Looking across to the snow capped alpine mountains seen from the back seat of a motorcycle

Home Repair And Restoration

CBF125 Regulator Rectifier Issue (At 99,000 Miles)

Fix Date 11 Feb 2023

By Ren Withnell

99 thousand miles. 99 blumming thousand miles. Pffffft. Could the reggy reccy (regulator rectifier) just not hang on another 1,000 miles? Grrrrr.

It's probably not too advisable to be riding a 99 thousand mile 125cc 14 year old motorcycle along a motorway at 2200 hours on a Friday night. With a maximum speed of 60mph now and realistically a maximum cruising speed of 53mph there's a large speed differential between myself on the said 125 and the high speed taxis trying to maximise their earnings, the over enthusiastic youths in their hopped up Type-Rs and Scoobies, and the half-baked drunkards. 

It's even less advisable when your lights appear to be getting dimmer. 

It's done this a few times recently. The brightness, the intensity of the headlight and clock back lights seems to come and go. Loose wire? Bulbs on their way out? Nah, doesn't make sense. Meh, it's fine. Until tonight. This is the first motorway run for a while and the first motorway run at night in quite a long time. The lights are no longer getting a bit dimmer then a bit brighter, they are getting ever dimmer.

You old wise mechanical sages will have read the title and understand the problem. Except you don't. You see you're thinking the reggy reccy is no longer charging the battery and the battery is going flat and that's why my lights are dimming. BUT - the lights on a CBF125 are NOT fed from the battery. They are fed from 1/2 of the single phase wave from the alternator - there is no direct connection to the battery. (FYI - this applies to the headlight, taillight and clock lights. The brake light and indicators are fed from the battery.)

So - I'm asking myself - knowing this what the hell is going on? It seems obvious that the high engine revs of the motorway are not helping so I pull off the motorway and onto the slower roads. At which point my lights appear to recover their joie-de-vivre. When the speeds go up the lights get dim, so I keep the engine revs down using careful route selection and get myself home safely.

With the seat removed I check the battery voltage. 13.9v. Errrr, no. No. 12.6v is right for a typical lead acid battery. Still high from being charged while riding (residual charge)? No, this is the next day, at least 12 hours after the ride. Start the bike up - at tickover I'm getting 13.5, that's OK for charging and the computer, fuel injection and high pressure fuel pump will be pulling the battery voltage down. Rev the bike up and I get 17v. Errrr, NO! At the most 14.5v is more than enough for charging.

A friend suggests before shelling out for a reggy reccy I clean all the earths. It's a good call. Electricity is complicated, far more complex than you might imagine. Why were my lights going dim on 1/2 of the alternator's sine wave output when the battery is apparently receiving more than it's share on the other half of the alternator's sine wave output? 

Ren's hand holds the old regulator rectifier showing the rear mount has been cleaned
Remove the oxidisation to see if we can fix this.

I've seen bad earths on car tail lights - you get weird effects like when the indicator is on the brake light comes on too - all at half-brightness. A bad earth causes the angry pixies to take strange routes that they wouldn't normally take. Anyway, there's no harm in trying. Natch, no benefits after an hour's filing and wire-brushing. I still maintain it was worth a shot.

A new none OEM regulator rectifier can be had for £33. That's not bad actually. It arrives, I fit it and all is well once more. Tickover sees around 12.9v, full throttle ramps up to 14.1v, perfectly acceptable. 

The new regulator rectifier
Shiny and new.

I am concerned the over voltage to the battery will have fried it. So far - seemingly not. Either the problem hasn't been a problem for long enough to kill it or this battery is made of Adamantium with Wolverine's self healing mutations. This battery has been on the bike for a VERY very long time.

Only another 850 miles to go at the time of writing...

Advertise here - contact ren@bikesandtravels.com

Reader's Comments

Upt'North ¹ said :-
Well done Ed.
I think I'd have gone.
But I think I'd have checked, like you did what the battery was up to, I know it's a different set up on the 125 but iffy batteries do weird stuff.
Electrotrickery and Witchcraft.
You're the wizard.
28/03/2023 08:12:20 UTC
Ian Soady¹ said :-
I've never liked that sort of arrangement. In the dim and distant past when I was using a Suzuki SP400 for commuting - excellent bike for the job by the way, much better than the usually highly praised XT500 that replaced it - I rejigged the electrics to use a cheap full wave rectifier from Maplins and a Zener diode. This is what I'll be using for the Grumph although that will be much easier. I can't now remember the exact details of the SP's setup but I had 12 volts and everything except the ignition (I think) running off the well charged battery. I like zener diodes - OK they're not very efficient but they are simple. Unfortunately they're becoming somewhat rare.
28/03/2023 16:33:23 UTC
Ren - The Ed¹ said :-
The "1/2 wave for lights, 1/2 wave for everything else" is perhaps "suboptimal". It'll be a cost thing as the coils will be one single winding and not 3, only 2 wires from the coils not 3, no need for any light switches as once the engine stops the lights go out (scary at night if you stall at the lights), cheaper reggy reccy as fewer diodes required, I'm sure there'll be more corners cut. But as with many things it doesn't need to be perfect just good enough (that's what I keep telling Sharon).

Considering how many of these bikes have been sold and how long mine has lasted - yeah it is only just good enough, but it is good enough. There's quite a few forum posts and guides out there explaining how to convert the CBF125 to full wave all connected to the battery operation. I did consider doing this but then, nah, it is good enough.
28/03/2023 17:23:27 UTC
Snod said :-
Look at this guy, fitting NEW parts. A part that is even meant for the job and didn't need slimy tape wrapping around twisted-together wires.

Someone should inform him there's a cost of living crisis going on.
28/03/2023 18:20:44 UTC
Ren - The Ed¹ said :-
Oh my word! Oh Snod you are so right and I'm so sorry! I feel like I've let you down, myself down and countless misers down too. I am a shameful spender of money in a time when money is incredibly tight, I shall go and flagellate profusely in self hatred. No-one will ever take me seriously as a bodger or scrooge again.

Money is tight. I was in a car park the other day among the endless BMWs and Mercs and Audis and Jags when I noticed just how many of these vehicles had balding tyres. It must be hard to afford to replace those £200 per corner tyres on your £70,000 car when you have to shell out for Tarquin's private tuition and Grismelda's pony lessons.
28/03/2023 22:03:06 UTC
Bogger said :-
Well done our Ed.

Now it's about time you got a NEW bike wrapped around your new Rectifier.

30/03/2023 19:50:17 UTC
Ren - The Ed¹ said :-
We are presently at 99,515 miles Bogger so I reckon it'll be time to consider new options soon. There's a lot of "if"s and "but"s at the moment.
30/03/2023 21:06:48 UTC
Glyn said :-
What happens at 100,000? Do we all need to surround the bike and sing Auld Lang syne? Will the Ed be placing a deposit for a new machine or is that just a silly thing to write?
31/03/2023 07:10:37 UTC
Bogger said :-
Nah more like Chopins funeral march.

31/03/2023 10:19:28 UTC
Ren - The Ed¹ said :-
What happens at 100,000 miles? I'd like to think I'll become momentarily famous as that blithering idiot that rode a 125 for far far far too long. In reality there'll be a post on here, I may receive a "well done" from one or two understanding acquaintances and that will be the end of that.

I have considered letting Honda know but I doubt they'll be interested. Owd knackered 125s are not their target market.
31/03/2023 12:46:16 UTC

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