alternator not charging DC

[bavareze]

Hello everybody.

I had this 91 K75 for over 7 years.

This is what happened recently:  the battery/charging warning lamp remained ON when the ignition is turned off.  Then, when I turn the ignition ON , the lamp is off (normally, it is supposed to turn on).  Then when i start the engine, it remains off but no charging


I assumed it was the voltage regulator (the piece that comes with the brushes) so I replaced that but the behavior remains the same.

What i noticed is that, with the ignition OFF, when I remove the 2 wire plug from the alternator, the charging warning lamp turns off.  But then obviously no charging at all.

Any tips? 

Anybody with a diagram that could help me understand what happens?

Thanks!

[johnny]

longshot here...

http://www.motobrick.com/index.php/topic,7600.0.html

i think it is crud inside your cluster... butts im an idiot savant...

im betting rbm and ngk and scott_ will know what the problem is...

j o

[rbm]

What's happening:

An alternator will not generate an output current without an input field current, but there will be no field current until there is an alternator output! To solve this chicken-or-egg dilemma, alternators require a temporary source of field current called the excite current.

When the ignition switch is turned on, current flows from the battery, through the charge lamp to the alternator auxiliary terminal, and then through the field coil. As soon as the engine starts, however, the alternator begins generating current, which then makes the voltage at the auxiliary terminal 0.5 to 1.0 volt higher than the battery voltage. With only 0.5 to 1.0 volt across its terminals, the 12-volt charge-indicator lamp goes out. If it stays on or comes on again, you know that the alternator in not producing current.

Your problem:
Possibility 1: There is a problem with the ignition switch.  Maybe cleaning it will help.   The charge light should only get power with the kill switch normal and the ignition on.  Check to see if the choke light comes on with the ignition off; that would verify power on Pin 6 of the cluster which is normally off unless the ignition is on.
Possibility 2: Might be a blown diode on the diode board.  Except for Possibility 1, the only way the charge light can come on with the ignition off is for current to leak through the diode board on the alternator.  This is assuming that no aftermarket mods or devices are attached to your motorcycle.  Is it completely stock?

Is it possible to get ahold of an oscilloscope to probe the alternator output?  You're looking for AC voltage on the output of the alternator; there should be none.  Another alternative is to use your multimeter on AC voltage range to measure the AC component of the alternator output.  If it is high, you are probably looking at a blown diode, probably shorted.

An alternative is to disconnect the alternator and bring it to an automotive repair center that specializes in automotive charging systems to get it tested.

Diagram and explanation:
http://www.buchanan1.net/charge.html

[bavareze]

thanks for this explanation.

my bike is stock, no mods at all.

I just replaced this piece I bought off ebay

http://www.ebay.com/itm/321845220760?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

to me it looks like this is an assembly that has brushes as well as the rectifier diodes.  Correct me if that is not the case.  But if this is the case, then the blown diode could not be the source of the problem, as they can't be both bad.

I see the plug connecting to alternator has 2 wires, one small one big.  I assume the small one is the field, so I if I start the bike, field it directly from the battery and then the alternator works, it must be something wrong in the dash/ignition switch area.

What do you think?

[rbm]

That's the regulator assembly you've purchased.  The diode board (rectifier) is a separate FRU inside the alternator.


The large connector is the B+ connector (the output that charges the battery) and the small connector is the D+ (the field windings).  D+ should have a Blue wire, B+ should have a large red wire.  If you can
1) disconnect the harness plug from the alternator
2) temporarily connect the small tab to the battery's positive terminal through a 120 Ohm, 3Watt resistor to simulate a charge light (very important)
3) temporarily connect the B+ to the battery positive terminal with a large gauge wire
4) start the bike and measure the battery voltage at >2500 RPM to see if the alternator is outputting 14V or so.

If this test works, then the problem lies in the dash/ignition switch area.

[bavareze]

ok, i'll do that tomorrow.

now here's another question, just for my knowledge:  what happens if I field the alternator directly to +12V, without a resistor?  Will that burn the Voltage regulator as the current from D+ to D- would be to high?

[rbm]

Yes, probably.  The charge light limits the field current.  If you eliminate that resistance, the current  to the field coil will not be limited.

BTW, this is what's built into that regulator with the brushes.
http://retro.co.za/rsre/boschvreg.html

[bavareze]

so i replaced the whole alternator off ebay - cost $40

everything back to normal!

thanks for your support!

[Gio]

Another fine example of great help (and good info / education about how things work) from one 'bricker to another - johnny, please send rbm another medal for his collection!

[johnny]

rodger that...

i been sending him weed butter cookies... notice how he is unintelligible for a few days... then he is superstar for a few days...

just goes to show you that weed butter cookies will make even canadians supernatural...

j o

[Gio]

Indeed. As a fellow Canadian, and for purely scientific reasons, I would request that you send me some also so we can test your theory ...

Cheers!

[rbm]

Thanks for the weed butter cookies Joohnny.  I guess I shouldn't have smothered them in that maple spread, topped with back bacon.  The salt/sugar fix send my brain into a tail spin and the hallucinogenic properties of weed butter just put me over the edge. Never go online while in a weed butter craze; you see the results.