Topic: CB750K4 Fuse Question

[jumpnkill]

Hey guys!  I'm in the middle of my build and I'm having a hard time understanding the fuse set-up.  I've seen some that have a harness, some that seem to have three tubular fuses, and some that just have a basic single in-line fuse with no harness.  Can anyone please tell me what the proper set-up should be for my bike?  Pictures and wire descriptions would be awesome.  I have the diagram from the work manual but it doesn't show much but the color of the wires.  Am I missing something?  Also FYI, I bought a replacement harness from CB750 supply that I'm trying to wire up.  Thanks in advance.

[Bowswell]

http://www.grovestreet.com/PicPage.do?id=1259795

http://oldmanhonda.com/MC/WiringDiagrams/MCwiring.php#class

start with this!

[Bodi]

I'm not sure what a K4 has, sorry. 
Basically, the early bikes had one main fuse. This is pretty simple: the battery + goes to the fuse, the fused power goes to the ignition switch, and most everything connects to the black wire from the ignition switch. There are two output terminals on the ignition switch, one for the tail light and one for everything else: the ignition switch only powers the tail light in "PARK".
Later bikes are more complicated, especially the tail light circuit... but to simplify, the battery goes to the main fuse again, then the main fused power goes to the ignition switch. From there you will see a black wire, this goes to the ignition coils plus a bunch of other electrical stuff plus one end of the HEAD fuse. The other end of the HEAD fuse goes to the headlight. I'm not going to try to explain the tail light circuit now.
The reason for this change is to reduce total electrical failures that can be somewhat fatal if they happen in fast highway traffic. A headlight in the process of burning out can blow its fuse: if you only have one fuse then when it blows your ignition goes dead and you are without engine power. Adding the headlight fuse means your ignition still works and you keep riding when that happens. Same with the TAIL fuse, it means a short circuit or a damaged bulb doesn't kill the ignition. I have a 66 CB77 with one fuse and I've had it blow while riding at night in highway traffic. Luckily I (obviously) survived but it was fooking scary.
Using a later harness on an early bike is possible but you have to come up with a triple fuse block and mount it, plus the engine connector is different I think, and you'll probably have other little issues to figure out. I think the horn changes from "always powered with a switched ground" to "always grounded with switched power" for example, which causes grief if you don't install a late model switchpod.

[Johnie]

Our member hondaman is making blade type fuse holders which are far better than the glass tube fuses. Here is a pic of my K4 with the new hondaman fuse box which includes the connection to the harness. He is still making them if you want one. They are under $50 shipped. If you want a pic of the OEM fuse box let me know and I will get one for you. Click the pic to enlarge it.

[jumpnkill]

Thanks for the info.  The hondaman unit looks nice.  The harness from CB750 supply doesn't tell you what each connector goes to...does the hondaman plug right into the harness?  Does it address some of the issues previously mentioned about one fuse blowing and you lose everything?  Also, I was looking at the diagram posted and saw a starter safety switch.  My bike didn't have one...probably because the d-bag before me jacked that up too.  Am I supposed to have one?  What do they do?  Do they plug into the harness as well?  Thanks again everyone.

[Johnie]

A starter safety switch keeps the starter from engaging while the engine is running. How do you know he took it off? The hondaman unit does plug into the harness by the white connector you see on my pic there. If the main fuse blows you are correct, you will lose all power. It is a 15 amp fuse. The smaller fuse is for the lights.

[Johnie]

Here is a pic of the OEM K4 fuse holder. There is a 15A main, 7A headlight and 5A tail. Has six wires with male end. The wires are 2 red for the main, 2 are solid brown for the tail, 2 are brown with red stripe for the headlight. Hope this helps answer some questions.

[jumpnkill]

A starter safety switch keeps the starter from engaging while the engine is running. How do you know he took it off? The hondaman unit does plug into the harness by the white connector you see on my pic there. If the main fuse blows you are correct, you will lose all power. It is a 15 amp fuse. The smaller fuse is for the lights.

I know the starter safety switch has been removed because the only items attached to the battery holder were the regulator, rectifier, and the starter magnetic switch.  I guess that's two new things I need to find.  Thanks for the awesome pics too! 

