I went to Oregon Motorcyle Parts and ran the following tests. Seems like a bad regulator as the bike will not charge when rpm increase to 4000.
I still don't understand the resistance values from white to green wires at 8 prong plug? Charging system diagnostics for 1969 ~1978 Honda SOHC Fours.
An analog meter is preferred but digital meter is OK for these tests but you'll have to take into consideration the static resistance
in the meter. This article assumes that you have basic proficiency with an electrical multi-meter.
Start by removing the left side cover and unplug the 8 or (9 pin on some models) plug from the alternator windings to the wiring
harness. Set the meter to the lowest resistance setting, Rx1 on an analog meter or 0 to 200 ohm scale (sometimes only labeled
200) on a digital meter. On the engine side test the resistance between the yellow wires in all combinations. You should get .5
to 1.2 ohms resistance here. 0.7 plus or minus a couple tenths
Next check from the yellow wire to ground and you should get infinite resistance (or the same
resistance as air.) Infinite
Next check the resistance from the white to the green wire. The spec here is 4 to 6 ohms but I've found that a little out either
way is OK but more than 1 ohm out is usually bad. 0.7 sounds bad??? Next test either the white or green wire to ground and you should get
infinite resistance. Infinite The green wire plugs into a ground connection but when it's not connected to the harness it shouldn't be
grounded when unplugged.
If all this checks out, move to the wiring harness side of the plug and Test the green wire to ground. You should get no
detectable resistance here or in other words: the same resistance as when touching the meter probes together. If this is all good,
plug the 8 (or 9 pin on some bikes) plug back together. Good
Now set the meter to DC volts and unplug the voltage regulator. Turn on the main ignition switch but set the handlebar switch
to off. Check the voltage from positive to negative across the battery 13.0 V then check the voltage from the black wire that would
have plugged in to the voltage regulator to ground. 12.9 VIf there is less at the black wire than at the battery, you have a wiring
problem. This can be tested by making up a fused jumper wire to run from the positive side of the battery to the black wire on
the regulator. Then run the bike and check the voltage output.
Assuming the power to the regulator OK: plug everything back together; turn on the main switch and the handlebar switch off.
Either hand a paper clip from string or use a .002 feeler gage blade and lay it up against the end of the alternator case (where
the screws are that hold the field coil are) then pull it away. The magnetism of field coil should bend the gage or hold the paper
clip when you try pull it away. This test proves that at least the regulator is working a little. Yes good magnetism
Now test the rectifier as per article #3 on my FAQ page.
How do I to test a rectifier?
Start with one lead of the meter (or test light) to the positive lead on the rectifier. Touch the other test lead one at a time to each of the
AC terminals of the rectifier. At this point you will either have continuity or not but it should be the same with all the AC terminals on
the rectifier.
Swap the test leads (still working with the positive terminal of the rectifier) and repeat the test. This test should have the opposite
result as the previous test. Again the result should be the same for each of the AC leads.
Move on to the negative lead off the rectifier and repeat the 2 previous tests. This is test is easier with a test light than with an
electrical meter. The point is to check that power flows one way but not the other and the exact numbers isn’t as important. If your
meter has a buzzer for continuity, this works very well too.
This will catch a bad rectifier 95% of the time. The rest of the time they only fail under load and will usually get pretty hot.
Rectifier seems good
Set the meter back to the lowest resistance setting and test the resistance from the white wire to ground. You should get the
same reading here as before when you did the test from white to green at the plug. 0.7 sounds bad??? Any discrepancy here is a wiring problem.
Unplug the rectifier and check the resistance between the yellow wire and once again you should get the same resistance as at
the alternator plug. 0.7 plus or minus a couple tenths
If this is all OK then remove the ground wire from the battery then test the resistance between the red wire where the rectifier
plugs into the positive battery cable. 1.9 ? Is this high? meter set at 20M Then test from the red wire to the rectifier to the red wire at the ignition switch. There
should be no detectable losses here. 2.0?
Since the ignition switch is unplugged, test the resistance from red to black wires in the on position. Any detectable resistance
here is bad. 2.1?
If you resolve all this and you bike still doesn't charge correctly, there is one more thing to do to prove the problem is in the
voltage regulator. Start by running the bike until it is warmed up enough to idle properly. Use a fused jumper wire and jump
power from the positive side of the batter directly to the white wire that plugs into the regulator. This will give the bike full
charging all the time so do not run the engine over 1500 RPM or you risk boiling the battery and blowing bulbs. With the engine
running test the voltage output at the rectifier to ground with the engine running at idle and 1200 RPM. If you get good charging,
you need a new voltage regulator. Increased battery voltage from 11.9 to 13.8V Also does the same when I jumper white and black
regulator wires together and raise the rpm. remove the jumper and no charge at 4000 rpm. Bad Regulator???
