Diagnostic information and technical articles
Diagnostics & Technical Articles
About OMP
I've written all these articles.  I think I've done my best but I'm open to suggestions to make them better and I'm also open to putting up original articles from my customers.  No copyrighted material please...
1 - Where to start if you have charging system problems.

Before you can suspect any charging system component of failing you first should check all the easy stuff.  First make sure all the cells in the battery have electrolyte up to the full line.  Use a hydrometer (a fancy name for a tube with floating balls in it) and check all the cells after the battery has been charged.  You can get a hydrometer for only a few dollars at most bike shops.  The ones made for cars are way to big to use on 99% of bike batteries.  Once you are
sure you have a good battery...

Next check all the connectors. The battery cables and all the other wiring connections should all be clean and tight.  A weak or dirty connection can cause poor charging system output.  A melted plug is also a sign of a loose or dirty connector.  The heat comes from the resistance of completing the circuit through oxide (or oil and dirt) on the surface of the terminals.  If you use spray cleaner to flush out any plugs check for signs that the plug is being damaged from the solvent and discontinue using it if there is.  Also use an air nozzle (or caned air) to blow out the connectors after cleaning them.  Some of the spray lubes like
WD-40 are good for cleaning out connectors but I do not endorse them as lubricants. Again check to make sure the spray lube is not damaging the plastic plugs or insulators.

Bikes with mechanical voltage regulators may also need to have the points in the regulator cleaned or better yet replace it with an all electronic voltage regulator.

If you still have a charging system problem after this, then start working your way through the charging system with an electrical meter as described in the repair manual for your bike.  If something doesn't make sense, ask me and I'll try to make it clearer for you.
2 - Do I need an expensive test meter to figure out what's wrong with my bike?

Good test meters are no longer expensive and there's really no good reason to not have one.  I use an analog meter from Amazon that costs around $14 and while I have several meters, I go with this one first.  If you can't find it, send me and eMail and I'll send you a link to it.  Analog meters are far more useful for bike stuff and it's worth the few extra minutes to learn how to use one.

For testing rectifiers, a small battery and a light bulb wired together will give you an adequate test light.  So long as the light goes on when you touch the ends of the leads together, it'll work for testing connections and rectifiers.  Back in the bad old days when evertyhing had ignition points, a battery powered test light was a very good way to set ignition timing.  Before LED flashlights, it was common to make one by wiring test leads into a cheap flashlight.  The same type of thing was more than $50 from Snap On...
3 - 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. If the rectifier fails all tests with a digital meter, it may not be the rectifier but that you're using the meter set on the wrong scale. Set the meter to the diode test function and test it again.  If your meter doesn't have this, it's time for a new meter. 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.
4 - Charging system diagnostics for older Honda singles and twins ( mostly Pre-1978)

First off, read the above articles.  These things all need to be dealt with when you have a charging system problem.

There are 3 wires from the stator and depending on the year of your bike they will  be different colors.  From 1969 on the colors were pink, yellow and white.  The pink wire connects to a full charging coil, the yellow to a 2/3rds coil and the white wire to a 1/3rd coil.  The yellow and white wires connect in the headlight switch when the lights are turned on.  What this means to you is that the resistance from pink to white will be a little less than pink to yellow.  You should get around 3/4 to 1 ohm from yellow to pink and somewhat less from pink to white.  None of these 3  wires should have continuity to ground.  Check the stator with an ohm meter and if it passes these test then move on to checking the connections.

The white and yellow wires from the split coils are connected together (in the headlight switch) when the headlight is turned on directing more power into the charging system.   If the switch fails the charging system will not give full output.  To  make this more confusing, most pre-1969 Hondas use the same system but with different wire colors from year to year and sometimes model to model.  On these bikes the brown wire usually represents the full charging coil and yellow and pink wires are from the split charging coil.  The best way to be sure is to check the wiring diagram in a repair manual for your bike. The best fix is a new switch but they are usually no longer available.  Some people bypass this and connect the white and yellow wires together at the rectifier giving full charging all the time.  This is only a good idea if you ride with the headlight on all the time. 

