Topic: Really stumped on charging/battery issue...CB750K7

[seanbarney41]

So I have done all the tests to the best of my ability and there has to be something I am not grasping.  Bear with me.  Not very good with electrical.  I don't understand how to check the rectifier.  Everything tested ok but here is what is going on.  I have 2 different batteries  have been working with.  Both are old and used but work well in my other bikes.  One fully charged reads 12.6 at rest.  As soon as the key is switched on it drops about a full volt and then continues to drop rapidly!...2 o 3 tenths of a volt per second, even when I pull the headlight fuse.  Start the bike and voltage drain slows but does not stop even when revved up.  Also, no magnetism at the 3 screws in the alternator cover when key switched on.  I have switched out the regulator, alternator, and rectifier in that order, with suspected good used parts.  Nothing made any difference.  I also cleaned all the connections in the charging circuits and the harness and connectors all look to be in good shape.

Ordinarily, I would suspect bad battery here but these batteries work ok in a K4.

To me, the bike acts like something is shorted, except that most everything works properly (except the horn and the right turn signal does not blink...typical) and it is not blowing fuses.  So I thought maybe the alternator was shorted even though resistance and continuity tests looked ok.

[scottly]

Also, no magnetism at the 3 screws in the alternator cover when key switched on.

Try jumping the white wire at the regulator directly to battery +. If still no magnetism, disconnect the white wire from the reg and measure the resistance from the white wire to battery -; it should be about 7 ohms.

[seanbarney41]

Also, no magnetism at the 3 screws in the alternator cover when key switched on.

Try jumping the white wire at the regulator directly to battery +. If still no magnetism, disconnect the white wire from the reg and measure the resistance from the white wire to battery -; it should be about 7 ohms.

that gets me magnetism!

[seanbarney41]

Also, noticing the jumper sparks a little on initial contact and the green coil on the back of the reg starts getting hot even before key switched on

[seanbarney41]

Resistance between white wire and batt - IS exactly 7 ohms

[scottly]

Measure the voltage at the black reg wire with the key on; should be close to the battery voltage. (with the jumper removed)

[seanbarney41]

Its about .75 volts lower than battery voltage which is down to 7 volts with key on!

[scottly]

You need to charge the battery back up before further testing, but your measurements sound mostly ok. With a charged battery, key on, not running, check for magnetism. If good, start bike and check again.

[scottly]

I don't understand how to check the rectifier.

Read reply #331 on this thread:
http://forums.sohc4.net/index.php/topic,32093.325.html

[seanbarney41]

Ok, 3rd battery, newest of the bunch and has been working well for at least a year.   Very weak magnetism at the 3 screws.  12.9 at rest, 11.9 key on, 11.6 running and does not rise at 2500, 3500, 4500 rpm.

[bryanj]

Sounds to me like crudded up connectors behind the sprocket cover going to generator. These go loose then hot and soft and then fail

[scottly]

Very weak magnetism at the 3 screws.

What is the voltage on the white reg wire key on, not running? Key on and running?

[seanbarney41]

Sounds to me like crudded up connectors behind the sprocket cover going to generator. These go loose then hot and soft and then fail

i initially suspected this as well.  Connections on first stator were thoroughly cleaned and inspected.  Second alternator has brand new connectors crimped on by me connected to previously cleaned harness.

[scottly]

Sounds to me like crudded up connectors behind the sprocket cover going to generator. These go loose then hot and soft and then fail

i initially suspected this as well.  Connections on first stator were thoroughly cleaned and inspected.  Second alternator has brand new connectors crimped on by me connected to previously cleaned harness.

The 7 ohms from the white reg wire to battery - included those connections for the field coil, which is responsible for the magnetism.

[seanbarney41]

Both rectifiers used now test out after reading Scottly's link. ...meter reads  .0L in one direction and from .453 to .488 in other direction

[seanbarney41]

Very weak magnetism at the 3 screws.

What is the voltage on the white reg wire key on, not running? Key on and running?

   .031 both key on and running

[seanbarney41]

Doesnt seem to be critical but, just noticed the rear brake switch has been stuck on for god knows how long, when i diengaged it, the volts at white reg wire went from .052 to the above reported .031.  With the brake light off, battery voltage does climb slightly when the engine is revved.

[scottly]

Very weak magnetism at the 3 screws.

What is the voltage on the white reg wire key on, not running? Key on and running?

   .031 both key on and running

That's waay too low; should be the same as the black reg wire, key on, not running.
Try disconnecting the white reg wire, jump it to battery +, start the bike, and check for charging.

[seanbarney41]

Scottly, trying to jumper this up and getting lots of sparking at the white wire.   Is this normal?

[scottly]

With the white disconnected from the reg, there shouldn't be a lot of arcing, but some. The current drawn by the 7 ohm field coil is only about 2 amps.

[bryanj]

Try checking ohms value white to green, then disconnect green and try white to ground  BOTH WITH POWER OFF

[seanbarney41]

Well the arcing is pretty severe.  With the bike running, that setup is overcharging, seeing spikes up to 16 volts at around 2000

[scottly]

Resistance between white wire and batt - IS exactly 7 ohms

[bryanj]

If it charges like that with the white connected to battery with engine running there is no problem with charging circuit but you have a wiring/connection problem in the loom between battery positive and regulator black.
This could be ignition switch or any of the bullet connections, it is also possible the points in the regulator are corroded

[scottly]

Well the arcing is pretty severe.  With the bike running, that setup is overcharging, seeing spikes up to 16 volts at around 2000

Does the field still measure 7 ohms?
The jumper bypasses the reg, turning the charging system on full. This means the field, stator, and rectifier and their associated wiring are OK.