OK I've got one here that's just got me stumped. Maybe some
old-timey Honda mechanic might be able to help

1980 Honda 750K.

I put a new battery in it start of the season, and I ride it most
every day. The problem is that after about a week of riding,
the battey has very little charge in it - just enough to start the
engine if the engine starts in about 3 revolutions. If the engine
doesen't the starter gets slower and slower and stops. Then
the bike will not start.

If I pull the battery and put a fresh one in, the bike starts
immediately. If I leave the fresh battery in, the bike works
fine for a week then the battery starts getting lower and lower
charge in it and the same thing happens.

I've traced the problem down to the bike isn't charging the
battery unless the engine is 2500rpm or more. I see the
battery terminals jump about a half-volt when the bike is
started, to about 12.5 volts, but I don't see the voltage hit
13.8 at the battery terminals unless the engine is revved up.

At idle, I measure almost a full 12 volts going into the field coil
and about 9 volts AC leg-to-leg on all 3 legs coming out of the
alternator.

I've replaced the regulator/rectifier with no change in
behavior. On both units, the diodes passed the ohmmeter
check.

Both rotor and stator pass the resistance checks in the
FSM. There are no shorts to ground on either one. This
is true whether the bike is cold or warm. I've cleaned all
the electrical contacts. The rotor brushes both have plenty
of material left on them. Power output of the alternator
does not change whether the bike is warm or cold.

My guess is that either the rotor or stator has some windings
that have broken down. Unfortunately, the FSM has no
listing of what voltage is supposed to be on the field, and
stator, at a given RPM.

Ted

"Ted Mittelstaedt" <(E-Mail Removed)> wrote in message
news:newscache$3bs62k$l21$(E-Mail Removed)...
> OK I've got one here that's just got me stumped. Maybe some
> old-timey Honda mechanic might be able to help
>
> 1980 Honda 750K.
>
> I put a new battery in it start of the season, and I ride it most
> every day. The problem is that after about a week of riding,
> the battey has very little charge in it - just enough to start the
> engine if the engine starts in about 3 revolutions. If the engine
> doesen't the starter gets slower and slower and stops. Then
> the bike will not start.
>
> If I pull the battery and put a fresh one in, the bike starts
> immediately. If I leave the fresh battery in, the bike works
> fine for a week then the battery starts getting lower and lower
> charge in it and the same thing happens.
>
> I've traced the problem down to the bike isn't charging the
> battery unless the engine is 2500rpm or more. I see the
> battery terminals jump about a half-volt when the bike is
> started, to about 12.5 volts, but I don't see the voltage hit
> 13.8 at the battery terminals unless the engine is revved up.
>
> At idle, I measure almost a full 12 volts going into the field coil
> and about 9 volts AC leg-to-leg on all 3 legs coming out of the
> alternator.
>
> I've replaced the regulator/rectifier with no change in
> behavior. On both units, the diodes passed the ohmmeter
> check.
>
> Both rotor and stator pass the resistance checks in the
> FSM. There are no shorts to ground on either one. This
> is true whether the bike is cold or warm. I've cleaned all
> the electrical contacts. The rotor brushes both have plenty
> of material left on them. Power output of the alternator
> does not change whether the bike is warm or cold.
>
> My guess is that either the rotor or stator has some windings
> that have broken down. Unfortunately, the FSM has no
> listing of what voltage is supposed to be on the field, and
> stator, at a given RPM.
>
> Ted
>
>

You might get more suggestions if you describe what type of alternator it
has. There's plenty of electrical people about who don't know your
particular bike.

Mark Olson wrote:
> Ted Mittelstaedt wrote:
>
>> OK I've got one here that's just got me stumped. Maybe some
>> old-timey Honda mechanic might be able to help
>>
>> 1980 Honda 750K.
>
> [snip story of failure to charge battery]
>
> Follow the fault finding chart at electrexusa.com. Do all the
> checks in order, don't skip any, and you will find the problem.
>
> Did you measure the AC voltage open-circuit or with the stator
> connected to the rectifier? You should disconnect the stator
> from the rectifier for the AC voltage test.
>


I like that chart, somewhat similar to the diagnosis instructions at

http://home.earthlink.net/~trinomial/DOHCcharge.html

which links from http://www.motorcycleproject.com/

The author, Mike Nixon, helped me with a similar problem on a 750C. I
notice both his write-up and the electrexusa.com chart say to check the
rotor before the stator. In my case, swapping the rotor fixed it,
luckily we had a parts bike. We also swapped the aluminum cover - you
could see the scuff marks on the bad rotor, it seems that the cover
housing on these engines can easily distort even if the bike is just
leaned against a wall, let alone a tip-over, causing rotor mis-alignment
and scraping.


