Honda-SOHC
SOHC.co.uk Forums => CB350/400 => Topic started by: Gaz_hay on May 02, 2026, 10:31:34 PM
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Good evening,
I have recently purchased a Honda CB400 Four
1977
I’m planning on restoring bits myself , but I have no idea where to start on the charging issue, I have use of a multi meter and some basic knowledge on using it…
But I have zero knowledge on the cb charging set up , and never had any knowledge or experience with the stator especially with a “field coil”
Hopefully some gurus on this group can help me out , any information or ideas where to check would be much much appreciated 👍👍👍
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I assume when you mention charging issue that it's not charging.
I would start with checking the first connector block from the alternator, it might just be corroded terminals in the block.
Next I would check the connections to the voltage rectifier and the voltage regulator.
Also check the main earth connections, loom to frame, engine to earth strap etc.
If the above do not work then its worth using the search facility on here for checking the alternator field windings etc its a pretty regular area of discussion so you should come up with plenty of threads/posts on the subject. I have never had to go beyond the basics with my 400 so I can't advise on the more complex aspects.
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There is a 6 page bulletin explaining the 3 phase charging, i have tried to screenshot and post
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3 wiring diagrams to go with details
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If the field coil is energised it should attract a metal spanner if you hold it close to the honda logo on the alternator cover with just the main switch on
If that doesn't happen it narrows down which connections it might be.
Charging at low revs the field coil is fed the full 12v from the regulator. As the revs go higher a contact opens and the field coil is fed from a wire wound resistor under the metal back cover on the regulator. So if your getting little or no charge at lo revs that suddenly bursts into life as the revs increase, there may be a grubby contact in the regulator. At 15v another contact shorts out the feed to the field coil.
The shop manual downloadable from Ash's Dropbox might help regarding what revs the regulator should switch over.
As mentioned in a previous post , spraying the connectors with wd40 or duck oil followed by plugging unplugging a few times to wipe the contacts might work if the connectors are grubby. There is also the 3 spade connectors that go to the regulator.
Regards
Dave
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The reg does not switch at revs but at a prest voltage, as the generator output is based on both the field coil power input AND the revs it means that the reg allows charging till the battery voltage hits a specific level, its all in the pages i attached
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Thanks everyone for the info / replies
Will try and get my head around diagrams etc that have been attached, I’ve also checked all the plugs on the charging system and all in good condition not corroded.
I will check the magnetised idea on side cover later ….
Thanks for baring with me I’m not the best on electrics or even using the internet forum.
Much appreciated 👍👍👍
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The reg does not switch at revs but at a prest voltage, as the generator output is based on both the field coil power input AND the revs it means that the reg allows charging till the battery voltage hits a specific level, its all in the pages i attached
I was trying to simplify things a little, the transition voltage where the resistor is switched in series with the field coil should be in the manual.
At 15v the contact does short the field coil feed, now through the resistor to ground. Once the voltage drops below 15 the contact opens again. This off/on happens very fast and essentially modulates the feed to the Field coil to keep the voltage under 15v.
Dave.
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I have no field coil activity by the looks with the ignition on the coil is not magnetised, does anyone know what the resistance between the green and white wire to the coil should be or any other tests I should do before swapping out the field coil
Thanks
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All in the Honda manual which i think is downloadable from Ashs dropbox or Alladins cave
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I have no field coil activity by the looks with the ignition on the coil is not magnetised, does anyone know what the resistance between the green and white wire to the coil should be or any other tests I should do before swapping out the field coil
Thanks
Have got the side cover completely off and added 12v down the white and earth down the green and my coil is working fine,
I’ve checked white wire out of the regulator and no 12v is being sent to the coil hence no charging (I’m guessing )
By reading up…. I’m sure the black wire to the regulator is the 12v ign feed and also shows the regulator the charging status …. So next test would be to see if I have any voltage on the black wire with the ignition on… thinking this is correct 👍
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Black wire is switched 12v that supplies 12v everywhere when the ignition switch on
With the regulator connected up, ignition on you should be seeing battery voltage on both black and white wires. IE full 12v going out to the field coil.
With all wires disconnected from the regulator you should have continuity between the spade terminals on the reg for black and white wires.
From the black wire you should have continuity to any other black wire on the bike even with even with gnition off.
Are you missing 12v from anywhere else? Inside the wiring harness where the black wires are joined is by a soldered connection so if you have lost 12v at the regulator I would expect it to be missing somewhere else, or the evidence is pointing the individual wire and spade connector feeding the reg.
With your spanner close to the coil give the black connector a wiggle with ignition on.
Dave.
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Sounds like the bullet connectors down by the neutral switch to me
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Thanks guys, will double check the black wires tomorrow and white as mentioned above, wil also double check all connections and bullet connectors … will update you all tomorrow 👍 thanks all for the input would be lost without the help much appreciated
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The Honda charging system is very robust and i have never seen a field coil or stator fail unless damaged in an accident, i have seen a small number of regulator fail, espicially where bike have been stood for a while.
The only difference between 400 regs and all the other fours is the angle the spade connectors are at.
DO NOT be tempted to play with the adjusting screws on the coils and contacts they need to be set on a regulated power supply, mine died and i dont have enough reason to replace it now
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Sorry about the delay, finally had time to dig the old girl out and do more checks - thanks to your lot I’m finally getting somewhere can’t thank you enough
I’ve tested the black wire with ignition on and I have 12v …. When. It’s connected to the regulator/ rectifier there is no 12v coming out of the white wire to the field coil (my understanding this should have the 12v the black supplies )
Forgot to mention mine had the new style regulator / rectifier all in one not the one you can adjust …
Have attached picture (to show what it looks like offline ) 😇
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It may be toast. If it's getting power and not giving any out to the field coil.
If it were me I would try to find a used original. As Bryanj says they're pretty robust mechanical devices. Usual problems are oxidised contacts if they haven't been used for a long time. There is a rubber seal around the cover so moisture shouldn't have an easy way in. Downside for you if you go that route will be needing a separate rectifier as well.
I have a mistrust of no-name electronic devices from the far east but everyone is different.
Regards
Dave
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Perhaps one from a later Honda
https://www.ebay.co.uk/itm/277950477570 combined unit from OEM supply, would need individual "adopted" loom end into existing SOHC plug.
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Electrex do a combined reg/rec for the cb400. Seem to recall it uses Mosfets as well, which is usually a sign of a decent one. Connectors seem to be plug and play as well.
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Thats not a "later" one its an aftermarket one, they can be very unreliable and it only take a fraction of a second of reversed polarity to wreck them
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Thats not a "later" one its an aftermarket one, they can be very unreliable and it only take a fraction of a second of reversed polarity to wreck them
It's not from SOHC F2 but later twin cam 750 (not the 70s replacement for SOHC motor either) that was original fit "OEM" and Japanese supplied component.