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CG125 ‘81 Charging Issues

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scotto75
Renault 5 Driver



Joined: 13 Mar 2016
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PostPosted: 22:27 - 19 Jun 2019    Post subject: CG125 ‘81 Charging Issues Reply with quote

Hi, I noticed today that my indicators weren’t working very well on my 6V CG125. I checked the battery and whilst running it was down to 3.96V. Obviously there’s no charge getting to it. I then checked the headlight but nothing happened. I’ve run some tests, here’s the results.

1 - I removed the two pin rectifier and checked for continuity. One way I got no reading, the other way I got 4.17 MOhms. Only getting a reading one way is fine, but the reading seems a little high?

2 - I checked the voltage across wires from the stator whilst the engine was running. I got 0.24V on yellow, 0.1V on pink and 0.86V on white and yellow. They seem really low and I thought they should be the same?

3 - I checked continuity across the coil and from the coil to the ground with the points open (White/Yellow to ground, black to ground and white/yellow to yellowas per the Haynes) and got continuity across all three. This means the coils are okay?


I would like to get a second opinion on the results and any other checks I should do next. I’m guessing the rectifier is okay but maybe on its way out. The stator coils are fine, but maybe grounding and so the output is low?
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Honda ‘73 CB350, ‘80 CG125, ‘88 CG125, '73 Triumph T140V
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Easy-X
Super Spammer



Joined: 08 Mar 2019
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PostPosted: 10:53 - 20 Jun 2019    Post subject: Reply with quote

Most likely one stator pickup goes to the rectifier to charge the battery and another runs the lights with a regulator that dumps excess power to earth.

Multimeter on the battery terminals, give it some revs, are you getting >6 volts? If not then you either have a duff regulator, bad stator or iffy wiring.

If there's not even a slight glow from the headlight when you give it some revs? Again, probably either the regulator, stator or wiring.
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Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter
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WD Forte
World Chat Champion



Joined: 17 Jun 2010
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PostPosted: 15:38 - 20 Jun 2019    Post subject: Re: CG125 ‘81 Charging Issues Reply with quote

That bike has no regulator, just a silicon diode so the battery gets DC
any regulation was done by the applied load of the lights and battery
which acts somewhat like a capacitor.

If the diode is fooked and it sounds as if it is, then the battery is getting unrectified AC and they dont like it up em.
the other tests are a bit too vague to make sense out of

Not sure how much an original plug in diode is, I'd save money and stick a
10A TO220 type in myself
They're cheap and easy to source if your prepared to do a wee bit of modding and add a heatsink

https://www.ebay.co.uk/itm/10A-1200V-Fast-Recovery-Diode-VS-10ETF12-TO-220AC-Rectifier-310nS-Multi-Qty/302808571852?hash=item4680cc1fcc:m:mKLcXpfsKQxZvq_JAOUUArg
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scotto75
Renault 5 Driver



Joined: 13 Mar 2016
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PostPosted: 23:53 - 20 Jun 2019    Post subject: Reply with quote

Thanks for the replies. Yes, there’s no lights at all and the battery voltage does not increase when the the engine is running or revved. It’s presently at 3.8V and doesn’t change when the engine is ticking over.

I admit that I don’t really know what to make of the AC voltage readings of the stator wires. I know the rectifier is reading one way and not another, and stator coils have continuity like the should.

I will look into the zener diode.

On a side note, when I loosened the flywheel cover a ton of water came out from the bike being out during torrential rains. Also, the thread on the flywheel appears to be knackered so my puller won’t screw on proper! Fun & games.
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Honda ‘73 CB350, ‘80 CG125, ‘88 CG125, '73 Triumph T140V
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WD Forte
World Chat Champion



Joined: 17 Jun 2010
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PostPosted: 00:30 - 21 Jun 2019    Post subject: Reply with quote

Not a Zener diode!
Just a diode as a rectifier ( AC to DC)
Zener diodes were used as simple regulators on old bikes to keep the voltage within bounds
They are similar but are used and connected differently
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TUG
World Chat Champion



Joined: 12 May 2007
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PostPosted: 01:20 - 02 Jul 2019    Post subject: Reply with quote

the CG is a simple machine, if your charging system is testing fine with the multimeter then it will be a faulty earth on the loom, so you'll have to see whats going on.
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