Topic: Electrical issues '73 CB750

[Mark K]

Removed battery ground cable to frame, checked continuity, cleaned both ends really good, reinstalled frame end of ground cable (battery out of bike currently). Cleaned frame area where ground attaches so its completely bare metal, no paint.

Removed frame ground wire, cleaned the frame where frame ground wire attaches (behind the seat latch) and reinstalled. Problem still remains.

Have not attached the brown w/black stripe wire to black power yet, as suggested by HondaMan, hope to get to that today.

Forgot to mention, also have a brown wire w/red stripe (comes from the fuse block as well), does not have anywhere to plug in.

One other thing, the PO had a ground wire between the top of the rectifier going to ground. The wiring diagram does not show that, as there is already a ground wire (green) coming off the rectifier (along w/ 3 yellow wires and 1 red wire).

Suggestions on the brown w/red stripe wire and the extra ground from the rectifier?

Thanks!

[HondaMan]

Have not attached the brown w/black stripe wire to black power yet, as suggested by HondaMan, hope to get to that today.

Forgot to mention, also have a brown wire w/red stripe (comes from the fuse block as well), does not have anywhere to plug in.

One other thing, the PO had a ground wire between the top of the rectifier going to ground. The wiring diagram does not show that, as there is already a ground wire (green) coming off the rectifier (along w/ 3 yellow wires and 1 red wire).

Suggestions on the brown w/red stripe wire and the extra ground from the rectifier?

Thanks!

Yep...the Brn/Blk and Brn/Red wires are (usually) the 2 ends of the center fuse in the fuseblock. Normally, the Blk/Red comes from the START button: in the UNpressed position it is supposed to send 12 volts to the center fuse, and that power returns to the headlight on the Brn/Blk wire, which then goes to the Lo/Hi headlight switch and also to the (left handlebar) Turn Signals' switch to power those when the switch is ON one side of the other. In some of the K3 bikes (notably those made near 3/72 production) the Blk/Brn was changed to Black/Yel, which it later became in the K4 wiring harness also.

But, a caveat here: this info is Honda OEM wire colors. I have seen slight variances in the colors in the aftermarkets harnesses, and especially when someone bought the wrong (i.e., usually K4/5 type) wiring harness. Honda changed the Blk/Brn scheme then.

It their wiring 'language' (some called it 'parlance' back when) the Brown family of wires belongs to lighting power circuits. Black wires are always 12 volt Ignition power-related. When the original source (like, Black for +12v) is 'interrupted' in the post-K2 harnesses by a switch or fuse, the source color is retained (i.e., Black) indicating "+12v Power" and the circuit that it is powering is added as a stripe. Such the wiring is Brn/[circuit], like Brn/White for Taillight power (and the front running lamps in K3 and later). The Red color designation was "power supply" to other circuits in a continuous fashion, while Yellow color indicates temporary (we might call it 'momentary') power, like the Start circuit. In the K3 with its unique Safety Module gadget, this got messy with the power from the Start Button where it was originally Yel/Red (and supplied handlebar Gnd when pressed) until K3 started: this changed to Blk/Yel in some K3 (and Blk/Red in others), going into the Safety Module, and then LtGn/Red going from the Safety Module to the Start Solenoid. In those bikes (and later ones) the opposite wire on the Start solenoid then went to Gnd, where in the early bikes it was tied to Blk instead (i.e., the wiring at the Start Solenoid reversed the coil connections in the K3 and later bikes).

The wiring diagrams from the early/late K3 period are not delineated as "pre/Post 3/72", so they are often wrong with the Blk/Red. Blk/Yel, and Brn/Yel when taken from the Internet, too. The original Owner's Manuals (that came with the bike) for the differing K3 bikes WERE correct, and we often asked the owner for it if we had to work on wiring for them in my shop.

[PeWe]

PO had improved grounding on rectifier.....

I have done that mod too after reading an old Hondaman post where he mention that and why.
That explains why so many old rectifier wirings are melted, green wire plus the connector sit loose in contact housing that need to be fixed with epoxy.
The green wire will suffer when engine grounding is bad, electrical starter will find other ways ...
No problem on my K6 I have own since early 1979.

Another thing with old rectifier I found for my K2 build, plus an extra unit.

Photo where a thick grey wire is added, goes direct to battery -.

[Mark K]

Double checked all wire connections. Battery connected, circuit tester jumper on 15A fuse location, key on, light is on. Disconnected everything in head light, light remains on. Removed wire harness from bike, started removing black tape/covering. Found this. Could this be what is causing the issue? I have not stripped black covering any further.

Should these black wires connect with bullet connectors or be soldered together?

Thanks.

[Airborne 82nd]

Double checked all wire connections. Battery connected, circuit tester jumper on 15A fuse location, key on, light is on. Disconnected everything in head light, light remains on. Removed wire harness from bike, started removing black tape/covering. Found this. Could this be what is causing the issue? I have not stripped black covering any further.

