Topic: CB750 barely charging. Looking for next steps.

[Tugboat]

I barely made it home last weekend after a ~100 mile ride on my K5. Started out with a fully-charged battery. I knew I had charging problems when I stopped to get gas about 50 miles in and the starter & horn didn't work when I went to fire it up. It would kickstart though. The headlight didn't get noticeably brighter when I revved it, so I rode home w/ no headlight, no blinkers... and as I pulled into my driveway the bike died. Lucky me!

Checked the battery: 8.8v

So I checked the stator output and it's around 1.4v. What are the next things to check? Photo for attention

[bryanj]

Go to the UK site and look under tricks and tips/faq and i wrote a comprehensive testing document

[Jerry Rxman Griffin aka MuthaF'er]

Check input and output at your ignition switch. It probably carries more current than anything else.

Remember to pull the headlight fuse in this instance. 

[HondaMan]

Check the condition of the white plug where the engine plugs into the frame, by the left side cover. Make sure the contacts are not all rusty, as many are. This can stop the field coil from powering up the alternator. Also: check the 5 bullet connector to the alternator, under the sprocket cover on the engine's left side. They are often cooked and burnt form getting wet from overzealous washing of the sprocket area to clean it up, then they rust, too. You can replace all of these with modern brass contacts that won't rust again.

[scottly]

With a fully charged battery, turn the key on, and check the magnetism of the field coil at the mounting screws. If the field is too weak to support bolts, you have a regulator or field wiring problem. If the bolts stick, you have a rectifier or stator wiring problem.
 

[Tugboat]

I tried the bolts-to-cover trick and it didn't work. So I did as Hondaman suggested and took a look at the 8-prong connector from the motor to the harness. I recently had this motor out to fix leaky pucks, and when I put it back together I'd cleaned this connector as best I could with a Dremel brush. There is considerable corrosion on the female bit; male bit looks ok-ish. Also took a look at the bullet connectors going to the field coil. They were  in pretty good shape - no corrosion on either side. I then took a good battery from my KZ400 and applied power directly to the two field coil wires, and then stuck three bolts to the cover. Cleaned all the connections with electrics spray and put it back together and the bolt trick still works. Haven't had time to take it out and fire it up yet to check power across the battery, but at least I think I'm on the right track. Thanks for the input y'all!

[ekpent]

 So maybe the fix was the "good battery" from the KZ400 ? Did you try it with the other battery you used before ?

[Tugboat]

Just wanted to follow up here. Turns out it was the main 8-blade plug coming from the motor. Both sides were corroded and one of the female bits had basically fallen apart. I replaced the plug with a new one from Oregon Motorcycle Supply and now I have 13.1v at idle and 14.2 at 4k rpm. Had to cut off a couple of blades, which meant I had to cut them all off, resulting cable length is about 1/2” shorter and man - that makes the air box install really tight. But it fits. And it works.

[ekpent]

 We should all probably give those major plugs and others wires more love and attention then many of us probably do. Definitely guilty of neglect here.

[bryanj]

Thanks for comming back with the fix, something else to try and remember

[scottly]

I tried the bolts-to-cover trick and it didn't work. So I did as Hondaman suggested and took a look at the 8-prong connector from the motor to the harness. I recently had this motor out to fix leaky pucks, and when I put it back together I'd cleaned this connector as best I could with a Dremel brush. There is considerable corrosion on the female bit; male bit looks ok-ish. Also took a look at the bullet connectors going to the field coil. They were  in pretty good shape - no corrosion on either side. I then took a good battery from my KZ400 and applied power directly to the two field coil wires, and then stuck three bolts to the cover. Cleaned all the connections with electrics spray and put it back together and the bolt trick still works. Haven't had time to take it out and fire it up yet to check power across the battery, but at least I think I'm on the right track. Thanks for the input y'all!

The easy and fast bolt magnetism test divides the charging system in two parts, making it easier to narrow down the fault than a shotgun battery of voltage and resistance checks of every component in the system.

[Spanner 1]

 With respect, when I saw the pic of Tugboat's 8-way connector with a whitish corrosion on every spade/ female, I straight away went to the very bad practice of spray washing these old bikes in the DIY car wash. That high pressure detergent was never supposed to penetrate parts of our bike where it would wreak havoc with the electrics. Those connectors get water logged with a detergent mix that simply corrodes the #$%* out of these terminals. Please don't do that. The connectors on our old Honda's need to be unplugged every 6 months and examined for corrosion, loose contacts, contacts 'backed out ' of their connector, filled with dielectric grease and re-plugged firmly.  Always do this before a long trip. It's about the only check I do anymore before a 600-700 mile weekend apart from adjusting the chain, rant over !

[AlekStooge]

With a fully charged battery, turn the key on, and check the magnetism of the field coil at the mounting screws. If the field is too weak to support bolts, you have a regulator or field wiring problem. If the bolts stick, you have a rectifier or stator wiring problem.
 

That's a cool tip.