Another tip I found somewhere is that the rear axle is just the right
size to use as a puller to get the rotor out. In my case, I put a cheap
electric torque wrench on it. Seem to remember that we put the bike in
gear to torque it back on it, not sure if that's a risky thing to do.

On Jun 9, 12:17 am, "Ted Mittelstaedt" <t...@toybox.placo.com> wrote:

> I've replaced the regulator/rectifier with no change in
> behavior. On both units, the diodes passed the ohmmeter
> check.
> Both rotor and stator pass the resistance checks in the
> FSM. There are no shorts to ground on either one. This
> is true whether the bike is cold or warm. I've cleaned all
> the electrical contacts. The rotor brushes both have plenty
> of material left on them. Power output of the alternator
> does not change whether the bike is warm or cold.

I fixed a drop in charging by replacing a set of brushes
even though the old ones had only about 50% wear.
Not sure if the brush was beginning to hang up or if
it was just the increased pressure but at $9 for new
ones, it was well worth swapping them as an experiment.

Even if they're not obviously bad, replace the brushes if
they're near 50% gone. Might not hurt to swap in a new
regulator as well. You could check this by jumpering
out the old one to constantly energize the rotor. On mine,
there's a standard 3 wire automotive regulator and it's
a simple jumper across two connections.

> My guess is that either the rotor or stator has some windings
> that have broken down. Unfortunately, the FSM has no
> listing of what voltage is supposed to be on the field, and
> stator, at a given RPM.

There should be a known resistance value for both
the rotor and stator that you could check. Resistance
values should be available on an owners list.

You could also check the A.C. on each leg from the
alternator and on the rectified output of the diode board.
A difference in voltage between the A.C. legs would
point up a problem. A significant A.C. current after
the diode board would indicate a bad diode.

On Jun 9, 7:49 am, "ian field" <dai....@ntlworld.com> wrote:

>
> You might get more suggestions if you describe what type of alternator it
> has. There's plenty of electrical people about who don't know your
> particular bike.

Sounds like an energized rotor energized by a regulator,
probably a three wire stator, and diodes to rectify the
A.C. output. What other info did you have in mind ?

I am an old Honda mechanic and my best guess is that the rotor is going
bad. A quick check is to hold a long (8 inch or more) feeler guage so
that the lower end of it is about 1 inch away from the alternator cover.
Turn on the key and the feeler guage should move over to the cover. My
guess is that it will only move a little. Check the resistance of the
rotor. If it is about 4 to 5 ohms it is OK, if it is below 2 it is bad,
I have probably changed 50 of these "back in the day". If you are gong
to buy a used one be sure to check it out.
Bruce

Ted Mittelstaedt wrote:
> OK I've got one here that's just got me stumped. Maybe some
> old-timey Honda mechanic might be able to help
>
> 1980 Honda 750K.
>
> I put a new battery in it start of the season, and I ride it most
> every day. The problem is that after about a week of riding,
> the battey has very little charge in it - just enough to start the
> engine if the engine starts in about 3 revolutions. If the engine
> doesen't the starter gets slower and slower and stops. Then
> the bike will not start.
>
> If I pull the battery and put a fresh one in, the bike starts
> immediately. If I leave the fresh battery in, the bike works
> fine for a week then the battery starts getting lower and lower
> charge in it and the same thing happens.
>
> I've traced the problem down to the bike isn't charging the
> battery unless the engine is 2500rpm or more. I see the
> battery terminals jump about a half-volt when the bike is
> started, to about 12.5 volts, but I don't see the voltage hit
> 13.8 at the battery terminals unless the engine is revved up.
>
> At idle, I measure almost a full 12 volts going into the field coil
> and about 9 volts AC leg-to-leg on all 3 legs coming out of the
> alternator.
>
> I've replaced the regulator/rectifier with no change in
> behavior. On both units, the diodes passed the ohmmeter
> check.
>
> Both rotor and stator pass the resistance checks in the
> FSM. There are no shorts to ground on either one. This
> is true whether the bike is cold or warm. I've cleaned all
> the electrical contacts. The rotor brushes both have plenty
> of material left on them. Power output of the alternator
> does not change whether the bike is warm or cold.
>
> My guess is that either the rotor or stator has some windings
> that have broken down. Unfortunately, the FSM has no
> listing of what voltage is supposed to be on the field, and
> stator, at a given RPM.
>
> Ted
>
>

"Rob Kleinschmidt" <(E-Mail Removed)> wrote in message
news:364d1772-745f-4241-a6c8-(E-Mail Removed)...
> On Jun 9, 7:49 am, "ian field" <dai....@ntlworld.com> wrote:
>
>>
>> You might get more suggestions if you describe what type of alternator it
>> has. There's plenty of electrical people about who don't know your
>> particular bike.
>
> Sounds like an energized rotor energized by a regulator,
> probably a three wire stator, and diodes to rectify the
> A.C. output. What other info did you have in mind ?

