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1980 cb650 no headlight and voltage drop off

2439 Views 16 Replies 5 Participants Last post by  dingolong1987
First post and happy to help when I can. I've ran across many issues as you ass have as a motorcycle owner but this one has me very puzzled. I am doing some minor restoration on a 1980 CB650 and after getting it running and road worth I have put about 200 miles on it give or take. I noticed the headlight wasn't working at all but all other lights and blinkers work fine. The healight (low and high beam) do not work at all. I checked voltage to the headlight plug and had battery voltage (12 volts at the time). With the meter on the back side of the plug, still reading 12 volts, I plugged the headlight in and immediately lost the 12 volts. Once I unplugged the head light plug from the head light, with the meter still on the back side of the plug, I read around 2 volts. After a while the voltage will climb back up to battery voltage but it's the same issue each time. Another odd thing I've noticed is that I don't read any voltage to the high beam lead that goes to the headlight plug at all. Sometimes if I'm checking the plug and reading battery voltage, I will cycle the high/low beam handlebar switch and all the sudden the voltage drops off from battery voltage to 2 volts or random voltage.

I have taken the low/high switch off of the handlebar, inspected it, cleaned it and checked it with an OHM meter and it checks out ok. It seems to me that the switch is a very simple set of "pot switch" style contacts and they appear to be working so I don't think the issue is the switch but I'm all ears.

I wasn't 1000% convinced that the switch was working properly so I started taking voltage from the plug that enters the headlight housing from the main wiring harness. Taking voltage from this plug would bypass the low/high switch and tell me if the issue is coming from the main wiring harness or if it down stream like the low/high switch or head light or harness on that side. I am reading the same voltage drop off from the main plug on the main wiring harness that feeds the electrical into the headlight housing. I used two fuses in order to plug my power side and ground side into the main wiring harness plug. I used alligator clips to clip onto the fuses and my meter leads and I read battery voltage which was around 10.5v at the time (this was a week or so later). With my 10.5v power going from the main plug to my meter, I tried hooking it directly up to the head light and the voltage dropped out. Once I took the leads away from the headlight the voltage came back up to about 2.5 volts and started slowly climbing.

So, I'm assuming I have some part or something partially shorted??? I'm all ears and I'm a professionally trained technician (mechanical/electrical) but I figured someone might have seen something like this before. I thought I saw a capacitor behind the seat which is going to be the next thing I look at but before I start buying components online and guessing at the thing until its fixed does anyone have any input?

Sorry for the long post and thank you very much in advance.
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What happens if you run a ground from the headlight and a hot lead from the battery bypassing all the wireing? Does the headlight work then?

Start with the basics check all your grounds and then check all your connections, cleaning everything as you go.
Great advice but yes. I ran the headlight directly to the battery and it worked on low beam and high beam. What's bizarre to me is that when I hook the wires up directly to the plug coming from the harness and I get battery voltage, when I hook it to the light I lose my voltage. I can't find any grounds that are loose that are supposed to be plugged in but just to eliminate the grounding issue I've tried the same test (hooking up the light directly to the harness) except I grounded to the frame instead of hooking up my ground to the harness and I got the same results. I don't think my issues is grounding I think it has something to do with the hot or positive side. I'm wondering if maybe I have a part that is partially failed. Do you guys know if these bikes have a capacitor on them? I saw what looked like it may have been a capacitor under the seat.
Hello there Dingolong1987,
A capacitor? that sounds weird, it would be more for an electronic item, holds a charge for a little while. Seems to me that once a wire comes off a fuse or circuit breaker, in this case it might fork off to different loads, BUT one wire would go to the dimmer switch and from there, two wires, 1 for HI, 1 for Low and the plug on headlight MUST have 3 wires,1 of which is wired to ground. On some bikes, there is a so called bonding jumper to make sure any lamps mounted on steering has a good ground. make a jumper lead, hook one end to main frame and other end to forks.

If possible, open up dimmer sw. put one meter lead on handlebar or good ground, with other lead check for volts at the dimmer, there must be 3 wires at that sw.. 1 has 12volts+ at all times with key on 'ON' then with sw. on HI one of two other wires should have 12 v.+ on it, put sw. to lO and other wire should now have voltage on it. see how it goes then let me know. glad to help when ever I can.
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I have opened the low/high handlebar switch and it appears to be working correctly. It does have one constant 12v supply and then one leg goes to low beam and the other leg goes to high beam. My issue is the voltage being supplied to the switch. It seems that after sitting for a while I have the appropriate amount of power being supplied which is whatever the battery voltage is. But when I apply the headlight to the circuit my voltage drops down to 0 volts. Then when I disconnect the headlight again the voltage will be at around 2vdc and slowly climb. It seems like something is dragging the voltage way down when the headlight is hooked up but I've tried with another headlight as well and it does the same thing.

Someone mentioned to me that the voltage regulator may be bad but I didn't think the regulator had anything to do with the lighting but more of a charging system component. Where is power supplied from the bike to the low/high beam handlebar switch? It seems my issues is upstream of the handlebar components which would be through the main wiring harness to the components under the seat.

