Stator or regulator/rectifier?

I have a 1999 Honda Shadow 750. Recently, it began starting harder and
harder until finally it wouldn't turn over at all. Sounds like classic
battery problems.

I threw a charger on the battery, and it charged right up. It was turning
over nice and strong, even a few days later.

So I began investigating the charging system. Even with the engine revved,
I was still getting less than 13 volts on the battery. The shop manual
says 14 to 15 volts.

I disconnected the three stator wires and mesaured the voltage across
them. I was getting over 50 volts AC when it revved, and the waveforme
looked good when I hooked up my scope (to my untrained eye).

When I hooked it back up to the rectifier/regulator, the voltages on the
stator damped down to around 10 volts, and the output from the
rectifier/regulator was (as I said) less than 13 volts across the battery.

This was true even with a known good battery I borrowed. Since the
rectifer/regulator is a single unit, I don't know how to check whether
it's good or not.

I'm thinking the stator is ok, and the rectifier/regulator is bad. I would
like to be more sure before I plunk down the cost of a new one (but it's
still cheaper and less invasive than trying to dig into the stator).

Does my analysis sound right? I'm either looking for validation or
possible other explanations.

Thanks!

- Jay

 

That's a permanent magnet alternator, isn't it? Power output would
depend on the strength of the magnets, the RPM of the rotor, and the
number of turns of wire in each coil of the stator. The stator should
put out more than 50 volts, it should be around 100 volts open circuit
if the engine is turning around 5000 RPM. If the engine is turning fast
enough, there is the possibility that the bolt holding the rotor to the
crankshaft is loose and the rotor is slipping on the end of the crank
and that's bad news, it can chew up the end of the crank and ruin the
rotor. That happened on my Suzuki GS-1100, ruined the end of the
crank...

The other bad news possibility is that the stator is internally
shorted. It should have extremely high resistance from each phase to
ground, maybe a megohm or so. And the resistance from phase to phase
should be very low, but it should be the same from any phase to the
other two phases, like around 1 or 2 ohms...

A low power motorcycle alternator hooked to the battery via the diodes
in the rectifier regulator will behave as you describe. Even when it's
working perfectly, the alternator can only raise the battery voltage up
to about 17 or 18 volts.

Check for continuity through each of the three AC inputs to the
positive lead (or pin if the RR has a plug). It will probably be around
15 to 20 ohms, depending on your ohmmeter. All three AC inputs should
read the same to the positive lead (or pin). If you don't get any
reading on any of the three AC inputs, reverse your leads and try
again. You should get the same 15 or 20 ohms to the positive RR output
lead (or pin). Then reverse your leads and check the three AC inputs to
the ground lead (or pin) on the RR. You should get the same results, 15
or 20 ohms to the ground lead or pin. If you don't get any reading on
one AC lead to positive output or the ground wire, that diode is blown
out. If the RR has no ground lead, it may be case grounded and the
readings would be to the case. Also, the RR may need a better ground if
it's case grounded...

The only way to check the zener diode and the SCR shunt inside the RR
is to run the engine up to about 5000 RPM and watch the voltage rise to
a peak of about 14.5 or 15 volts and then sharply drop off to battery
voltage.

If you roll the throttle off and repeat the process the zener diode
should trigger the SCR to shunt one phase to ground over and over and
over ever time you roll the throttle on. This shunting of one phase to
ground cuts the alternator output in half.

Check into aftermarket parts made by Electrex USA (they have a website)
before you buy any OEM Honda parts...

Your open circuit stator voltages sound very low. Could be bad
insulation and an internal short in the stator. The stator is not
supposed to be grounded. The low resistance of the stator windings
makes it hard to guess whether the stator has internally shorted
coils...

OK. You're a good person, Jay. People like you.

You're welcome ;-)

 

Well, if the stator is giving voltage in-range and the RR is not, then
the problem is most likely the RR. The RR check for my 1994 VFR750 goes
like this:
Use a meter to check the resistance (basically you're testing diodes, so
use that mode if your meter's got one) between the pins of the meter.
Resistance should be infinite EXCEPT for the following configurations:
Pos. lead on pin #1 and Neg. lead on
Pin #2=0.5-10 kOhms
Pin #3=0.5-10 kOhms
Pin #4=0.5-10 kOhms
Pin #5=0.5-10 kOhms

In addition, Pin #5 on the negative lead should read 0.5-10 kOhms when
the positive lead is attached to ANY of pins 1-4.

Pins are numbered, ON MY RR, starting from the lower left with pin #1
and #5 to the right of it. THat's the bottom row. The top row is 3 pins,
left to right: 4,3,2.

I don't know the numbering of your pins, so my procedure may not apply
to your RR. Check to see if there's more information specific to your
model online. You can also often get RR's online somewhat cheaper than
from Honda (popular models appear on eBay all the time). If you've
modestly good with electrics, swapping in an RR from an entirely
different bike (e.g. one with better heat-sinks for the VFR) is not too
difficult.

HTH

--
Later,
John



'indiana' is a 'nolnn' and 'hoosier' is a 'solkk'. Indiana doesn't solkk.

 

Well, either I was tired the other day or things got worse, but I went
back out and measured today, and I was only getting around 10V AC even
without the regulator attached. So it looks like the stator is bad.

I seem to have dodged that fate. Things looked ok when I pulled the stator
out.

I'll do that!

Yeah, it looks that way.

And that's all that really matters... (Thanks

I appreciate the help.

- Jay

 

Had you checked all three legs ? Where were you measuring the
10V A.C. ? I recall once finding a single leg shorted out and being
able to repair the insulation on that one lead.