Should these black wires connect with bullet connectors or be soldered together?

Thanks.

Who ever makes that harness should NOT be doing any electrical work at all.No telling what else they have done wrong. My 10 year old grandson can do better than that. I would send it back.

[RAFster122s]

Any wires not crimped or soldered found just twisted together are a hack of a P.O. or shoddy mechanic that has bodged something previous to you…

[HondaMan]

If that splice had been done properly, it would have 2 wires coming in from two opposite sides into the copper crush-splice for a 4-way splice - but with very little bare wire showing. That one looks like someone might have forgotten a wire, then tried to twist it onto the bare ends by the splice? (Dunno, can't quite tell from the picture?) But, the branch connections are correct. Those are what the splices in these harnesses look like. It is just unusual to see them all in one place, as the OEM Honda harness limited any such junction to 3 wires maximum into the splice, which most often was found in the form of a stripped spot on the main "backbone" wire (usually the largest wire of that color in the bundle) with one new wire branching away from the site to the load, like to the coils, or to the voltage regulator.

[Mark K]

Any wires not crimped or soldered found just twisted together are a hack of a P.O. or shoddy mechanic that has bodged something previous to you…

This is a new aftermarket wire harness with some of the black wrapping tape removed, as I was looking for shorts and/or places where the wire harness may have been rubbing. Since it was just installed on the bike a few weeks ago and the bike isn't running, there are no places where the harness has bare wires touching anywhere on the frame caused by rubbing.

[RAFster122s]

If they were just twisted together, then my statement stands... "Mechanics" sometimes aren't very good mechanics...
I will never buy a car from a mechanic. Some of the poorly maintained or cared for vehicles I have seen were from guys who were mechanics for a living.

[bryanj]

Thats because we know exactly how far it will go before dying and fix it just before!

[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.

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,

Removed/stripped harness, soldered all the splices. Hooked everything to the harness, same result, unplugged everything in headlight bucket - light remains on.

Something I had not checked before was disconnecting the black lead from the regulator. Unplugged black wire - turned the light off, as did unplugging the regulators green wire.


Is this an indication the regulator is bad?

Is there a way to test the regulator off of the bike?

Thanks for any and all help.

[PeWe]

Headlight still on without the 7A fuse (for headlight)?
Your 1973 has the later fuse box with 15, 7 and 5A right?

Wrong connection in headlight bucket from light switch?
12V from "system 12V" feeding ignition etc to headlight, not from the 7A circuit.

[bryanj]

If headlight works via regulator something is very wrong

[TwoTired]

Is your test light an led type?  I always use a filament bulb, stop lamp, or headlight. 

The regulator connects a 7 ohm load on the black distribution whenever It thinks the battery needs power.  That’s nearly a 2 amp load drawing through that test light, but not enough to blow a fuse.  If you suspect the regulator, use an ohm meter to measure the black connection point and green/frame ground. It takes a bit lower than 1 ohm to melt the fuse.  When using a meter to measure ohms this low, be sure to use the lowest range available on the instrument, and subtract from the test readout whatever  is displayed when only the two leads touch together.

If you measure ohms too low on the reg’s black connection, you’re on a good track. Disconnect the white from the regulator.  If the black terminal to green still measures low ohms, the regulator has a problem.  Need to know what regulator you have for further guidance. The stock has much better known internals than aftermarket.

If your regulator proves not to draw excessive, then leave it disconnected while reconnecting the remaining black connections.  A filament bulb will show quite bright when a short is connected.

[Mark K]

Headlight still on without the 7A fuse (for headlight)?
Your 1973 has the later fuse box with 15, 7 and 5A right?

Wrong connection in headlight bucket from light switch?
12V from "system 12V" feeding ignition etc to headlight, not from the 7A circuit.

PeWe,

Yes '73, 3 fuse fuse box -15A, 7A, & 5A. 15A jumpered, 7A & 5A still in fuse box while testing. Headlight will not light at all. I have a 12v black wire feeding the ignition in the right hand control.

[Mark K]

If headlight works via regulator something is very wrong

Headlight not working at all.

[Mark K]

Is your test light an led type?  I always use a filament bulb, stop lamp, or headlight. 

The regulator connects a 7 ohm load on the black distribution whenever It thinks the battery needs power.  That’s nearly a 2 amp load drawing through that test light, but not enough to blow a fuse.  If you suspect the regulator, use an ohm meter to measure the black connection point and green/frame ground. It takes a bit lower than 1 ohm to melt the fuse.  When using a meter to measure ohms this low, be sure to use the lowest range available on the instrument, and subtract from the test readout whatever  is displayed when only the two leads touch together.

