• By
  • On 7th July 2016

Here is a quick guide to troubleshooting your HONDA CB550 (or CB500, CB750, etc.) electrical system and testing all of the system’s components with regard to the question “Why is my battery not charging?“.  While this guide may be of interest to all curious readers who want to find out how to test alternators, voltage regulators and rectifiers, it is intended to help those who have a battery that is not charging and who have ensured that all 3 of the following conditions apply: 1) the correct type and size of battery is installed and 2) all original electrical system’s components are present, and 3) the battery is not charging when the bike is running.

REQUIRED TOOLS: A multi-meter (voltmeter, ammeter, ohmmeter).



Use your voltmeter and check the battery’s voltage.  A good battery should read 12.6 V or more.  If the voltage is less than 12.6 V,  charge the battery using the correct battery charger.

If the battery is not charging, skip the rest of this post and get a new battery.

(It may be possible to “rescue” the battery.  Check the fluid/acid level. If a little low and the battery is relatively new, say a few months old, top up to the correct level using distilled water and try charging the battery again.  If the level is too low, say half way, and the battery is about a year old or older, “rescue” is unlikely.)

If the battery is charging properly when connected to a battery charger, skip to the CHECK THE WIRING section.

If you don’t have a battery charger, you may want to operate the horn (or headlight in hi beam) for about 30 seconds.  The horn will sound loud (the headlight will not fade noticeably), if the battery is good.   Repeat that a day later.  A good battery will operate the horn (or headlight) since it will re-charge itself to some degree overnight.  A bad battery will not.


Start the bike and let it idle.  Connect the voltmeter to the battery’s terminals.  It should read around 12 V, more if the battery is fully charged.  Increase the RPM.  The voltage should gradually increase to 14 V or slightly more at 4,000 RPM and above.  Switch the multi-meter to AMP mode.  At idle, the reading should be around 6 A and that reading should decrease as the RPM increase going to about 1 A at 6,000-7,000 RPM.  NOTE: If the headlight is on, the initial reading at idle speed may be 2-3 A.

If both the voltage and amperage readings are as given above, the electrical system is functioning normally.  If the battery is not charging, skip the rest of this post and get a new battery.

If the readings are not as shown above, read on.


Inspect all the wiring connecting the battery, the voltage regulator, the alternator and the rectifier to the extent possible.  You’re right. That’s much easier said than done.  A good deal of it is not easily visible, let alone accessible.  Repair or replace any cut, broken, worn out, fraying or damaged wires.  Check all connectors and ensure all terminals are intact and clean. The idea here is to make sure all components of the electrical system have the means to “communicate” with each other via good clean electrical wires and connections. Perform the CHARGING TEST again.  90% of charging issues could probably be solved after checking the battery, the wiring and all connections and connectors.  If the problem persists, read on.


As you may recall from the ELECTRICAL SYSTEM BASICS post, the alternator generates electricity by means of a magnetized coil (the field coil) rotating inside a stationary coil (the stator coil). The coils, as their name implies, consist of long copper wires coiled around some piece of metal sometimes in strange shapes.  The longer the copper wire the greater its resistance and we can measure that.

Disconnect the wiring harness that comes from the alternator at the connector located at the electrical box behind the left side cover.  You will know it is the correct one because there will be 3 yellow wires, a white wire, a green wire and a couple of other wires in it.  In the image below, the purple arrow points to it:

Electrical Schematics Charging System HONDA CB550F

Connect one of the leads of the ohmmeter to the green wire and the other to the white wire.  (You’ll recall that the electrical power that magnetizes the field coil arrives from the voltage regulator via the green and white wires).  You will be measuring the resistance of the field coil.  It should be around 5 Ω.  The original HONDA specification is 4.9 Ω +/- 10%.  If the reading is 6 Ω, there is no need to panic.  The measured resistance is probably greater than the actual resistance since the connections between the terminals in the connector and your ohmmeter are not perfect and some additional resistance is coming from the wires. If the resistance is infinite or extremely low, somethings is wrong.  You might want to disassemble the alternator assembly and see if the resistance measured at the field coil itself is within spec.  Maybe there was a bad connection or a wire was shorted when you took the readings at the connector?  Clean and repair the wires and connections as needed. If the reading is still out of spec, replace the field coil.  It’s very easy to do.

