Topic: Dead short CB750 K2

[disco]

I just fitted some new Euro handlebars to my 750 K2. Everything was working OK. However I had a blinker connection in the headlight that would manage to fall apart regularly. I opened the ‘rats nest’ and got it going again. However, not long after it blew the main 15A fuse. I’ve done a continuity check and I have continuity from black to earth. Where do I even begin looking for the short? The wiring within the headlight all appears correct.

[PeWe]

I should do a visual inspection on all visible wires and remember everything that was affected by the handlebar change.
- Open the handlebar switches and look for a short to bar.
- Anything you disconnected- connected wrong at the handle bar change?
- Will it short when activating anything like headlight, blinkers, horn or is it constant with everything off, except ign switch on?

Since it started after handle bar change, disconnect both switches and measure if any of those wires are shorted to ground.
Mark the connections with tape where not obvious will help when reconnecting.

Disconnect connectors step by step one way to see when short will disappear.
Battery + disconnected, ign switch on, buzzer connected to black and ground.

It will stop buzzing when disconnecting the affected area.

[newday777]

Was this a one time fuse blown or does it continue to pop fuses? If it was an original fuse did it show popped in the center of the fuse?
I've had my main fuse loose total power on one of the first rides after I got it on the road after a 12+ year hibernation before I got the bike. I put in the spare to get back home, then ordered some new fuses from the local Honda shop. No problem since after 4 years. I chalked it up as an old fuse.

[pjlogue]

First thought is the wiring inside the handlebars.  If you pulled wires through the bars there is a high probability you damaged the insulation with the sharp edges of the new bars.  I did that many years ago when I put new bars on my bike.  Another possibility is in rewiring the rats nest inside the headlight bucket wires got pulled and it caused chafing in the harness and hence a short.  Easiest thing is to isolate each circuit and test for a short to ground.

-P.

[TwoTired]

There is normally connectivity between black and green, as all the bulb filaments make a connection pathway.  Normally, the bike draws about 10 amps and @ 13v this represents a resistive continuity path of 1.3 ohms.  A “short” would show a significantly lower resistance,  but few test meters will show much accuracy, or even truth, in attempting to read lower resistances.  Do you have access or know how to use a millimeter?

Is your fuse blowing in center of fuse?  Or, near the ends of the glass?  The latter is unlikely an actual over current.

If you do have a consistent/ persistent  short, one technique of locating is to wire a 12 bulb to the fuse terminals.  A true short (key on) will have that wired in bulb glow bright.  Then you wiggle various bits , turn bars, etc., until it goes dim, indicating short located.


Cheers,

[disco]

Hi all thanks for the replies.

PeWe; I did a visual and don’t see anything out of place. It was all working fine, before I went into the headlight to fix the loose blinker wire. I’ve labelled a lot of the wires, because I’m using the CB400F controls from Yamiya (with Aussie style lighting controls ie ‘park’ and ‘headlight’) and wire colours don’t match.

New day; I’m popping fuses continuously. Right now I’m waiting on new ones to arrive.

PJ; new controls which I greased up slightly before feeding through the bars. So hopefully it’s not in the bars.

TwoTired; im not familiar with a ‘millimeter’. Fuses are blowing in the center. Can you please elaborate on the 12V bulb method? So I leave the fuse out and connect the bulb wires to either side of the fuse holder?

[TwoTired]

I meant to type milliohmmeter.  (Was probably auto corrected) It’s a Not too common test device.  And you probably need some training to use it effectively.

If you have a dead short that parts fuses immediately, then a 12v bulb wired to the fuse terminals will limit current to safe levels in the shorted circuit to keep it from making smoke and flame, allowing you to disconnect things until the lamp dims.  A reduction in current makes the test lamp dimmer, indicating you’ve removed the short circuit.  Then you investigate the branch you just disconnected to find the physical connection from black to green (or frame bits).

A simple instrument or running lamp incandescent bulb will work fine for this.  Just wire a couple of alligator clip, and solder them to the test bulb.  I usually just use the same bulb and wire clips that I use for timing the points.

Some common short points to scrutinize are where the bar controls mount, and the wire bundle wrapped near the steering head.  The latter sometimes manifests as intermittent when the bars are turned.  But a good visual sometimes reveals the chaffing bit.