If you measure ohms too low on the reg’s black connection, you’re on a good track. Disconnect the white from the regulator.  If the black terminal to green still measures low ohms, the regulator has a problem.  Need to know what regulator you have for further guidance. The stock has much better known internals than aftermarket.

If your regulator proves not to draw excessive, then leave it disconnected while reconnecting the remaining black connections.  A filament bulb will show quite bright when a short is connected.

TwoTired,
Appreciate all of your help. My test light is a filament, 3watt bulb.

Regulator is stock, pictures attached.

I do not want to assume anything, so I will ask, your advice is based on regulator still mounted on the bike and connected, correct?

Also, I may have misunderstood your earlier post, I thought when the short was unplugged, the light would go out?

The light dimming when I unplug something shows the short?

I really appreciate everyone's patience with me, electrical issues (along with plumbing) are my nemeses.

[TwoTired]

The regulator tests I described are with the regulator mounted.

The regulator cover should have color labels for wire attachment. Disconnect the black lead only to measure the resistance to green there at the reg terminals.

The test lamp illuminates when there is current passing through it.  Its brightness can vary with voltage and current in a series circuit.  With the black circuit activated, the fuse or test lamp will carry current.  A 3 watt bulb, will limit current in circuit to about 1/4 amp max.  All wiring should survive this without smoke.  Additional resistance will further limit current to less and bulb would be dimmer than if placed directly on battery.  But, it is relative, and 3 watts isn’t much in current value.  I like to use a headlight bulb for more current and brightness effect, while still protecting wiring.  Using a 3 watt bulb, you may have to probe with a voltmeter.  A short will have the black wires at zero when the test light is on.  A more normal load allow some voltage to appear on the black wires, other than zero, which normally dims the bulb in ratio to the added load on the circuit.  The light should be brighter when the short is present.  I’m not certain you’ll see that with a 3 watt bulb.  I know you do with a 40 watt bulb.

Some circuit theory:
The black wires distribute voltage in parallel to all its loads. All loads see the same voltage and drain current as needed.  The bulb adds a series component which drops voltage commensurate with its resistive value to all attached to the black wire.  A 15 amp fused circuit can tolerate  a system load resistance of 0.8 ohms.  That isn’t far from a zero ohm short.  With circuit current limited to 0.25 amps and a 3 watt bulb resistance of 48 ohms, small wonder you don’t see much difference in brightness, with/ without short.  Your light is just telling you there is a circuit path, not resistive quality of that path which all loads will have.  The 40 watt bulb has a filament resistance of 3.6 ohms, which explains why dimming is more apparent.

There are many ways of troubleshooting.  If you wish to use your test light, I suggest you use that to allow power on the black circuit and use a voltmeter to monitor that while you disconnect/ or connect things from that black distribution circuit. When what you disconnect sees a rise in voltage off zero, that’s the circuit to investigate.  Or, when adding another connection to the black and voltage goes to zero, that’s the branch with the problem.

[Mark K]

TwoTired,

Thank you for all of the information. I "reassembled" the harness today after I soldered all the internal splices yesterday. Reinstalled harness on bike and will continue testing. I'll hook up the headlight bulb in place of the circuit tester. I'll post my results here, once I figure it out or hit another stumbling block.

Thanks again.

[TwoTired]

If you use the headlight bulb, remember it is drawing power from the battery, which limits your time for troubleshooting, as it depletes.   I keep my automatic battery charger, good for 6-10 amps while the lighting and circuits are powered.  The automatic and float charge design will keep from overcharging the battery if the circuit loads get too small.  I normally wouldn't like to use a charger with more than 2 Amp capability.  But, A good automatic design will taper off and trim the voltage and current down as the battery gets full.  Do periodically check your battery voltage, if it gets below 11 volts, suspend your investigation until the battery is restored to 12.6V resting.

I mention this because, I can probably find issues faster than some who are inexperienced.  My techniques work, but there was a learning curve that made the troubleshoot time shorter with practice.  I'd rather you didn't kill your battery while hunting down the overcurrent issue.

There is an alternative.  If you can locate a 15 or even 20 amp 12V circuit breaker, you can clip that into circuit where the fuse goes.  Then with all disconnected from the  black wire, adding one connection at a time will tell you which circuits are ok.  And when you add the bad one, it will trip.  I would then connect all the others to verify that branch alone draws excessive current.  Remember all electrical problems with proven designs are actually a mechanical/physical problem.  Something connected where it shouldn't that allows current to flow where it shouldn't.  That includes a switch that when closed bridges Black to Green.

Have you tried opening every switch you can find?  If you have a supply of fuses and can afford it, that maybe another option for isolation.  I can't imagine not using a known applicable wire diagram to aid troubleshooting, like what is found in the Honda Shop manual and its supplements even though they are not in color.  The errors in them are quite rare.

