Topic: 1978 CB750F Charging System Questions

[BrianShaw]

1978 CB750F Charging System

I need a little help. My bike does not seem to charge the battery.
Is this correct.

Testing the field coil (green and white) I should be getting 7.2 ohms. Is there a range I should be getting between?

Testing the stator (3 yellows) I should be getting .2 ohms. Should I be able to pick any 2 of the 3 wires and get the .2 ohms? again is there a range.

Testing the regulator volts, it only goes to 11.38.
but my readings for the field coil and stator are all over the map. So I think that is what I need to get working first.

Any help on this would be greatly appreciated.
Thanks,
Brian Shaw

[TwoTired]

Ohm readings should be made with the tested device disconnected from the circuit.

Tolerances are 10%.  Be sure to factor in meter tolerance and test lead resistance.

Testing the regulator volts, it only goes to 11.38.

Where are you measuring and under what conditions?

Cheers,

[BrianShaw]

Ok, the stator was removed from the bike and it still tested at very high ohms on one of the leads. Cut the connections and put new connections on at they range from .4 ohms to .8 ohms. Directly touching the wires on the stator.

Ohms accuracy states +- 1% of reading +5 digits.
So does that mean .2 ohms would read .7 ohms?

Is this good, or does the stator need to be replaced still?

Regulator was tested from idle to high rpm, no tach.
never goes over 11.5 even at high rpms.

This is all being done in the drive way.

[dtmmil]

Not sure, don't have any help on this, but if you need a stator I have one from my 78k.

[Spanner 1]

I think your stator is fine..... Honda says 0.2 ohms, I would say 40 years later 0.4 ohms is good !!
FYI the stator and field coil on the 750's very rarely go wrong, nor do the stock regulators.... the culprit is the rectifier IMO with 1 or 2 of its diodes either shorted-out or open circuit = same result 1/3 of the alternator giving 0 output = 11volts max. You can easily test the rectifier's 6 diodes with your meter....... do a search for a good explanation.... Good luck .

[skamagedon]

+1 on a diode in the trio dying. it would give 12v tops with 1 dead> if the bike runs fine without the headlight on, thats it

[BrianShaw]

I soldiered the connections for the stator. They are all pretty good now. Showing .4 to .6 ohms at the wiring connection. Given the age and the resistance in the meter itself, I think I am good. I was seeing numbers around 50 ohms on one of the leads prior to looking into the issue. The motorcycle seems to run fine write now, just not charging the battery. There is no way to turn off the head light except to unplug it. Has Hi/low, no off. I ordered a rectifier, just waiting for it to show up. I figure the stator wiring issues killed the rectifier.

Hopefully I should be good now.

[TwoTired]

You can turn off the headlight by removing the headlight fuse.

Have you tried the regulator bypass test?

Disconnect the white wire at the regulator.
Jumper that white wire directly to the battery POS terminal, and rev the bike.  What voltage do you see on the battery?

You MUST check the charging system with a known good and fully charged battery.  Otherwise, the voltage level will remain low until the battery has reached full charge, witch can take 10-15 hours using the bike's alternator.

[BrianShaw]

Regulator bypass test yields the same result as unplugging the regulator and/or the rectifier. Seeing 11.8 volts. Should the bike run with both of those unplugged?

Had the battery tested today and it tested good.

Is there a way to test the AC power coming from the stator?
Is it possible that the field coil is bad?

Is there anyone out there that would not mind talking to me on the phone about this.
I could really use the help.

[TwoTired]

You can't unplug the rectifier and expect the battery to charge.  It is in direct line with power from the stator to the battery, converting the Alternating Current the stator makes to DC power that the battery can use.

Has you stator been crashed/bashed/mashed?  If not it is really rare for it to go bad.
Fudging up the connection wires is pretty common.

[BrianShaw]

Nothing is smashed. All the ohm tests check out as well.

Is it possible to test the AC current from the stator?
Then test DC current coming out of the rectifier?

[Spanner 1]

So, with your meter connected across the battery and the bike running at idle thru' 5,000 rpm, the voltage reading stays at 11.8v , or starts-out lower and climbs to a max. of 11.8v ?( This is the very first test of a charging system to do on your bike)...... voltage stays 'static', climbs a little or goes to about 14.5v are the 3 possibilities...... seems like yours is not 'climbing' from 11.8v.
Still need to test the rectifier....... results ? Not knowing the condition of the rectifier will just delay the repair of the charging problem.
Honda does not give any specs. for AC voltage on the yellow wires, only DC voltage output to the battery. vs rpm.