5 - Coping with a weak charging system
(like on pre-1979 Honda singles and twins and T-series Suzuki 250cc and larger twins.)

Even with a properly functioning charging system, many older bikes with a balanced charging system will have a battery low enough to where the electric starter will fail to start the engine in a week or so of regular riding with the lights on.  The problem is made worse by installing a headlight that is actually bright enough to actually see where you're going. Replacing the selenium rectifier with silicon rectifier will help but not solve the problem entirely. Selenium is an inefficient semiconductor and has a high resistance when compared with silicon.  You can take advantage of this and gain a few percent in charging system output. All of the Oregon Motorcycle Parts rectifiers use silicon semiconductors.

The obvious solution is to make the alternator produce more power.  However this is much more difficult than it sounds.  There used to be several places that would Rewind the stator to produce more power but then a regulator/rectifier unit like our VRRPM2 is mandatory not optional.  The problems with rewinding is is very involved if you want to do it yourself.  The information is out there but I've never done it.  If you go with sending it out to be rewound, there's very few that do this
anymore and it's expensive when you find someone that does it.

The better and cheaper solution is to reduce the power draw as much as possible and use a larger battery if there is space for it.  Using a larger battery takes some research but often there is a taller battery with the same length and width and then you just have to make sure it clears the bracket and whatever is over the top.  This isn't always an option but sometimes it really works out.

Swapping out the tail and signal bulbs with LEDs will save some power but the voltage regulators on these old bikes weren't very good and without installing a regulator/rectifier unit like our VRRPM2 to protect them, LED lights won't last long. 

Other options include using a low wattage headlight bulb in the daytime.  The low wattage bulb trick was a cheap and popular solution back in the days.  It involved drilling a hole in the headlight reflector and then using a die grinder (a Dremmel tool will work) and make an opening large enough for a small light bulb socket.  Usually a pop-in one from either an old car turn signal or dash light then wiring it to a hidden toggle switch on the back of the headlight shell.

Another approach was to mount a small light under the regular headlight.  Some 1970 and older Honda cars had a small rectangular back-up light.  One of these mounted to the under the regular headlight does not look too out of place.  Another advantage is that with this set up a halogen light can be installed in the sock headlight shell for better night vision.  If you go this route, most states have some regulation on the distance a headlight must be visible.  A turn signal bulb meets the minimum requirement without a problem in most states.  For some people using a trickle charger once in a while or at night is a cheap and easy solution.

In the end your bike just might not make enough power for all the accessories you'd like to put on them and you will have to decide which ones are the most important to you.

I considered carrying LED lights here at  OregonMotorcycleParts.com but they're so plentiful and cheap on eBay I felt it wasn't worth my effort.  Needless to say I've bought a bunch of them to play with and someday I may put up and article about this.  There are now LED headlights that are brighter than the best conventional headlights and draw less power too.  If you have upgraded your voltage regulator, one of these would be worth considering.
6 - All of the components in the charging system on my bike test out OK but it still doesn't charge. 

This is almost always caused by dirty, corroded or weak terminal connections. Corrosion on the terminals acts as a resister dropping current flow and creating
heat.  The heat often ends up melting the plugs making the problem even worse.  The only fix is to clean all the connections and coat the terminals with
dielectric grease to prevent them from corroding again.
7 - Diagnostics for 1979~1984 Honda 750~1100 DOHC fours (and SOHC CB650)

The following information also applies to the CB550 Nighthawk but the resistance on the field coil (black to white wires) is lower.  Honda has released an updated stator/field coil assembly for this bike (this is the only bike I know that uses this arrangement) and I  recommend calling the Honda dealer for this information as the manual may be wrong.

The first test is to test the voltage between the red wire on the rectifier / voltage regulator unit (do not unplug it for this test) and ground with the bike running.  If you're only getting battery voltage then you have a problem.  If you're getting over 15 volts then the regulator might be bad or there is a wiring problem (see article 9 if you have this problem.)