...I'm trying to be as clear as possible without writing a book. Tomorrow I'll try to post some pictures in order to reference what I am talking about. I appreciate the responses.
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Don鈥檛 just check for loose ground wires but good grounds. You may need to clean the ground.
Hi again,

O.k. so we know the headlight is good, Good. the meter reads 12 volts at plug for the headlight, but when you plug in headlight into plug connected to harness, no light and no voltage. Do you have the bike running when you do these tests? I had an R/R rectifier/regulator go out on me so I know that they go bad, either they overcharge the battery or they don't charge it by putting out less than 12 volts. I have not worked on Honda's but I do feel that the charging system is pretty much like all others, a magnetic rotor spinning over or inside a stator with bobbins of magnet wire, the leads go to a solid-state rectifier and regulator, If the battery voltage goes down to say 10 volts or so, the regulator allows current to flow to the battery, if the battery volts are up around 12.5 charging current is reduced. I would really like to help you but I don't want to cause more troubles than you have. I guess the best thing is to get a shop manual with a wiring schematic, and with multi-meter start checking everything that looks like it is in the headlight circuit.

I know that larger Honda's , the goldwings have more of an automobile type charging system, instead of a rotor with permanent magnets, it is a would rotor and power must be applied by means of brushes on the rotor's slip=rings to it to be turned into a spinning magnet. I doubt this is the case with your bike.
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I haven't tried all these testings with the bike running yet but the headlight didn't work while the bike was running before I started working on it. I am going to get the carbs put back in this week and try running the bike while checking everything. Again just for a point of reference, I have tried testing everything with a jumper grounded directly to the frame so I am confident that it is not a grounding issue. I sure wish it was though because that would be an easy fix. I'll get the bike running so that I can check the regulator/rectifier as well. I appreciate the input sirs. If I get it running and read the same stuff then I'm going to start taking the wiring harness apart to trace wires. I've read through the wiring diagram and even though I'm very familiar with industrial type equipment wiring diagrams with multiple voltages and controlling devices, the wiring diagram for the bike doesn't make a ton of sense to me. I'll get back after more testing.
Well the fun begins now. Sounds like you need to trace the wiring from the battery to the light. You will need to separate, clean and test each and every connection until you find the voltage drop.
Great advice but yes. I ran the headlight directly to the battery and it worked on low beam and high beam. What's bizarre to me is that when I hook the wires up directly to the plug coming from the harness and I get battery voltage, when I hook it to the light I lose my voltage.

Just for the hell of it, take the CENTER wire from headlight socket, hook to a longer wire and hook to battery - or a damn good ground on main bike frame. If you had or have voltage seeing it on a voltmeter, that is not the same as putting the headlight into the socket, the voltmeter draws NO current but the headlight does, I see current trying to flow to lamp through steering bearings and grease, I am sure there is a wire in headlight bucket that hooks or hooked to a good ground, this jumper ground by-passes ground of steering head, so being VERY low resistance or .01 ohms passing current through such a low resistance makes for hardly any voltage drop and you should have all of 12 volts, putting meter probes into socket shows voltage but pulling current through a bad-ass ground gives a BIG voltage drop. Please let me know what happens, I have not had this happen but I have seen these grounding jumpers shown on bike wiring diagrams and not having a good ground would /could result in large voltage drop, the more amps you pull, the greater the drop. . .Then again if you also have running lights/dir sigs, they might not be as bright if they don't
have a good ground.a ground jumper can really make a difference.
Just bumping this real quick to let you guys know I'll take a look and try the troubleshooting that Ray recommended. Sorry for the delay. Been out of the country for a while.
This sounds like Honda is switching the ground side instead of the hot side, or indeed you have a bad ground somewhere. Inspect each ground connection by taking it loose and clean it with sandpaper if it isn't shiny..
Seems way too familiar, this story the way I remember it, battery showed 12 volts on a meter but was 'So shot ta hell' when bulb
was hooked up, battery volts crashed through the floor.seems no one EVER did a LOAD TEST on the battery.
I could give that a shot. The bike cranks and runs fine though. The battery is relatively new like within the year and I kept it on the charger while I had it. I know my buddy let the battery die though after I sold it to him. I've totally seen stranger things. I'll go get it tested tomorrow. I'm planning on working on it tomorrow evening.
Is this thread for real? or accidentally repeated from a few months ago?
I started it a couple months ago and then work got busy and I went out of the country after that. I should have just waited until tomorrow but I was thinking about it today and checked the thread out and commented after that. I'll give you guys updates tomorrow and what I found after checking the grounds and trying your troubleshooting method. I'm also going to run the battery over to Auto Zone and have them test it.
Soooo... the headlight works again and not really sure why. I didn't run the bike at night but it seemed a little dim during the day. I had to replace some oil seals and rebuild the carbs again due to fuel rail orings and float bowl gaskets leaking. After reinstalling the carbs, doing a sync and some rough tuning we got the bike running awesome and then noticed that the headlight started working again... One thing I noticed is that while idling all the lights (including headlight) are on but they get way brighter when the RPM's increase. I think this is normal although I am still a little concerned that the battery may have some issues so when I get a chance I'm going to go get it tested. Also, I'm going to remove the headlight and start playing with wires to see if I can find a bad connection or something. It works right now but I don't want to put my buddy on a bike that could potentially leave him in the dark without a headlight. Not safe. I wonder if I may have a rectifier issue if it's not a battery issue? My gut feeling is that my connections and grounds are good but I may have an electrical component that is failing.

Wish I had found the smoking gun by now but I wanted to give you guys an update anyway. I'll post the smoking gun when I find it.

On the upside, if there was ever a CB650 that could do wheelies, I've got it. Bike runs like a champ.
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