[TwoTired]

Is it possible to test the AC current from the stator?

Of course.  What test equipment do you have available?

Then test DC current coming out of the rectifier?

Have you done the 12 diode tests of the rectifier? 
If you have a DMM, set it to the diode check selection.  Unplug the rectifier and test it separate from the bike. Record results of the following:
Probe each yellow to red, then reverse probe polarity and probe again.
Then probe each yellow to green, then reverse polarity and probe again.
A good diode will have a low reading ~ 200-500 ohms with one probe polarity, and infinite with the other probe polarity.  Read your meter manual to learn what you meter displays when resistance in infinite.  And, don't confuse it with zero resistance.
Note there are six diodes in the rectifier, hence 12 tests.

Measuring current require  breaking the circuit and inserting a current meter.
Somewhat easier, is to measure the voltages, to discern what the circuit is doing while all connected together.

With the circuit being tested all connected and in operational mode (alternator expected to make power), set your DMM to measure AC voltage.  With one probe on the green wire of the rectifier, measure each yellow wire from the stator at the rectifier connection.  You can expect 30 volts AC or more for a meter reading.

With the battery disconnected and the rectifier unplugged from the bike's wire harness, set your meter to measure ohms (Maybe you have a 200 Ω scale?).  Place one probe on the battery POS cable, and the other probe on the Red/wht wire terminal in the plug that connects the Rectifier.  Report measurement.
Then place one probe on the battery NEG cable, and the other probe on the Green wire terminal in the plug that connects the Rectifier. Report measurement.

Cheers,

[BrianShaw]

Meter I am using is a Innova 3320.
I changed the rectifier to 2 radio shack 276-1185. They are testing good with the correct ohms, wired correctly.

Red/white from rectifier to pos battery terminal is between 0 and .1 ohm
Green from rerctifier to negative Battery terminal is between 0 and .1 ohm
Field coil magnet test work, turned into a magnet with 12v battery power
Green voltage regulator connection to green connection on harness to field coil is between 0 and .1 ohm
White voltage regulator connection to white conection on harness to field coil is between 0 and .1 ohm



Background on the bike.
It was a wiring nightmare, replaced the wiring harness with one off ebay in good shape. Replaced all the male/female bullet style ends on it with new, did not replace the large plastic multi-connection ones but cleaned them. The original rectifier was bad, on of the diodes was bad.
Replace with a redio shack home build. There was horribly spliced wires coming from the stator. Cleaned that up and soldered good wires in place. Getting correct ohm readings from the stator.
Getting correct ohm readings from the field coil. The ignition coils  and plug wires were replaced as well.

All the readings I get from the battery indicate that it is not getting any current from the voltage regulator (or getting the same voltage as the battery?). Readings from the voltage regulator are the same as the battery. Disconnect the voltage regulator and the battery readings are the same.

Battery was tested good and it is at 100% charge before I begin tests. Although it does drop off during testing (not getting charged).

Meter I am using is a Innova 3320.

[TwoTired]

The Innova 3320 can measure up to 10A current.  Isn't your bike fused at 15A?  You'll have to be very careful what current branches you measure, or you'll be replacing internal fuses in the meter.

The voltage regulator does not feed the battery directly.  It senses the battery voltage and then controls the alternator field coil power to strengthen or weaken the alternator output so as not to overcook the battery.

Checkk the voltage on the black wire of the Vreg.  It should be withing .5 V of what the true battery voltage is, even with all the electrical/key switch on.
  With the battery in a low state, the White wire to the field coil should have full battery voltage on it, too.  The white wire voltage should only be less than battery voltage when the battery is fully charged (14.5v Max - 13.8 V).

Cheers,

[BrianShaw]

I am at a loss here.
What exactly charges the battery? I would like to test that wire.

I would think that it works this way.

black from battery to regulator, green/white regulator wire to field coil, field coil magnatizes, creates ac current to stator, stator 3 yellow wires go to rectifier, rectifier converts ac to dc, from the rectifier green goes to ground and red/white goes to the battery. Thus the red/white should be putting out dc power. What am I missing here.