Now, connect one of the leads of the ohmmeter to any yellow wire in the connector and the other lead to another yellow wire. Do all possible combinations.  OK, let’s make this a little clearer.  Let’s call the three yellow wires Y1, Y2 and Y3.  Connect one of the leads of the ohmmeter to Y1 and the other to Y2.  Record the reading. Keep Y1 connected, but now connect the other lead to Y3.  Record the reading. Disconnect the lead from Y1 and connect it to Y2.  Keep Y3 connected where it is.  Record the reading.  That’s all.  All readings should be around 0.4 Ω.  HONDA’s specification is 0.35 Ω +/- 10%. I normally get readings around 0.5 to 0.6 Ω and I am not worried.  The higher reading is, again, due to imperfect connections between the ohmmeter and the terminals in the connector, and hence some additional resistance. If the resistance is infinite or extremely low, somethings is wrong.  You might want to disassemble the alternator assembly and see if the resistance measured at the stator coil itself is within specification. Maybe there was a bad connection or a wire was shorted when you took the readings at the connector?   If the reading is still out of spec, replace the stator coil.  It’s very easy to do.

Perform the charging test again.  If there is very low or no electrical current generated by the alternator, it is possible that the voltage regulator is not sending any power to the field coil, which in turn cannot get magnetized and hence no electricity is generated.


Where is this thing? Just follow the white and green wires and they’ll lead you to it.  In the image below, it is the gizmo on the left with the black-white-and-green sticker on it.

Voltage regulator identified

Disconnect the black white and green wires, undo the bolts/screws that fasten the voltage regulator to the box and remove it.  Then undo the screws holding the cover and remove it.  You should see this:

Voltage regulator with cover removed

There are two gaps that need to be checked, cleaned and adjusted, if necessary.  Refer to the picture above and check the core gap.  It should be 0.6 to 1.0 mm.  Adjust if necessary.  Check the point gap.  It should be 0.2 mm.  If the points are pitted, clean them and polish with 600 grid sandpaper prior to adjustment.

You can verify that the voltage regulator sends electrical current to the filed coil by applying a 12 V electrical current (directly from the battery, for example) to the black (+) and green (-) wires.  A voltmeter connected to white (+) and green (-) should indicate that electrical current is coming out of the voltage regulator.  Depending on the amount of current going in, the secondary resistance circuit will remain open or will close and you will see that if the cover is off.  If no current comes out of the voltage regulator, replace it.

Perform the charging test.


If the my-battery-is-not-charging problem has not been solved yet,… something must have gone wrong while performing all previous checks, inspections and adjustments described above. You might as well start over.  Chances that the rectifier has failed are so slim indeed.  Rectifiers have no moving parts and are so simple in principle (consisting only of a few diodes) and in construction that the likelihood of them failing is next to nothing.  Check for physical damage, broken connections, broken ribs in the housing, anything that would indicate the problem is on the outside.  It is located at the bottom of the electrical box behind the left side cover.

Electricla box with rectifier identified

If you must test the rectifier, disconnect the rectifier connector.  There will be 5 wires: three yellow, a red/white and a green.

To test it, first take a look at the rectifier’s schematics and try to remember how it works.  Wires 1,2 and 3 in the image below are the three yellow wires coming from the alternator and carrying alternating current. The diodes convert that alternating current to direct current and that current goes to charge the battery via wire #4.

rectifier schematics

The testing principle is this:  there should be continuity in the “normal” direction, i.e. in the direction of the black arrows and no continuity in the opposite direction.  To be clear, if current is coming in through wire #1, it should come out of wire #4 (and only out of #4; it cannot go elsewhere).  Your ohmmeter will send an electrical current in order to read the resistance to that current.  So, if you see no continuity when you connect #1 and # 4 together through the  ohmmeter, reverse the leads and there should be a beep and continuity.  If there is continuity in both directions or no continuity in both directions, the rectifier, incredible as it may be, is not functioning properly and should be replaced.

I hope this helps.