Motorcycle Forum banner
1 - 18 of 18 Posts

· Registered
Joined
·
35 Posts
Discussion Starter · #1 ·
Hi everyone,

Wondering if you can provide me some guidance...

I have a 1975 Honda cb125s. It is a 6 volt bike, and also important to note is that it has an on/off switch for the headlight (old school stuff, right?).

I've discovered that when riding with the headlight off and not using blinkers, the bike runs fine. However, as soon as I use the blinkers and/or turn on the headlight, the bike bogs down and stalls.

I got my multimeter out. With the bike idle, voltage starts around 7.20 and steadily climbs.

When I rev it with no lights on/in use, voltage jumps up and tops off around 8.30.

However, as soon as I turn on the headlight, use blinkers and brakes, etc., voltage drops to around 5.80, and the bike stalls out. Also, none of the blinker lights flash, they just remain solid.

Not being an expert, but also wanting to save some money if I can, might this be a "simple" fix, such as: replace light bulbs and retest? Replace regulator rectifier and retest? Stator, etc.? Might these things just be worthwhile to replace anyways?

Any insight would be greatly appreciated, thanks so much!
 

· Premium Member
Joined
·
6,196 Posts
The battery should prevent Voltage swings that wide, so if the battery passes testing, you have a high resistance between it, and where you are measuring. Which brings this up: just where are you probing with the meter?
 

· Premium Member
Joined
·
6,196 Posts
Have you checked that there is still water in the battery? A flooded 6V lead-acid battery starts to boil above about 7.2V. Take it to a shop to be tested; I expect it is fried.
 

· Registered
Joined
·
35 Posts
Discussion Starter · #8 ·
Have you checked that there is still water in the battery? A flooded 6V lead-acid battery starts to boil above about 7.2V. Take it to a shop to be tested; I expect it is fried.
I did check, and the fill level is RIGHT on the high line. Too much?

If the battery isn't overfilled and it tests fine at the shop, what's the next most likely culprit? Other posts and information I've read point at regulator, stator, the whole harness in general, bad bulbs, etc.
 

· Premium Member
Joined
·
6,196 Posts
I am probing the battery posts with my multimeter.
Do you mean the actual posts on the battery, or the cable ends that clamp to it? Try removing those cable ends and cleaning them until you have bright surfaces, both on the cable ends where they contact the battery, and the battery posts too. Then when you measure, push the probes into the seam between the terminals and battery, if you can.
 

· Registered
Joined
·
35 Posts
Discussion Starter · #10 ·
Do you mean the actual posts on the battery, or the cable ends that clamp to it? Try removing those cable ends and cleaning them until you have bright surfaces, both on the cable ends where they contact the battery, and the battery posts too. Then when you measure, push the probes into the seam between the terminals and battery, if you can.
I mean the actual posts on the battery. I did just get back from testing the battery at the bike shop, and they said that the battery looks great.

They recommended to start cleaning connections, etc.--so I'll do that, including the cable ends and battery posts, and work my way through different connections, then retest.

Any other thoughts in the meantime would be greatly appreciated.

Thanks all!
 

· Retired twice: Navy and as a govt contractor
Joined
·
12,410 Posts
No thoughts at the moment other than I think you are on the right track.
 

· Premium Member
Joined
·
6,196 Posts
Yep, cleaning all terminals is the best thing. Those 6V systems are more sensitive to Voltage drops than 12V systems.

BTW, the wiring diagram I found has no regulator; it would be nice to have one.
 

· Registered
Joined
·
35 Posts
Discussion Starter · #13 ·
Yep, cleaning all terminals is the best thing. Those 6V systems are more sensitive to Voltage drops than 12V systems.

BTW, the wiring diagram I found has no regulator; it would be nice to have one.
Yeah, I'm working my way through, but so far nothing looks corroded or grimy, etc.

I did find what looks to be a regulator, so maybe previous owner(s) one put in...

Another discovery I made is that with the headlight off, the blinkers blink--but as soon as I turn on the headlight, blinkers go solid.
 

· Premium Member
Joined
·
6,196 Posts
Coming on but not blinking is the 'bulb out' indication, because the current through the flasher is reduced. When your Voltage drops far enough, the same thing will occur. Either your generator is lacking power, or the headlamp and tail lamp are drawing too much power for it to keep up.

The wiring diagrams I have found show that, like my CB450K, your generator stator has two sets of coils, one for lights off operation, and the other added to the first when the headlamp is turned on. It could be the wiring to the lighting switch is disconnected, or the switch needs cleaning. Also, the old-school selenium rectifier gets worse with age, limiting the current to the battery. If you could find a modern rectifier/regulator for 6V, you could bypass the switch and apply full generator power all the time. Good luck with that, though.
 

· Registered
Joined
·
35 Posts
Discussion Starter · #16 ·
Electrical stuff is my weak point when it comes to bikes, so, I caved and brought the bike to a shop. They replaced a relay, so now the blinkers blink correctly...but apparently that's all the shop did, because the bike still loses voltage (RPM won't go over 4,000 when any lights are being used) and stalls when you are using too many lights (headlight, blinkers, brakes, high beam, etc.). Needless to say, that's a big problem when the sun starts going down.

I will re-test to see if voltage continuously climbs when bike is idle and no lights are being used.
 

· Registered
Joined
·
35 Posts
Discussion Starter · #17 ·
So a couple of quick updates:

As it currently stands, when the bike is sitting idle with no lights on, it hovers around 6.15 volts. And when I rev the bike, voltage tops out at 6.22 and does not go any higher...so it seems that going up to 8 volts (and beyond) is no longer happening. Hopefully that holds true as I continue to test.

But, if I use any lights, voltage drops below 6 volts and bike stalls.

Being a bit annoyed that the bike shop didn't seem to address the main problem of voltage dropping and bike stalling when lights are used, I did start digging into the wiring a little more bravely...and after taking much of the bike apart and taking the harness half out, I found that multiple wires are frayed, cut, poorly spliced, and are literally hanging on to plugs and connectors by just a couple strands in multiple spots.

I'm not an expert, but this can't be good, and I'm hoping that is is the main cause of the voltage drop. Either way, I'll be working on cleaning it all up, redoing plugs, etc.
 

· Premium Member
Joined
·
6,196 Posts
I'm not an expert, but this can't be good, and I'm hoping that is is the main cause of the voltage drop. Either way, I'll be working on cleaning it all up, redoing plugs, etc.
I suspect cleaning up the wiring will take care of your problem, at least most of it. The charging system is really weak at idle, so I wouldn't be surprised if the Voltage slowly drops with the lights on, though. This happens with my CB450, even after replacing the tail/brake light and indicator lights with LEDs. My LED headlamp draws 30W, more than the stock low beam but less than the stock high beam (25W/35W), so it's not really a player.
 
1 - 18 of 18 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top