If I test the red/white from rectifier and connect to a ground (not flowing back to the battery) I do not get a good current reading. I would think I should.

I am still at the same point I was a week ago with countless hours of testing.

[Spanner 1]

Brian, your charging description is correct..... one quick test, por favor. See what voltage is between the white wire and green wire on the regulator ( don't disconnect anything ) with the bike running. This would be the voltage (or lack of ) to the field coil. If ,say. 11v present then were back to the rectifier.... could certainly be bad, even with it being replaced. I'm still thinking it's the rectifier 

[BrianShaw]

11.8 volts from the regulator. Same as with the rectifier completely disconnected.
How do I check if the stator is bad?
Can a rotor go bad?

[Spanner 1]

You already confirmed that the stator read 0.2 ohms ( actually 0.4 ) any yellow to yellow, and that the field coil measured 7ohms, so a.o.k. there ( or did I read that wrong ? ). So, back to rectifier IMO. You could look for AC voltage from the alt. ( yel/yel/yel ), but if the rect. is bad then y/y/y may read 0v and the stator still be good !! You would have to have the field coil 'on' ( white/green ) and the 3 yellows disconnected from the rectifier to 'prove'  AC from the stator..... can't be done without snipping wires !!
Time to test the 'new' diodes, I think.

[TwoTired]

What exactly charges the battery?

The charging system.

I would like to test that wire.

You need two connection for current flow.
Power to charge the battery comes from the Rectifier's Green and Red/Wht wires.
You can measure voltage on these wires/connections by simply touching probes to them.
However, if you want to measure current, you must break a wire and insert an ammeter into the circuit.  And the ammeter must be capable of handling the current passing on the that wire or something is going to blow in the test equipment.

I would think that it works this way.

black from battery to regulator, green/white regulator wire to field coil, field coil magnatizes, creates ac current to stator, stator 3 yellow wires go to rectifier, rectifier converts ac to dc, from the rectifier green goes to ground and red/white goes to the battery. Thus the red/white should be putting out dc power. What am I missing here.

That's a reasonable working description for a layman's understanding.  (The inaccuracies aren't that important for these discussions.)

If I test the red/white from rectifier and connect to a ground (not flowing back to the battery) I do not get a good current reading. I would think I should.

 
The red/wht wire should be the same potential (voltage) as the Battery POS terminal)
You cannot measure current via a parallel test connection.  Current is measured with test equipment inserted in series.
The same current that flows in the Red/Wht to battery POS connection will be the same current magnitude that flows in the Green to Battery NEG terminal connection, just opposite directions.  (Electrons flow in a loop, regarding these DC circuits.)

I am still at the same point I was a week ago with countless hours of testing.

Do you understand the difference between testing current and testing voltage?  The terms are important.
Do you understand ohm's law?  I = E/R
If you have skipped the fundamentals of electricity, that would explain why you are getting frustrated.  I just can't tell at this point.

Cheers,

[bollingball]

Brian It is good to collect info. from everyone. However if you find someone you trust to be smart it may be better to PM that person so you don't get sidetracked. Some times it gets confusing with five or six people coming from different directions. I know everyone means well. Just a thought.

[BrianShaw]

I wish I knew who to trust and who would want to spend the time.

The diode test checks out. First 6 tests are all about 4.7 - 4.9 second 6 tests are 0 on my meter.

So as far as I can tell it is a problem with the stator/field coil.
Even though when I test it now it is at .2 ohms from the white wire connection point. And the field coil does magnetize when 12v are applied to it.

Or maybe it is the voltage regulator only seeing about 11.8v come across that.

[BrianShaw]

Can a new stator even be purchased?

[Spanner 1]

Wait now !... the field coil needs to read very close to 7ohms to be good.... you say 0.2ohms white wire to? (green or ... ). Clarify ; test field coil = white and green wires (disconnected from the reg.) must read 7ohms or very close ( meter set on lowest 'ohms' scale ) Stator = 0.2ohms, any yellow to any yellow.( big plug disconnected).

And, about your diode test, it's not reliable to try test a diode without applying a voltage to it... so that's why your meter has a diode setting... with the 'beep' function ... beep one way , silent the other way... must use that.

[BrianShaw]

No that is the 3 yellow wires from the stator, outside the case. the (white) plastic 6 wire connection point.