[Mark K]

Thanks again for the additional information and tips. I am using the FSM wiring diagram along with the color one from the library here and the one from the Old Man Honda website. Hope to get to do some troubleshooting this week after work in th evenings.

[HondaMan]

The regulator: it sounds like it is OK. The Black wire on it goes to the coil, and to the upper relay contact. The Green wire is the ground. The White wire is the current to the alternator's field coil. The Black will always pass power to the Green, that's the normal operation. There is a resistor mounted to the back of the relay: that is the "mid-charge" current limit to the field coil. This is not a "relay" in the normal sense, as the coil pulls the upper contact downward in proportion to the voltage on the Black wire.

The way it works (for your future reference) is:
-When the voltage falls below (about) 12.4 volts, the coil does not pull hard enough on the moving arm. This lets the upper contact on that arm meet the +12 volt (Black wire) terminal. The moving contact, which is connected to the White wire, then receives a full +12 volts to send to the field coil and puts the alternator in full-charge mode.
-When the voltage on the Black wire rises to about 12.8 volts, the coil pulls more strongly on the moving contact and disconnects it from the upper (Black) wire's contact. Now, the current on the moving contact, which is connected to the wirewound resistor on the back of the regulator, is connected to +12 volts only through that resistor. This lowers the current to the field coil, reducing the alternator's output to about 60% of full power.
-When the voltage on the Black wire rises above 13.2 volts (K0/K1/K2 bikes) or 13.6 volts (all later SOHC4 750s) it pulls harder on the armature, which moves further down and connects the White wire to the lower contact, which is the Green wire, or Ground. This stops all current to the field coil and the alternator output drops to about 1/4 amp.

So, it's not really a 'relay', although it looks like one. And there is always a connection (the coil) between the Black and Green wires, which in your situation is enough of a load to light the little 3W bulb (or an LED type bulb tester).

There is often some confusion about this regulator, mostly because the earliest sandcast 750s used one that acted like a relay as it did not have the 3-position arrangement - only FULL charge, and LOW charge. In those, when the voltage exceeded 12.8 volts it pulled the moving contact open, which dropped the field current to 0 (these did not have the big resistor on the bottom). It did not ground the contact, it just left it open so the alternator dropped to 1/4 amp output. These were all recalled early on (in 1969) for new regulators because they made the headlight flicker dim-bright-dim as it cycled, quickly (some called it "strobing" when observed on the hiway at night) and some State Police entities thought it "looked too much like the flashing lights on our Police motorcycles". Touchy bastards, back then....

So, we ended up with a unique, one-of-a-kind, "linear relay" voltage regulator in the CB750, which was never seen before, nor since!

[Mark K]

Thanks HondaMan, that's a lot of great information. I've been taking screenshots of all the great information members have been posting here and compiling it for reference. Hope to get back to work on it so I can get this issue put to bed and move on to finishing this project.

Thanks again!

[PeWe]

New main harness wich means all connections to-from handle bar switches are just connected.

I should disconnect everything.
With help of Honda circuit diagram and the often simplified  colored diagrams verify all wires to-from each switch.
Easy with a buzzer, operate the switch and see which wires. This include to follow up some main harness wires too, if ends where it should for  the function.

NOTE!
There is a jumper wire between black and brown something (white?) to get headlight to work, or maybe gauges illumination.
This jumper wire is not part of the new main harness, so if you have archived the old, the jumper wire is lost. You can see this in the circuit diagram when looking for it when kniw its existence.

 Mark wites with tape where you note the function.

I noticed that aftermarket switches had different colors on some wires, the main harness too.

I did this on my K2 and got it right direct.

I thought it was wrong due to scheisse quality LED pilot bulbs that lived several minutes.
This wasted an hour fault tracing until I gave the bike what it wanted, stock type bulbs.

[Mark K]

New main harness wich means all connections to-from handle bar switches are just connected.

I should disconnect everything.
With help of Honda circuit diagram and the often simplified  colored diagrams verify all wires to-from each switch.
Easy with a buzzer, operate the switch and see which wires. This include to follow up some main harness wires too, if ends where it should for  the function.

NOTE!
There is a jumper wire between black and brown something (white?) to get headlight to work, or maybe gauges illumination.
This jumper wire is not part of the new main harness, so if you have archived the old, the jumper wire is lost. You can see this in the circuit diagram when looking for it when kniw its existence.

 Mark wites with tape where you note the function.

I noticed that aftermarket switches had different colors on some wires, the main harness too.

I did this on my K2 and got it right direct.

I thought it was wrong due to scheisse quality LED pilot bulbs that lived several minutes.
This wasted an hour fault tracing until I gave the bike what it wanted, stock type bulbs.

Thanks PeWe, great suggestions and will keep them in mind when I get back to work on it, don't usually have much time during the week to work on it, life gets in the way 😂