Topic: 76 CB750 - No headlights or rear running lights?

[Jim Harris]

Good morning folks!
Well, I went out for a test ride on the K6 we just picked up recently and everything ran great until I noticed that the headlight and rear running lights weren't working any more. I'm positive the headlight was on when I left and I'm 99% positive the rear lights were on as well. The front running lights are working as are the directional signals front and rear. I checked the fuses, all good. I checked the headlight, all good. I opened the headlight switch on the handle bar and cleaned the contacts, no change. Now I'm thinking that I must have a break in a common ground. Any suggestions would be greatly appreciated.
Thanks! 

[goldarrow]

old glass tube fuse could look good, but may actually be bad.  test the fuse with meter.

[Jim Harris]

Yes, they are the original fuses and they look old, but they are showing full conductivity with the meter. I'll go ahead and replace the fuses for new ones anyway just in case its an intermittent break in the filament. I'll let you know if that works.
Thanks!

[calj737]

Also worth testing the voltage at both ends of the fuses to the metal clips. And then check the backside of the fuse panel to verify that current is reaching the soldered wire. Had this issue very recently with another bike. Front showed fine, back had 100% loss due to corrosion within the crimp.

[Jim Harris]

Here's a few more clues...
I changed the fuses, no difference.
I don't think its the ground anymore. I opened the headlight bucket again (all original connections, never been touched) and put the meter on to see if there was any voltage on the headlight "drive" lead or the "pass" lead when connected directly to a ground on the frame, no voltage.
Then I tried the ohm meter, connecting the ground from the headlight to a ground on the frame and have 100% connectivity.
It must be that the power lead that serves the headlight and rear running light has a short somewhere. I'll check the back of the fuse box next as suggested by calj737.
I'll let you know what I find.
Thanks!

[Jim Harris]

OK, we are on the right trail...
More clues...
I disassembled the fuse box to check the connections on the back as suggested by calj737. They were corroded but secure and working. Then I set the bike on 2500 rpm and checked the voltage at each of the three fuse connections. the top 5A fuse and the bottom 15A fuse were each getting 12+ volts, the middle 7A (headlight)  fuse was only getting .6 to .7 volts, so it's not getting enough power. Where does the voltage come from that serves that center fuse? The wires in the back are brown with a red strip.
Thanks!
 

[calj737]

http://manuals.sohc4.net/CB750K/CB750_K6_WD.pdf

If you are only detecting .6/.7v then you must have too much resistance due to corrosion along that circuit. When you said you inspected the backside of the fuse panel, did you measure for voltage there too? Or were you measurements taken from the fuse side?

Use the diagram above to trace your loss.

[Jim Harris]

I check both side of the fuse box, same .6 -.7 volts both sides.
I just looked up the wiring diagram and saw that the center fuse gets its power through the "combination switch" (ignition switch) so I took that off to see if it was working correctly. It has a red power lead, a black neutral, a brown lead and a brown/white lead. If I'm reading the wiring diagram correctly, the power activates the brown leads to serve the taillights when the key is in position 1. I put the ohm meter on the leads in the back of the switch with the following results:
In position one, I got conductivity between the red and black leads
In position two, I got conductivity between the red and brown, but nothing at the brown/white or the black neutral.
I'm sure that can't be right.
Do you think the ignition switch is defective?

[calj737]

In the ON position, you should have 12v on the black as it carries 12v switchedower thru your bike. If you are not getting 12v at black from the switch, the switch is either defective or in need of repair.

[Jim Harris]

OK, I'll order a new ignition switch, thanks!

[goldarrow]

OK, I'll order a new ignition switch, thanks!

You can try cleaning the contact points behind the switch. Water gets in the rubber cover and cause corrosion