Suggestions on a Honda charging system

 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 wrote:


[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.

--
'07 FJ13AW  '99 EX250-F13
OMF #7

 Mark Olson wrote:

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.

 

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 ?