Honda CB750K5 Restoration by Royhall

[royhall]

I have no voltage whatsoever at any of the yellow wires tested at the re/reg plug either yellow to yellow, or yellow to earth. Does that mean that the alternator is done. Think it's time to get the alternator cover off and have a look.

[AshimotoK0]

I have no voltage whatsoever at any of the yellow wires tested at the re/reg plug either yellow to yellow, or yellow to earth. Does that mean that the alternator is done. Think it's time to get the alternator cover off and have a look.

If you have decent resistance readings between yellow wires then the  stator is probably OK,  provided no leakage to the frame. You do need current in the field coil though to magnetise the rotor on the crank, which is controlled by the regulator.

[royhall]

The field coil resistance is 8 ohms on my meter, should be 7.2 but again take the leads 0.8 away and its correct. When I turn on the ignition the black wire on the rec/reg should give battery voltage then the white wire should go live to the field coil and then to ground. If that is correct I can test for voltage at those points (basically white to field coil goes live on ignition switch turn on). Am I correct with this? Thanks all for the help. As you probably guessed, this is a school day for me.

[Bryanj]

Check that the green wire from field coil goes to earth then temporarily connect white to battery positive then disconnected yellow wires should give the ac volts between wires THERE IS NO CONNECTION TO GROUND on this system.
If you dont get ac volts chances are the connectors at generator are bad

[royhall]

UPDATE: I have 12.83 volts at the battery. With ignition on 12.80 volts goes to black wire on the regulator, but no power on white wire out of regulator to field coil. Two possibilities, either the rec/reg is duff or the regulator is seeing 12.80 volts as a fully charged battery and has turned off the charging circuit. Now if only I had a duff battery with 10.5 volts to give that a try. No idea how you would test for that. I'm fairly happy that the stator and field coil are okay so can possibly reassemble it tomorrow. I suppose I could run the engine with lights and indicators on to drain the battery to below charging level and see if it cuts in.

MODE3(seeFig.3). Battery voltage is excessively high; ignition switch is closed (on). The regulator’s lower set of contacts are now closed. The current paths are: From positive battery through fuse and field resistor, through the closed lower contacts of the regulator and then to ground. No current goes through the field (0amps), and the alternator has no output; therefore, no rectifier output and consequently a lower battery voltage.

The above could be happening due to a fully charged AGM battery and there is no actual problem with the charging system? I'm thinking 12.40 volts is the field coil switch on point. I may be doing all this for no reason, except a good learning day.

[royhall]

Check that the green wire from field coil goes to earth then temporarily connect white to battery positive then disconnected yellow wires should give the ac volts between wires THERE IS NO CONNECTION TO GROUND on this system.
If you dont get ac volts chances are the connectors at generator are bad

Cheers Bryan. I will try that tomorrow when it's reassembled. Green wire checks okay to earth.

[BigAl (Alan)]

Not sure if your meter reads DC amps?   If so, you can connect a + fused lead from the meter to your battery + terminal (battery must be fully charged)
Then remove the white flag terminal from the regulator and fit to the common terminal on the meter (image attached)
Run the engine, the field coil will be fully excited, your meter may read 1.6amps (assuming a 7.5ohm field coil is fitted)
If no reading there could be a coil winding or earthing issue.

[royhall]

Thanks for that Alan. Had to read it three times but I think I understand that. Basically your connecting battery live through the meter in series to field coil (on) (white wire) and taking the reading. I think my meter is capable of connecting 20 amp circuit. Will have a go tomorrow. Got to put it all back together first. Thanks.

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[royhall]

I have had the the side castings back off the engine to check the alternator and the connections, and to check nothing got trapped. All looked correct and tested correctly and is now reassembled. So I then hooked up the meter as Alan suggested to read the current to the field coil (coil previously tested at 7.4 ohms) leads running through the meter connected from battery to disconnected white wire at the rec/reg. My second meter was connected across the battery set to volts DC. The static voltage on the battery read 12.81 volts, on starting the engine battery voltage jumped to 13.90 volts (revving the engine got it just to 14 volts) and the current on the white wire read 1.69 amps.

That proves the charging system is working fine. What happened here was a classic case of too little working knowledge of the system. Previously if I was changing something like a rec/reg they come with instructions about what plugs where and that's all I did. There had been nothing wrong with the system all along, but very much not a waste of time as I now have a good knowledge of why things work the way they do and why I am plugging which wires where.

Thanks to all that helped with this. Just got the little problem with the carbs to deal with now.

[Bryanj]

Thats the thing with electrics, methodical and thought out testing and not just diving in

[royhall]

Thats the thing with electrics, methodical and thought out testing and not just diving in

Your dead right Bryan, and it is nice to find out that it was right in the first place and that means I got everything working first time. When it comes to electrics that's a first for me, I must be getting better. My knowledge has improved greatly.

The carbs are now back off the bike and winging there way to Matt Harper for building properly. I think Gerben made an outstanding job refurbishing the outside of the carbs, but inside not so much. Matt will sort them out, his work is first class. So as Matt says it will be 2 months (good people are always busy) I have finished building up the bike and stashed it under a cover in the corner.

What's missing in these pictures?

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There wont be any more posts now on this for the next 2 months until the carbs are back then hopefully it will be a simple job to finish. Sometime whilst there away I will get out the paperwork and add up the final cost. I hardly dare look as it was over 10k ages ago. I guess it's money I wont fully ever get back but at least it's been fun (mainly).

[Bryanj]

Dave Silver once offered to tell me how much i had spent with him, the second word was off

[K2-K6]

Bike looks so good Roy, a lot of your work and money obviously, but with something really nice to own and ride as end result.

The electrics section in earlier Honda workshop manual is really good reference to working on the charging etc. Very good detail generally, that did appear to be similar in Ted's Clymer manual too, with good overview from the parts shown.

Nothing like step through hands on experience to help in understanding though, definitely a bonus to get your head around things.

[Bryanj]

Thats very true, electrics can seem daunting and i find electronics difficult as you cant see the "taps" that turn the elctrons on!

[royhall]

Forgot to say in the last posting that the replica seat from Silvers is pretty low quality. Obviously it's a pattern part so didn't fit properly and required packing out to open correctly. The chrome trim is badly finished, plus it looks too high to me. Who's the main man for accurate seat refurbishment these days, may just resurrect the old one.

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