Thread: Horn brake light issue revisited

My KO still doesn`t have a horn or rear brake light. I`ve posted last year as have a few others. To recap, the issue is this: lights/headlight works fine. Tailight works fine also. Apply either front or back brake and the tailight dims, no brake light, but the headlight still works fine. Tried changing the rear bulb twice. No good. There is no horn at anytime. Checked resistance...ok. These were working fine, then suddenly stopped. New battery, odviously the lighting coil works if the headlight works bright when revving a little.
According to the wiring diagram, these share the same circuit. How can I tell if there`s a short? I have a volt meter, but I`m no electrician.

I`d like to resolve this...trails are open here!

Ian

Sounds like you have a grounding issue.You will have to do some troubleshooting to eliminate step by step.Try this first,
1) run a new wire pos & neg from batt to horn to verify horn is good.
2)check for corrosion on connections
3)ohm horn wire out from button
4)make sure ground wire is from taillight to frame
5)check rear brake switch
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I had a similar problem last night.I had stop light but no taillight.Found i had replaced the taillight socket and socket was 360 degrees out making the polarity backwards giving similar problems with taillight and stop light.Also doublecheck your connections in headlight bucket making sure nothing came unplugged.

The ignition keyswitch can give you all kinds of grounding headaches. The most common issue is lack of metal-to-metal contact after a repaint. It doesn't take much to fix; scraping the paint from a couple of tiny spots on the edge of the frame hole does the trick.

There can also be internal grounding/contact issues. Don't forget, K0s are old enough have kids of voting age That's more than enough time for moisture and corrosion to wreak havoc. Check light operation with the switch in "run" and "light" positions. Obviously, the headlight gets removed from the equation, but you can check brake light operation this way. If key position makes a difference, then the switch is where I'd dig deeper.

If it does turn out that your switch has an internal problem, I'd replace it. I have disassembled and repaired these switches; it's fiendishly difficult and the results have been so-so (translation: "waste of time")

Ok. Well, the horn seems pooched. Very little sound with bike running and jumper wires connected. Yes, I tried to adjust the horn with the screw on the back. No luck. I`ll order a new one.
The brake light fails to work even with the front switch. I`m getting excellent voltage from all connections...would I still get good voltage if there was a short/ground problem?

Let me try that rear light socket trick...it`s possible they are reversed/shorting.

Thanks.

Ian

You might also try sanding the contacts inside the socket to remove any oxidation on them.I have had this problem also.
LOL,
Dennis

You guys should try Deoxit. Depending on the type, it eats away oxidation without sanding.

deoxit.com - Home of DeoxIT - CAIG Laboratories, Inc.

This would be too easy, but is it possible that the brake light wire and the tail light wire are swapped where they plug into the harness? It should be a Green/Yellow for the brake light and a Brown for the tail light, if the schematics are right. You might also try attaching a test lead to the tail light and a good ground point on the frame or engine and see of that makes a difference.

Otherwise I guess you need to break out the meter. You can download a jpg of the CT70 schematic from a couple of places I know of:

http://us.st11.yimg.com/us.st.yimg.c...1997_318808803

or

MC Archive

I prefer to start at the hot end of the circuit and work my way around to ground. If you look at the schematic, and the wires, and the meter, and your eyes start to glass over just let me know. I can write you a "one step at a time" list of things to check if you're stumped. I have to write technical manuals from time to time, so I'm getting pretty good at it. PM me if it seems a little too overwhelming and I'll post a troubleshooting "mini-novel" on this thread that might be helpful to others too. Just give me a day or so to write it. It can be a lot of work. Good luck.

-kevin

I really appreciate all the replies. I do have a schematic of the wiring. Thanks. I have taken apart the tailight and socket, testing both wires and confirming; no corrosion. I even tried the tailight assembly from my K1 and it does the same thing. I`m starting to think that maybe it`s the ignition switch. The headlight is bright (separate circuit in the ignition switch). I`m also not sure that the battery is charging fully. I swapped rectifiers last season with no change, so I don`t think it`s a rectifier. The horn and brake light seem to share the same circuit from the ignition switch. Seems to be the common ground. Thanks for all the help!

Ian

Hey Ian, I've been staring at the schematic (for like an hour, . . . I hate to be stumped) and I think I might be on to something. There are two paths to the brake light bulb, through the two brake switches, so power has to go to one of the two wires on each of the switches. But as I stared at the schematic something wasn't right. The output from the rectifier only goes to the rear brake switch?? That can't be, cuz that won't charge the battery. For the battery to charge the red wire coming out of the rectifier MUST to tie to the red wire from the battery, (I'm no genius, but I know charging systems inside out) so they must be connected where the red wires cross in the harness, even though the obligatory dot is missing from the schematic. The dot is also missing where the red wire splits off and goes to the ignition switch, so it isn't a stretch to make that assumption.

So here's the question - How does the ignition switch turn off the brake light? Answer - It disconnects the battery ground from the chassis - this breaks the circuit. From an electrical standpoint this is extremely unorthodox, but it works and it's not unsafe. If your switch is only making a poor (i.e. high resistance) connection to ground, the charging system will be weak and high current components, like horns and brake lights, won't work properly.

Without getting into boring theory (I like it but I'm not normal) I think that you should give this a try. Inside your headlight shell there is a Y connector where a blue wire and two green wires connect. Get a piece of wire long enough to reach from the headlight to the tail light, put alligator clips on both ends connect one end to the 1-blue, 2-greens connector and the other to the tail light housing. This will take the ground straight from the wiring harness to the tail light, bypassing the grounding portion of the ignition switch. If your battery is in good shape and has a decent charge your brake lights should work, even with the key off. If they do the problem is in your ignition switch.

I hope you try it and I hope it works.
-kevin

Wow! I think you may be right...especially about the dots. I`m no electrician either, but I`ll try your idea and let you know.

Ian