Some alternators have a brushless energised rotor - which would obviously
rule out worn brushes.

On Jun 9, 2:05 pm, "ian field" <dai....@ntlworld.com> wrote:
> "Rob Kleinschmidt" <Rkleinsch1216...@aol.com> wrote in message
>
> news:364d1772-745f-4241-a6c8-(E-Mail Removed)...
>
> > On Jun 9, 7:49 am, "ian field" <dai....@ntlworld.com> wrote:
>
> >> You might get more suggestions if you describe what type of alternator it
> >> has. There's plenty of electrical people about who don't know your
> >> particular bike.
>
> > Sounds like an energized rotor energized by a regulator,
> > probably a three wire stator, and diodes to rectify the
> > A.C. output. What other info did you have in mind ?
>
> Some alternators have a brushless energised rotor - which would obviously
> rule out worn brushes.

The OP had mentioned brush length. Which bikes use
brushless energised alternators ?

Agreed that it might have been good to put all that up
front instead of burying snippets all over the posting.

"Rob Kleinschmidt" <(E-Mail Removed)> wrote in message
news:05594247-d04a-4de5-bb4e-(E-Mail Removed)...
> On Jun 9, 2:05 pm, "ian field" <dai....@ntlworld.com> wrote:
>> "Rob Kleinschmidt" <Rkleinsch1216...@aol.com> wrote in message
>>
>> news:364d1772-745f-4241-a6c8-(E-Mail Removed)...
>>
>> > On Jun 9, 7:49 am, "ian field" <dai....@ntlworld.com> wrote:
>>
>> >> You might get more suggestions if you describe what type of alternator
>> >> it
>> >> has. There's plenty of electrical people about who don't know your
>> >> particular bike.
>>
>> > Sounds like an energized rotor energized by a regulator,
>> > probably a three wire stator, and diodes to rectify the
>> > A.C. output. What other info did you have in mind ?
>>
>> Some alternators have a brushless energised rotor - which would obviously
>> rule out worn brushes.
>
> The OP had mentioned brush length.

Things were getting a bit hazy by the sixth paragraph.

Which bikes use
> brushless energised alternators ?

2 that I can think of off hand are the Honda CB400F and the Yamaha XS250,
the alternator had a rotor with interleaved pole pieces but no coil, one set
of 3 poles went to the center spindle the other 3 went to the outer ring
thus making a co-axial magnetic coupling to a static field coil. It was
popular for a while but most manufacturers seem to have reverted to putting
the field coil in the rotor and using brushes again.

> Agreed that it might have been good to put all that up
> front instead of burying snippets all over the posting.

"Bruce Farley" <(E-Mail Removed)> wrote in message
news:wce3k.7286$(E-Mail Removed)...
>I am an old Honda mechanic and my best guess is that the rotor is going
>bad. A quick check is to hold a long (8 inch or more) feeler guage so that
>the lower end of it is about 1 inch away from the alternator cover. Turn on
>the key and the feeler guage should move over to the cover. My guess is
>that it will only move a little. Check the resistance of the rotor. If it
>is about 4 to 5 ohms it is OK, if it is below 2 it is bad, I have probably
>changed 50 of these "back in the day". If you are gong to buy a used one be
>sure to check it out.

Thanks, Bruce.

I have a parts bike, and a second bike, both with the same engine, and a
rotor puller.
(I bought the rotor puller when I discovered a slide hammer was no good for
removing these rotors - fortunately, Mr. JB Weld, Mr. Soldering Gun, and I
were able to keep the rotor usable, this is -not- the rotor in the bike with
the
problem, BTW)

The rotor resistance when I first measured it (measuring at the connector
which
included the resistance of the brushes) was more like 20 ohms. I then
turned over
the bike and the resistance fell to something like 10 ohms. I will measure
it again
with and without brush resistance, as well as measure the resistance of the
other
rotors in the other bikes that I have for comparison.

Brand new, the Honda dealership wants $452.20 and it is a special order
part.
(yes, that's four hundred fifty two)

There's a guy advertising on Ebay that rebuilds these for about $100. I
probably
can also get one rebuilt at a local automotive electrical shop (we have a
number
of them in town). I doubt I will find a used one, every parts bike I've
seen over
the years in the various bike wrecking yards has been missing it's rotor - I
had
always thought it was because of people dropping them and destroying the
rotor,
side cover and such, but now I know better.

Ted