Cheers,

[newday777]

Hi all thanks for the replies.

PeWe; I did a visual and don’t see anything out of place. It was all working fine, before I went into the headlight to fix the loose blinker wire. I’ve labelled a lot of the wires, because I’m using the CB400F controls from Yamiya (with Aussie style lighting controls ie ‘park’ and ‘headlight’) and wire colours don’t match.

New day; I’m popping fuses continuously. Right now I’m waiting on new ones to arrive.

PJ; new controls which I greased up slightly before feeding through the bars. So hopefully it’s not in the bars.

TwoTired; im not familiar with a ‘millimeter’. Fuses are blowing in the center. Can you please elaborate on the 12V bulb method? So I leave the fuse out and connect the bulb wires to either side of the fuse holder?

A good place to start is unplugging the 400F switches you put on. If not wired correctly(different colors and functions on the 400F switches than 750k2 has) you can get dead shorts there. The headlight cut out when starting and lights on all the time in the 400F right switch is a key possibility. We can't see how you or someone else modded the wiring so you'll have to go through each wire individually testing and unplugging so the incandescent bulb wired with alligator clips to replace the fuse is a good way to make the checks(back in the 70s I just used an extra bulb 2 wire socket out of an old light(or new at auto parts store) to make my timing light for the points to see when they opened and works in this case to hook in place of a fuse.

[Little_Phil]

That's the way to do it. If you have a dead short it's as if the load side of the fuseholder is connected to the frame or a wire that goes there. The 12V will light just about as bright as if you had connected it across the battery terminals. Remove the short and the light goes out or it will dim if there is some parts of that circuit that are drawing normal current.

[disco]

Newday; the CB400F controls were wired correctly. Everything was working just fine. It just started acting up after I delved into the headlight to tighten up a blinker connection. I followed TwoTired advice and hooked up a incandescent bulb across the main fuse and yes it glowed bright. I have the lights and blinkers off. My next move is I’m going to check the right hand control at the handlebars. Maybe the black power wire has chafed?

[newday777]

Newday; the CB400F controls were wired correctly. Everything was working just fine. It just started acting up after I delved into the headlight to tighten up a blinker connection. I followed TwoTired advice and hooked up a incandescent bulb across the main fuse and yes it glowed bright. I have the lights and blinkers off. My next move is I’m going to check the right hand control at the handlebars. Maybe the black power wire has chafed?

Unplug each individual connection in the headlight until you find the circuit to narrow it down to approximate location.

[TwoTired]

Newday; the CB400F controls were wired correctly. Everything was working just fine. It just started acting up after I delved into the headlight to tighten up a blinker connection. I followed TwoTired advice and hooked up a incandescent bulb across the main fuse and yes it glowed bright. I have the lights and blinkers off. My next move is I’m going to check the right hand control at the handlebars. Maybe the black power wire has chafed?

Look at the wire diagram for your bike.  There will be one or two black wires going to the right control on your bars a likely more to the left control.  I think these all connect in the headlight bucket to the black wire supply source.  I don’t have the diagram in front of me right now.  Unplug these black wires one by one until the lamp dims.  The one you just unplugged is the problem source.  Leave that one unplugged and reconnect the others.  If the lamp is still dim, you can put the fuse back in, and operate the controls to see what works and what doesn’t.  The non-working one is where your short is and what needs correction.  Reinstall the lamp while you track down the control wire that is causing issues.

Cheers,

[disco]

Thanks everyone! Last night I undid the RH control and moved the wires around & bulb remained bright. So today I unplugged the headlight, park light, blinkers, and taillight and the bulb went dim. So I put a fuse in and started re-connecting. It’s all hooked back up and is now working fine? Hopefully it stays that way! Thanks all for your help. I learned a thing or two out of this too.

[PeWe]

You have hopefully found weak points you sorted so you can ride in peace;)

Not fun with intermittent errors that suddenly can stop the bike on a remote road in the darkness
or during a car passage when all power is needed before meeting the cars in opposite direction.

[150mphbike]

I had an electrician tell me if a fuse is just black it is from over load.  If the fuse element splatters it is a dead short. Hope this helps.