At first all those wires look rather intimidating but once you break it down it's rather simple.  Unplug both of the connectors and use the rectifier test procedure above to test the rectifier part of the unit.  The red wire is positive and the green wire is negative and  test them against the 3 yellow AC leads. 

The next thing is to eliminate the voltage regulator as the problem.  With out an expensive regulator testing, the best next thing to do is to by-pass the regulator to see if the bike charges.  Unplug the connector from the reg/rec unit to the alternator and release the white wire from the alternator side plug.  Warm up the bike so it will idle.  With the bike idling, ground the alternator side of the white wire.  This should cause the alternator to give full output.  Have a volt meter connected to the red/white wire from the reg/rec unit to ground and you should see full charging (14.5V) around 1200 RPM.  If it charges this way but not under normal use, the reg/rec unit is bad.  If it doesn't charge with this test, either the black wire to the reg/rec unit isn't delivering power to the reg/ rec unit, or there is something wrong under the alternator cover

The next thing to check is the resistance on the rotor.  Remove the alternator cover and check the resistance between copper rings on the rotor.  There should be 4.5 ~6 ohms resistance if it's good.  Most of the time when rotors are bad there will be either infinite resistance (completely burnt out) or less than 2 ohms resistance (internally shorted.) Either way you're looking at getting a new rotor or having the rotor re wound.  Even if the rotor test good, is worth cleaning tarnish off the slip rings with something like a Scotchbright pad.

If the rotor checks out good the next step is to check the resistance between the black and white wires to the brushes. Wiggle and tug slightly on these wires during this test.  If the resistance changes at all during this test or is different than the resistance between the copper rings on the rotor, the wires leading to the brushes (black and white wires) will need to be replaced.  The wires to the brushes going bad is more common than one would think.

The next thing to check is stator.  There should be about .5 to 1 ohm resistance between the yellow wires from the stator and infinite to ground if the unit is good.

I've chosen not to deal in replacement stators and rotors but if you need a source for them, I know a place that does very good rewind work that I can refer you to..
8 -  Charging system diagnostics for 1969 ~1978 Honda in-line 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.  Next check from the yellow wire to ground and you should get infinite resistance (or the same resistance as air.)

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.  Next test either the white or green wire to ground and you should get infinite resistance.  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.

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 then check the voltage from the black wire that would have plugged in to the voltage regulator to ground.  If 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.

Now test the rectifier as per article #3 on my FAQ page.

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.  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.

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.  Then test from the red wire to the rectifier to the red wire at the ignition switch. There should be no detectable losses here.

Since the ignition switch is unplugged, test the resistance from red to black wires in the on position.  Any detectable resistance here is bad.

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.
9 - Over Charging on bikes with an electromagnet alternator

The problem is usually caused by a bad ignition switch or other weak or burnt connection between the rectifier output and the voltage regulator power input.

The way it works is the voltage is sensed on the wire that feeds power to the regulator but the power output is on the wire from the rectifier.  This is usually the main switched power lead but on some bikes have a separate wire from the fuse box to feed power to the voltage regulator.

These 2 wires connect when the ignition switch is turned on and if the switched power wire has low voltage, the regulator boost the output until it gets what it's set to produce. 

The way to test this is to check the voltage output from the main power wire from the main switch to ground.  If the voltage here is within spec but is higher directly off the rectifier to ground, it's a good indication there's a problem in the switch or other connectors.  It's also a good idea to unplug the main switch, turn it to the on position and then check the resistance from the where power goes in to the main power wire out.  A meter on the lowest setting shouldn't be able to detect any resistance here.

On Hondas the main switched power lead is black.  Yamaha and Kawasaki use a brown wire for this and Suzuki uses an orange wire
Don't hesitate to eMail me for tech support at  Tony@oregonmotorcycleparts.com

I can also be reached though oregonmotorcycleparts@gmail.com or from our Facebook page but eMails.