SOHC.co.uk Forums > CB350/400
Charging circuit test
Dave487:
Check where the 3 yellows exit from alternator cover. I recently had a similar problem and one of 3 had gone to earth at the bullet connector close to the cover and melted the plastic insulators on all 3. I fitted new connectors and all is well.
Lobo:
Steve... to check for AC voltage ..
(1) select a suitable AC Voltage range on your multimeter. (Eg 50v AC)
(2) With all connections ‘normal’, start the bike
(3) put your multimeter probes across a pair of the YELLOW stator windings... and check the AC Voltage
(4) Do this for the next pair
(5) Ditto.
Attached a YouTube clip, not necessarily what you’re after, but useful general knowledge stuff..
https://www.youtube.com/watch?v=zCz735rFcYU
stevush:
OK Lobo, Interesting, thanks for the info, I'll try that over the weekend sometime.
What readings would I expect to tell me if its good/bad ?
Thanks
Steve
mike the bike:
If you get continuity to earth on all 3 yellows, it doesn't necessarily mean all three are shorting to earth, it will read a short because of the low resistance of the endings. Check the insulation of the yellows where it exits the alternator casting.
Lobo:
Steve - I honestly wouldn’t know typical AC readings as this test is not described anywhere I read - rather the resistance of each STATOR winding is the typical check. (should be 0.2 ohms ‘ish)
(Whilst you’re at it, you should check your FIELD Coil winding... the coil within the STATOR. This has WHITE & GREEN wires - this resistance should be 7.2 ohms’ish)
You mention you’re a little uncomfortable with reading Resistances... set your meter as the below pic RED circle. (No matter which way around you use the probes). Set the scaling correctly... or just understand why it reads 0.72 ... or 72 ohms etc!
I’m not near my bike(s) to do such an AC output coil check on a healthy machine... if I had to guess I might say 16vAC ?
BUT. The point is, it is highly unlikely all 3 windings would have failed in exactly the same way... ie. just as when looking at cylinder compressions - you’re mainly looking for a difference. Such a thing will point to an issue ... if all 3 readings are give-or-take the same (and > 15vAC) I’d reckon you could move on from the stator.
As Bryan has already mentioned, one of the diodes may have failed within the Rectifier (fin air cooled thingy with 5 wires (3 yellow + Red + Green) coming out. Have you checked this?
If you study the 2nd attachment below (drawing schematic) you can see that ALTERNATING (AC) current is fed into the middle of 6 Diodes. (via the stator output, ie the 3x Yellow wires)
Diodes only allow flow in one direction ... and so you can see that (to the left side of all #3 inputs) the positive part of the alternating flow will be allowed to pass through the ‘arrows’... to the RED/WHITE wire, the negative part will not. Equally, to the RHS of all #3 inputs.. all positive flow will be rejected, whilst the negative portion of the cycling current will be allowed to flow to the GREEN wire.
And bingo... Alternating current is ‘rectified’ into Direct current, ie +ve at the RED/WHITE and -be at the GREEN wires.
So... if one of those diodes has failed... your charging will be degraded.
Set your multimeter to the Blue (ie Diode setting) circle, and put:
(1) The Black probe on the first YELLOW, and the Red probe on the RED/WHITE Wire. You should have continuity ... and if your meter so equipped, a B...E...E...P.
(2) keep the Black probe on the first YELLOW, and move the Red probe to the GREEN Wire. Should have zip.
(3) move the Black probe onto the second YELLOW, and put the Red probe on the RED/WHITE Wire. You should have continuity ... and if your meter so equipped, a B...E...E...P.
(4) keep the Black probe on the second YELLOW, and move the Red probe to the GREEN Wire. Should have zip.
(5) move the Black probe on the third YELLOW, and put the Red probe on the RED/WHITE Wire. You should have continuity ... and if your meter so equipped, a B...E...E...P.
(6) keep the Black probe on the third YELLOW, and move the Red probe to the GREEN Wire. Should have zip.
If any part of the above fails you’ve likely an issue with your Rectifier. (Don’t forget this issue could simply be corrosion in the plug etc - ie if it fails the test check the wiring / bullets etc)
Wrapping up... and having re-read your first post... the charging on these old bikes is minimal at best. If you’ve non-standard coils (?) which are ‘high performance’ there is every possibility you’re overloading the charging system with lights on etc. If you’ve non-standard lights, add-in electrics... ditto. Just sitting at traffic lights, foot on the brake, headlamp on, maybe indicators... you’ll be depleting the battery. Bottom line, don’t begin to compare this charging system’s output with anything modern you might own! Use your electrics sparingly, avoid main lights if pootling about town etc, get the starting bang-on... ie no-need of excessive cranking.
Good luck,
Simon
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