Topic: 1975 CB750K Headlight ?

[John G]

Headlight not working. As per the wiring diagram I have blue wire from the headlight connected to the headlight dimmer switch and the high beam indicator light. The white wire I have connected to the headlight dimmer switch. The green wire I have to a green ground. I've checked the fuses and they are all good. Can you tell me what I'm missing. 

[Kenzo1979]

I would check that ground wire you’re using with a multimeter for continuity to negative battery terminal.  If all of your fuses are good, your ignition switch is turned on, all other things are getting power like turn signals, etc., and you’re turning the headlight switch on, if it has one,  and not getting anything on high beam or low beam, I would think a bad ground would be the only thing keeping both filaments from working.  You can test each filament by jumping your battery + to the other two tabs of the headlight, keeping your ground tab on negative battery.  If it was working previously, I would think it would be odd for both filaments to have died on you.  You can also use the multimeter to check the blue and white from the controls to see if it’s passing voltage.

[Kenzo1979]

Check for any other wires that could be feeding the controls power as well..  with lighting sometimes it’s a brown wire, possibly with some kind of colored stripe, depends on the wiring diagram of your specific bike. Also, make sure any black wires are not loose from anything in the bucket.

[Don R]

 A burned filament can cross the other one taking it out also. A cheap test light can be purchased or made from an old light socket.

[Kenzo1979]

That’s why the multimeter will tell you everything you need to know.  A Must have when diagnosing electrical problems, lights and other components.

[HondaMan]

The 750K5 powers the headlight thru the START switch on the right handlebar, when the START button is NOT pressed. It thus turns the headlight OFF when the START button is pressed, to make more battery power available to the starter.

If the headlight was converted (in the past) to a high-power halogen bulb (80 watts or more), this melts the START switch (warps it) so it will not power the headlight anymore when it is not pressed. This is very common in the K4-and-later 750 (and 550, too).

Look at your fuseholder and note the 2 wires coming from the middle fuse: these will also be the 2 wires in the middle of the plug from the fuseblock. They will be (usually) Brn/Red on both sides or Blk/Red on one side and Brn/Red the other, depending on when your 750 was built (i.e., before or after about 3/75). With a voltmeter, test the middle fuse for power: if it does not get power when the key is turned ON, then the START switch is likely melted and warped.

If so, you can bypass the START switch by connecting the wire that is connected to the LEFT side of the middle fuse directly to the Brn/Red (or Blk/Red) wire in the headlight, and connect the wire from the other (left) side of that fuse to a BLK wire in the headlight (there should be a spare socket or two in there, or you can use one of the 2 that go to the Start switch - you'll know if you picked the wrong one because the START button won't work: swap them then. This will power the Lo/Hi switch on the left handlebar, but will leave the headlight on during START, making the battery's job a little harder.

If the headlight bulb is more than 55/60 watts, get a headlight that is within that power, or less, to prevent it from damaging other wiring in the future. Even the 55/60w type will slowly [re]melt a new START switch, even if it is replaced. I offer a special START SAVER RELAY kit to prevent this from happening with hi-power headlights, and it can be used in situations like yours if the START switch has melted and you don't want to change the switch.

[John G]

Wow. Thanks Guys. Lots to consider. I'll keep you post after I have time to mess with it again. Cheers

[newday777]

Headlight not working. As per the wiring diagram I have blue wire from the headlight connected to the headlight dimmer switch and the high beam indicator light. The white wire I have connected to the headlight dimmer switch. The green wire I have to a green ground. I've checked the fuses and they are all good. Can you tell me what I'm missing.

Do you have the original wiring harness and handlebar switches? Or aftermarket of either?

[John G]

I have replaced all the wire harnesses and switches but still have the original set of wire harnesses and switches.

[newday777]

I have replaced all the wire harnesses and switches but still have the original set of wire harnesses and switches.

The original you had to use a short jumper into 2 different colors, I don't remember what colors. It controls the headlight. I'm out on a jobsite away from my K5 so I can't check it to verify the colors.

[bryanj]

From memory black/red to brown/red but dont quote me on that it may be Honda only

[HondaMan]

From memory black/red to brown/red but dont quote me on that it may be Honda only

Yep, that should be it in a 750K4/5/6. The Brown/White then comes back from the [middle] fuse to the left-hand HI/LO switch.

Edit 5/10: I also just found an old K5 fuseblock that has the Black/Red and Black/White on the middle fuse (lights). Honda did this sort of thing sometimes: the wires could change colors in between the headlight and the fuseblock. This came from having differing vendors of their parts that they didn't make themselves. Electrical parts are one of those things.

[John G]

Interesting. Thanks. I hope to get to it ASAP.

[John G]

Okay; not sure what I did but I now have all light working except a couple of dash lights. I hope I can diagnose those. Ha

[HondaMan]

Good job!

[Tracksnblades1]

My Buddy’s 75K had a light switch. In town he could kill his lights and disappear.
Leaving the rest of us with 76+ getting the blame for his deeds. You could still track him with the old RC sidewinder.