays on and doesn't blink.

In one of the links above it talks about two brown/red wires but I only have one. I know my headlight isn't working because the low beam isn't tied into the main harness so I'll need to fix that, but I'm not sure how to do it yet. Horn still doesn't work. Neutral light doesn't work.  High beam indicator in the dash works when the plug isn't hooked into the headlight.

What do you guys think?

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[madmtnmotors]

So the blinkers used to flash and now they just come on and stay on?

Check the bulb in the dash for the neutral light first, make sure it's not blown.

[sethmcalister]

The blinkers worked perfectly before jumping the brown wire in the headlight into a black four way. Now, when i hit left the left side comes on and stays on and the same for the right side. Im going to pull the jumped cable out to see if that is the problem.

I'll check the neutral wire/indicator light when I get home today.

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[sethmcalister]

I made a little progress last night.  Taillight and headlight are working.  If I push the blinkers all the way, they stay on, however, if I hold it just before it clicks in they flash.

The starter, horn and neutral light still aren't working.  The starter solenoid works and the black wire in the wiring harness in the headlight has power.  Not sure what else I should look for here?

[madmtnmotors]

Have you verified that the bulb for the neutral light is a good working bulb?

Electrical diagnosis and repair requires a methodical, step by step process of elimination. I often find electrical diagnosis requires finding out what it's not.

[sethmcalister]

Yes, I've verified that a working bulb does not work in the neutral light.  Looking at the wiring schematics, the neutral light, horn and starter button are all tied together which would indicate there's some issue keeping all three from working.  Would that make sense in thinking that?

[sethmcalister]

I've replaced the harness with a new one so all of the connections should be stock now.  The horn is new (from vintagecb750).   Is it possible that the diode is no good?

Also, I can't figure out how my front stop switch is supposed to mount.  I know it belongs on underneath the master cylinder but how?  I can't seem to see how the brake lever actuates the switch?

[sethmcalister]

The winker relay is brand new and was working until I jumped the brown wire for the taillight into the black per MadMtn's posts from earlier.  They work, but not if I click them all the way over.

I've been trying to find a picture of how it's setup and I haven't had any luck aside from the schematic from csmnl.com.

[sethmcalister]

This was due according to this post from Hondaman:

http://forums.sohc4.net/index.php?topic=110284.0

[madmtnmotors]

For the turn lights, I'd suspect the winker relay is bad.

This may be partially true. You indicate that you have LED turn signals. There are numerous posts about getting a "proper" turn signal relay to work with LED lights. The LED lights often do not generate enough resistance to "activate" the stock turn signal relay. This could partially explain your inconsistent results with the turn signals.

[sethmcalister]

Correct, but I have an LED compatible electronic relay and resistors set up which,as far as I've read, is sufficient for these to work properly.

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[sethmcalister]

This is where my lack of wiring knowledge comes in...how do I test that?

The controls have not been refurbished, as far as I know. The starter worked with the old harness though.

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[madmtnmotors]

You will have to trace your problem circuits out one by one, systematically, until you find the problem(s). That's the nature of the beast with electrical troubleshooting. If you try to focus on or solve too many issues at once it's difficult to track your progress, or to determine if you've inadvertently introduced other problems. Fix one issue and move to the next. Using the aftermarket harness has introduced variables that cannot be solved by simply following the factory wiring diagram. The factory diagram now becomes a guide to determine the differences between your harness and the factory harness. One problem circuit at a time, using a multimeter (volts/ohms) to check for either continuity or voltage, whichever is appropriate for the section of wire you are testing, depending on whether you expect it to be energized or simply continuous (or connectivity as in checking the operation of a switch).

I see four options:

>Find an OEM harness
>Seek professional help (a lot of hacks out there though). This could also be expensive due to the non stock harness.
>Ply a friend/colleague (that can read a wiring diagram and use a multimeter) full of beer and steak to get them to help.
>Learn to read a wiring diagram and to use a multimeter.

There's no easy way out with electrical troubleshooting, especially with the variables introduced by the aftermarket harness. Patience and a process of elimination are the only way. Here is an interactive wiring diagram to get you started on your way to mastering the electrical system (and wiring diagram) of a CB750:

http://oldmanhonda.com/MC/wiring750K1.html

Now this is for a 1971 750 which is different (but very similar to) the 78' but should help you understand the basics of reading a wiring diagram.

[sethmcalister]

Thx, Wilbur.  Guess it's time I pick up a multimeter and plug away.  I like tinkering, but boy this is frustrating.  I'm close to the end and want it on the road...only so much riding time in New England haha!

[madmtnmotors]

I hear ya Seth. I have refurbished a couple 750 harnesses:

http://forums.sohc4.net/index.php?topic=137351.msg1549191#msg1549191

[sethmcalister]

Alright so the starter is working... Blew a fuse somehow. But, the GD horn still isn't. Can you guys offer any insight?

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[sethmcalister]

The controls are stock. The PO  had the light green that goes into the control attached to the gotten and the ground. The wiring schematic calls for light green and black. Neither combination works for me though. What is a ground interrupt?

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[sethmcalister]

Yes, the green from the switch runs to the horn. I don't know if the switch is grounded. However, based on what you're describing, it is the same setup.

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[sethmcalister]

There is a black at the horn.

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[sethmcalister]

I have not tested that. Should be red multimeter to black and black multimeter to green, right?

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