82' Honda CB750 Charging Woes

Here's the story: picked up an 82' CB750, runs and drives fine... but
the battery doesn't charge. First thing I did was buy a clymer manual
and go through their troubleshooting procedure. Everything checked out
ok, including the voltage RR, but since that was the most complex
component of the system and the readings were a bit "high" resistance
wise I decided to replace it anyways to the tune of $100 (ouch).

Still doesn't charge. What I've done so far;

1. New battery, fully charged (to note I've also hooked up a fully
charged car battery to see if that would provide proper excitation
voltage). On a motorcycle battery, eventually it'll run down to between
10.5 and 11.5 volts and just degrade worse if the headlight is
connected.

2. I cleaned up the connectors and then after that didn't work went
ahead and cut them off and put new ones on with clean connections to
remove any corroded wire ends.

3. Brushes on the stator cover look 'new', plenty of room left until
the scribe lines.

4. Rotor slip rings measure very low resistance between both hot AND
cold (~0.1 ohm). Visual inspection of the rotor was satisfactory, it
does not look discolored and has not sheared the key off the shaft. I
also cleaned the rotor surface. To note I also measured these readings
through the connectors as well and they are satisfactory.

5. Stator windings all measure very low resistance between them (on the
order of an ohm or less) and do not fault to ground. Upon visual
inspection, the stator enamel looked very good and the wires were not
discolored. Also measured windings resistance through connectors and
again, satisfactory.

6. Inspected battery cables, put new ends on to remove corroded wire
tips, but generally look "ok".

7. Frame ground looks "ok".

8. Replaced 30amp tin fuse on starter relay with piece of wire to
remove it from the equation. TO NOTE*: The fuse was getting extremely
hot (burnt my hand), actually the relay would smoke it would get so
hot, after replacing it with the wire it is now cool.

9. Disassembled the relay to the extent I could (couldn't get the
bottom anodized portion off) and cleaned the contacts. I still do not
trust this and suspect it may have internal corrosion, although visual
inspection indicated the interior is in "pristine" condition. Not quite
sure what the two tiny copper wires are that connect inside the
anodized portion, I would assume this is the starter solenoid itself.
Either way the bike starts fine.

10. Checked pigtails and connectors to the stator housing and the rest
of the bike. Everything looks fine with no grounded or exposed wires.
All insulation seems to check out ok along with resistance readings.

11. Measured AC output voltage from the 3 stator windings while the
bike is running--registers a low (10-12V) voltage.

12. Excitation wires measure battery voltage (low usually, 10.5-11.5)
volts, and 1.5 volts respectively.

And that's it, I'm actually an electrical engineer and I'm still
stumped by this problem. It's already almost July and I'd like to start
riding my bike!!!

Any help is GREATLY appreciated!

 

Couple more cents on this issue;

After reading that a shorted rotor would register a very low
resistance, I went and checked the reading again. The resistance is
about 5ohms, 17 ohms altogether through the brushes up to the wiring
harness for the regulator/rectifier, so it seems to check out ok.

So I know the problem is not the rotor, stator or RR.

I'm clueless.

 

I know you replaced it, but a little voice in my ear is still whispering
"reg/rec".

Mind you, I know eff-all about leccy things.

 

I've built my own custom regulator which I know works, along with a
brand new one which I know works, along with the old one, which I know
now also works.

It's not the RR. Something else is goofed up.

 

When you check the rotor, you have to isolate it from the rest of
the electrical system or you get bogus readings. You may have
already done this, but if you're not sure, that'd be a good test to
repeat.

I think I might do this:

Turn the bike on.
Check for +12 at the rotor.
Turn the bike off
Pull the +12 wire to isolate the brushes
Check resistance through the brushes to ground.

I read your note a little quickly, but this is where I'd start.

If you haven't found it already, electrexusa.com has a pretty
good troubleshooting flowchart. You need a newer version
of acrobat to read it.

 

On my 1981 (same exact electrical system I believe) I had a charging
problem. All the static resistance tests were in the range the book
gave me.

But in fact the rotor was bad. New rotor, problems more or less over.

More or less because this system seems kind of marginal for charging
even when it's working right. You want to have a good battery, and keep
it on a float charger when you're parking the bike for any length of
time. You do not want the charging system to do a lot of work to bring
up a worn-out or nearly dead battery.

 

Looks like your rotor is bad. The rotor should have 4-6 ohms
resistance.

Jesse

 

Gotcha, have to replace the rotor I guess.

 

Those rotors are really expensive to replace. The cheapest I've
found at a local bike wrecker, used, is $50. I would be inclined to
take it to an auto electric shop and see if they could rewind it.
You could also buy the wire and rewind it by hand, although it
takes a long time.

Also one other caution, under no circumstances use anything than
the correct puller tool to remove the rotor. The threads in the end
of the crank and the bolt are very tight tolerance, tighter than
a garden variety metric tap. (it's a 1.25 tap if you need to chase
the threads) I think that is so they can spec up to 75 foot pounds of
torque on the bolt to hold it on. If you use a standard screw puller
with the jaws, you can easily wreck the first couple threads in
the crank, then the bolt will never go back in without stripping.
Then you will have to use the metric tap and buy a grade 8 bolt and
torque it to the minimum and hope it holds together. I have one
of these engines that I bought used that the bonehead that sold
it to me had pulled the rotor with the jaw puller and I had to go
through this.

Ted

 

Hey Rj I have an 81 cb750K as well and like the rest of all of you, a charging system issue... I had replaced my rotor and RR not to long ago no cigar. She would run for a a couple runs but never would really charge I suspected the rotor may have been faulty when testing it with dirty slip rings the readings where all over the place 6.0-8.0 making me think this thing was junk, after sitting down and contemplating life on how much money I had spent on building this bike just so a simple charging system failure would be the problem that gets me I realized . No crap how bout I clean the slip rings... and what do you know once cleaned I got a consistent reading of 6.0 ohms does anyone know if this is okay the rotor is still under warranty so I’m able to return. After reading the promising reading of the rotor I moved on to the stator and had noticed some discoloration and some charging could this have shorted and is the culprit in my charging system woes ?????

 

Update about 2 seconds later the rotor is now readings 7-8ohma this system is going to be the death of me