81 CB750 Custom Charging Problems

 When my motorcycle is running and I check the battery with a
multimeter it is only registering 11.8 and will not go any higher even
at 5000 rpm I replaced the Regulator/Rectifier 2 weeks ago with a new
one and it was fine registering the right amount of voltage now its
back to discharging do you think it could be a bad battery or
something worse?
If the battery was bad would it still register the correct amount of
voltage across the terminals while running?
                                                 Thanks.......

 In article


There are several possibilities. First, make sure that all your
high-current connections are clean and tight: Battery +, battery -,
ground, starter. Next, the medium-high current connection from the
voltage regulator to the battery. Check every connection, including
fuses. I had a fuse blow once by burning a circle into the end of the
fuse which otherwise looked okay. That was an expensive tow bill and an
embarrassing call from the shop.

Next, check the alternator itself. Unplug the cable from the stator to
the rectifier/regulator. If it has three connections, the test is easy:
measure resistance across all three combinations of pairs of wires. They
should all be the same. If it has four, then one of them will be odd, a
different color or size of wire. Measure resistance from it to each of
the other three. They should all be the same. In either case, if one of
them is infinite or very low, then you have a fried winding in the
stator.

Put the battery on a charger overnight, then check its voltage. Try
starting the bike and running it for a while. The alternator should make
enough power to run the bike and charge the battery. It's hard to really
know the state of the battery if the rest of the bike is suspect.

--
Timberwoof <me at timberwoof dot com>
faq:  http://www.timberwoof.com/motorcycle/faq.shtml

 if it's putting anything out, it would be 12.8 to 13.8 volts .
connections on a bike this age are always a question.
also, is there any way to measure the alternator armature resistance or to
see if the circuit has continuity?
as i recall, this is the double cam model, isn't it?
i'm more familiar with the single cammers.
does the battery have enough juice to start on battery?
are there brushes in that model? they're always suspect.
any loose connection between the regulator and the battery will
often take the regulator out.
just some thoughts.
sam

 



Start with Timber's advice to check wiring and connectors, a common thing
that can happen is crash bars or fairing struts can work loose and chafe the
wires coming out the housing.

You can check the battery doesn't have a shorted cell by using a regular
charger, if it gets up to 13.8 - 14.4V try and see how long it keeps going
on the bike to check its holding its charge.

If you need to check the alternator its always worth a look at the workshop
manual to confirm what type of winding, usually its a 3-PH "delta winding".
The resistance of each winding is too low to make a meaningful comparison
with most multimeters and whichever coil you try to measure will have the 2
other coils in series across it so you probably wouldn't see it if one of
the coils is open circuit. There is a trick that can be used if you have a
12V wall-wart with AC output, use an indicator bulb to limit the current
which you pass through each winding in turn and observe any obvious
difference in brightness of the bulb - if 2 of the windings give dimmer bulb
than the other then the other is probably open, if 1 winding lights the bulb
brighter than the other 2 then that winding is probably shorted.

You should also check that the windings are not shorted to earth as this can
pass a loop current through the 3 earth side diodes in the rectifier pack,
this fault can obviously ruin the rectifier/regulator pack.

As long as the alternator uses a field winding the rectifier should be easy
to check, many DMM multimeters have a diode check function which displays
the diode forward voltage (usually 0.6 - 0.7V). Put the red prod to the
rectifier negative and the black prod to each AC pin in turn, then repeat
with the black prod to rectifier positive and red to each AC pin in turn -
all six tests should read about 0.6V, then switch to the highest Ohms range
and repeat all 6 tests with the prods reversed - all 6 tests should read
infinite resistance.

Naturally you will already have checked the slip rings under the brushes are
clean and the brushes are undamaged, the manual usually states the field
winding resistance and this is usually easier to measure.

 



***Correction - I got that bit back to front, 2 windings *brighter* means
the other one is open.

bulb