[HughL]

Question regarding the headlight fuse - 76 K6, my headlight fuse is blown but everything seems to work without it, high beam, low beam, tail lights, signals, horn, everything. I'm not sure its supposed to be doing that or if its wired differently from what it should be. My question is that is this normal?

[Stev-o]

Question regarding the headlight fuse - 76 K6, my headlight fuse is blown but everything seems to work without it, high beam, low beam, tail lights, signals, horn, everything. I'm not sure its supposed to be doing that or if its wired differently from what it should be. My question is that is this normal?

Does not sound normal!   Sounds as if the the fuse has been bypassed or

[MauiK3]

Hondaman's book is very helpful with such things. Best tool in the shop.

[BenelliSEI]

If your bike had a starter safety switch there will be a RED plug that went to it on the main harness.

[bryanj]

Might have the wrong rh switch unit on it

[HondaMan]

Question regarding the headlight fuse - 76 K6, my headlight fuse is blown but everything seems to work without it, high beam, low beam, tail lights, signals, horn, everything. I'm not sure its supposed to be doing that or if its wired differently from what it should be. My question is that is this normal?

This is accomplished by jumpering the wires inside the headlight bucket to bypass the Headlight fuse in the fuseblock. It often happened when the old fuseblocks started melting the fuses because the fuseclips have now lost their chromate plating: this makes them get very hot while carrying current, which then softens the headlight fuse, and the first bump you hit when riding breaks that fuse.

This was just one reason why I started making the fuseblocks that use modern ATC automotive fuses: they don't get hot and melt or break from bumps.
It happened to me in rush-hour freeway traffic at 55 MPH, killing the engine right at the pothole I hit!

[HughL]

Question regarding the headlight fuse - 76 K6, my headlight fuse is blown but everything seems to work without it, high beam, low beam, tail lights, signals, horn, everything. I'm not sure its supposed to be doing that or if its wired differently from what it should be. My question is that is this normal?

This is accomplished by jumpering the wires inside the headlight bucket to bypass the Headlight fuse in the fuseblock. It often happened when the old fuseblocks started melting the fuses because the fuseclips have now lost their chromate plating: this makes them get very hot while carrying current, which then softens the headlight fuse, and the first bump you hit when riding breaks that fuse.

This was just one reason why I started making the fuseblocks that use modern ATC automotive fuses: they don't get hot and melt or break from bumps.
It happened to me in rush-hour freeway traffic at 55 MPH, killing the engine right at the pothole I hit!

Interesting. I'm curious about how it was wired up then. Time to do some testing...it's interesting because the fuse randomly started blowing but it's not connected to anything.

[newday777]

Did you remove the fuse box and inspect the back side for melted plastic?

[HughL]

Did you remove the fuse box and inspect the back side for melted plastic?

I did, yes. The fuse box is intact.

[scottly]

Are you sure the tail light is working, and not just the brake light? What happens when you remove the the 5amp tail light fuse?

[HondaMan]

It's this very confusion that 'told' me to detail how it works in my book: starting in late 1972.
Here's what Honda did:
Up thru the 1972 CB750K2 bikes, there was just one 15A glass fuse that powered the whole bike, of a now-gone automotive ("SFA") type fuse.
The US DOT started leaning on Honda and Suzuki (first) to "protect the wiring better on these motorcycles to prevent gasoline fires in crashes" (like there was some sort of trouble? None we saw...).
So...
in 1973 the 750K3 Honda got a new wiring system that had a separate fuse for the headlamp and taillight and a 'safety system' to prevent the starter from working if the bike was in gear unless the clutch was also pulled in, or unless the transmission was in Neutral. The very first 750K3 models also had a sidestand switch that required the sidestand be UP to enable the electric starter - but this sidestand feature disappeared in a couple of weeks, and wiring diagrams for them are almost nonexistent today. These systems have a largish "Safety Module" under the left side cover, with a Red plug connector that requires matching wire harnesses to also have that Red receptacle. This was the "2nd generation" wiring systems (but we'll ignore that it was really the 3rd one, as the K0 was different from the K1, too, but no one sells the K0 harness anymore). These bikes have 3 fuses in their fuseholders, a 15A main (that powers everything) which feeds power to the Headlight all the time, interrupted by the [new] START button when it is pressed (the headlight goes OFF during electric START cycle). The HEADLIGHT fuse (7 amps) powers that START button. If someone installs a hi-powered headlight it can melt the START button a little bit, and then this feature has to be bypassed (or another START switch melted later), which often happens: this leaves the headlight (and taillight) ON all the time when running. The 3rd 5A fuse powers the taillight, either receiving its power via the same 15A main fuse in RUN or from the battery directly when the keyswitch is turned to the PARK notch.

