Topic: Testing CB750 K2 charging system

[AndyS]

I think that I am having trouble with the charging system on my CB750.
As usual my first port of call was the Clymer manual.
My problem is the very first test that they suggest;

"Locate the positive battery terminal and disconnect the red/white rectifier lead, and the red power lead. Connect both of these to the positive terminal of the ammeter.
Connect the positive terminal of the battery to the minus terminal of the ammeter with a wire. "

My problem is that my rectifier (and the wiring diagram in the manual confirms this) has only a red/white lead, three yellows and a green. I can't figure out what this red lead they are talking about is.
Can anyone shed any light on this, all I want to do is to check what sort of recharging I am getting at revs and in daylight and night time running conditions

[AJK]

Hi Andy,

I don't have a clymer manual, but i understand what they are wanting you to do. What they are testing is the average recharge current going back into the battery when the engine is running. You should find the red/white cable around your fuse box somewhere (perhaps behind it). One way to measure the charge current is to disconnect the red from the red/white and put the multimeter in between them (ensuring your multimeter is in amps/current mode and connected correctly). Then check what the current draw is against the book at say 4000 rpms. This is the average current that the rectifier is feeding back to the battery.

By all means you can run the above test, but it might be easier to start off with a general test instead by simply checking the voltage across the battery when running.
 Before starting this test, measure the voltage across your battery and make note of it with everything off. I.e. Just to get a snapshot of your battery voltage which should be ~ 12.5v if your battery is good.

With the multimeter still connected to the battery measuring the voltage, start the bike up & run it at the following revs and make note of the voltage you see at each rpm snapshot. If everything is working correctly you should have something like this.

1000 rpms, ~12v to 12.5v
2000 rpms. ~12.8 to 13.2v
3000 rpms. ~13.5v
4000 rpms and above. ~14.5v (then maxed out at 14.5v)

Open this pdf file
http://www.hondachopper.com/garage/troubleshooting_charging_system/troubleshooting_charging_system.pdf
and check out page 2

If your charging system is doing this, then you are probably ok. If not, then there is a screw you can play with on the regulator to correct it (assuming your system is ok). When checking at each rpms, the figures should be close to with 0.5v of that shown above, they don't have to be exact. Whats important is that the volts climb and max out at 14.5 volts over that rpm range.

Note: The above description only applies if your charging system is actaully working, but just out of regulation. If you have a problem with your charging system, like bad connections to the stator / field coils, bad diodes in the rectifier, etc. Then we will need to dig deeper to isolate the problem which will be another post.

Short riding runs on the bike can also take a lot out of the battery & sufficient ride time (say ~10 to 15mins) is needed to restore what you lost from the battery due to electric startup. The reason why bikes charge like this is to allow the bike to charge sufficiently for average riding, without overcharging (which kills batteries) or leaving you with a flat battery.

These bikes have very simple circuits, and with a bit of divide and conquer, you can get it charging correctly.

[AndyS]

Thanks AJK,
The charging system is certainly not working, and I have absolutely no recharge of the battery at any revs.
So it is now a case of working out why.
I had already made a copy of the Honda chopper how to guide, but was "doing the right thing" and using the workshop manual in the first instance.
The tests thus far seem to point to the stator or the alternator, so next step is put those on the bench, at least this will confirm if the units or the wiring is at fault.

Thanks again

[Vinhead1957]

The field wires are common problems. Check the bullet connections under the front sprocket cover and the spade connector to the rec/reg. The charging system regulates the power by turning the field on and off.   The ground is very important and a common weak spot check the green wire to frame ground.

[Vinhead1957]

Here is isolated diagram.   Corrosion is the enemy.   The insulators in the rectifier also are prone to breakdown but can be tested with a decent multimeter.

[strynboen]

dont use the starter.. vhen multimeter coppel in i a serie konneksion  i amp mode....it vill explode..or fuse blov...start motor use +100a..if you kick it vill work fine

[AndyS]

Thanks guys.
I was thinking along the same lines, stator/alternator or wiring. Wiring seems the most obvious, but less easy to pin point. Hence the reason to drop out the stator and alternator, bench test and rule them in or out.
The connections on this bike are all factory so 42 years old now, not surprising they are struggling, I only need to look in the mirror each morning to see first hand the affects of time

[AJK]

I agree with vinhead to first check for cruddy connections as likely cause. Some good things to rule out are field resistance and stator resistance. Then check the diodes in your rectifier.

Disconnect your battery for the following tests.

First, check the resistance across the green and white. This is your field coil resistance. It should be around 7.2 ohms. If this has a bad connection, or is broken internally (open circuited), then nothing in your charging system will work. The field coil is what drives the whole charging system.

Next, check the stator (3 yellow wires). Between any 2 yellow wires, it should read ~ 0.2 ohms (by the book). Mine actually read 1.2 ohms and my charging system system works just fine, so if its in the ballpark, thats ok. Say the 3 yellow wires are labelled 'A,'B' & 'C. Check combinations 'A' against 'B', then 'A' against 'C', then 'B' against 'C'. They should all read about the same against each other. If not, then your stator will likely need replacing or repairing. A break in any branch is no good.

Next, you can check all 6 diodes in your rectifier. Put your multimeter in diode mode and measure across each diode for a reading. They will only read in one direction. Check all 6 that you have readings for all 6. These things are pretty rugged, so i'd look connection/field/stator problems first.

Clean up any connections with wet'n'dry real good so its like new. Wipe over with metho. Do the inside female connectors as well with wet'n'dry on the end of a small screwdriver. In the end, it may just be cruddy connection(s).

cheers