In 1975 (late 1974 builds of the bikes) the Safety Module changed to a simple diode and a new wiring harness (again) that was slightly simpler, but still had all the features above. In these (and later) 750s the headlight is always ON when the key is in RUN position. The headlight power still goes thru the main 15A fuse, then thru the START button, then back thru the 7A Headlight fuse, then to the HI/LO switch for the headlight, and to the running lights. The last 5A fuse operates just like in the K3 bikes, running the taillight.

While the keyswitch got more complicated and was moved up into the instruments on the later 750, the wiring was like in the K5.

[Don R]

 I got confused by the 76 wiring issues being piggybacked into the K4 original thread.

 I've owned a couple 76 bikes, mine were F models, both 400F and 750F, the 76 has a start button that often falls apart inside. It has two contacts, when the start button is pushed one completes the start circuit, the other one opens the headlight circuit. When released, the spring pushes them to the opposite positions. I've seen various workarounds to make them run when failed, usually with an external switch. I'd look in the headlight bucket and in the right control.

[newday777]

Question regarding the headlight fuse - 76 K6, my headlight fuse is blown but everything seems to work without it, high beam, low beam, tail lights, signals, horn, everything. I'm not sure its supposed to be doing that or if its wired differently from what it should be. My question is that is this normal?

This is accomplished by jumpering the wires inside the headlight bucket to bypass the Headlight fuse in the fuseblock. It often happened when the old fuseblocks started melting the fuses because the fuseclips have now lost their chromate plating: this makes them get very hot while carrying current, which then softens the headlight fuse, and the first bump you hit when riding breaks that fuse.

This was just one reason why I started making the fuseblocks that use modern ATC automotive fuses: they don't get hot and melt or break from bumps.
It happened to me in rush-hour freeway traffic at 55 MPH, killing the engine right at the pothole I hit!

Interesting. I'm curious about how it was wired up then. Time to do some testing...it's interesting because the fuse randomly started blowing but it's not connected to anything.

The fuse is connected. In the K5 and K6, Honda added a short connector/jumper in the headlight wire spaghetti, (w/2 male ends). If plugged in the wrong female wires you will blow the fuse.

[HondaMan]

Question regarding the headlight fuse - 76 K6, my headlight fuse is blown but everything seems to work without it, high beam, low beam, tail lights, signals, horn, everything. I'm not sure its supposed to be doing that or if its wired differently from what it should be. My question is that is this normal?

This is accomplished by jumpering the wires inside the headlight bucket to bypass the Headlight fuse in the fuseblock. It often happened when the old fuseblocks started melting the fuses because the fuseclips have now lost their chromate plating: this makes them get very hot while carrying current, which then softens the headlight fuse, and the first bump you hit when riding breaks that fuse.

This was just one reason why I started making the fuseblocks that use modern ATC automotive fuses: they don't get hot and melt or break from bumps.
It happened to me in rush-hour freeway traffic at 55 MPH, killing the engine right at the pothole I hit!

Interesting. I'm curious about how it was wired up then. Time to do some testing...it's interesting because the fuse randomly started blowing but it's not connected to anything.

The fuse is connected. In the K5 and K6, Honda added a short connector/jumper in the headlight wire spaghetti, (w/2 male ends). If plugged in the wrong female wires you will blow the fuse.

Yeah, boy, was THAT ever a departure from their norm! Prior to the late 750K5 when they did that, their wiring harnesses were WAY better than any other Japanese bikes (Suzuki, are you reading this?). After that it seemed like their Production boys went into a free-for-all with anything they could get the bikes to work with?

When the START switch melted on the headlight side, we'd tell the owner to stop using 80w and 100w headlight bulbs, then jumper past the switch into the Black wire circuit (there was always a spare female in the headlight) so the headlight just stayed ON. But, my shop was in a college town" the kids never had enough $$ to have me (us) buy another Honda switch and install it: that would have cost $50! (1972 dollars were bigger than the current version...)