Need help with 1980 Honda CB750K charging system

[badunkafunk]

I would like to share with you the electrical problems I'm having with my 1980 Honda CB750K.  I'm about to throw in the towel and take it to a shop so any insight you have to offer would be greatly appreciated!

So the original issue was that the battery was not charging and the bike would die while riding it.  I have replaced every major component in the electrical system.  I'm currently getting a low voltage reading from the battery: 11.8-12 volts resting, 12.6 with throttle.  Right now I'm suspecting that the rotor I just replaced is weak.  Here are my actions taken and current readings:

- Replaced battery with a new agm battery - at fully charged it was 12.6 volts resting (it's gone down a bit from starting and driving)

- Replaced regulator/rectifier - tested good for continuity

- Replaced rotor - currently reading 4.5 ohms between rings (the old one was shorting to the case)

- Replaced stator and brushes - ac was consistent between the three yellow wires but hovering around 16-17 volts which seems low.  no other shorts.

[LukeM]

You didn't say at what engine speed you were when you tested your system.  At idle, the charging system is not charging.  It doesn't start supplying current to the battery until at least 2500 RPM.  My Haynes manual says to use 5000 RPM when making voltage and current checks.

When I had issues with my Shadow, I remember the yellow to ground circuit measuring about 18 volts or so at 2500 RPM.  I expect the CB750 to do about the same.

It looks like you've replaced just about everything in the charging circuit. 4.5 ohms on the rotor is about right (don't have the CB750 manual downloaded, and my Haynes manual doesn't mention that in their tests). If it isn't I expect someone else on the forum will chime in.

The diode array takes the voltage from each of the yellow wires, and converts it from AC to DC.  If your multimeter has a diode setting on it, you should see about .6 to .7 volts from a yellow wire to the red/white wire, and when you switch the leads (put the lead that was on the yellow on the red/white, and vice versa) it should show much more than that.  If you don't have the diode setting, you can use the lowest ohms scale.  You should see a low resistance measuring one way, and very high/infinite resistance the other way.  You should see roughly the same results measuring from each yellow lead to ground (either the frame, or the green lead in that area).

One more thing to check: make sure you have a good solid connection between the main ground lead (under your seat, if memory serves) and your frame. Any corrosion, or a loose connection, will give you a flaky electrical system.

I hate to say it, but welcome to the wonderful world of 30+ year old bikes. I'm running into the same issue with my '84 Shadow. Took me about a month to hunt down some parts for the front forks. That (and the rude intrusion of real life) made a one day job take about a month.  Still, it's a great feeling getting these old bikes running and running well. There's no shame in taking yours to the shop, if you've run out of ideas, and have the cash.  The end result is getting the bike working, right? :-)

I hope this helps.
Luke M

[eric moon]

Check out this to see if it helps


http://www.cb750c.com/publicdocs/charging_system/genesound_charging_system.html

[motogrady]

check every molex connector from stator to regulater,
heck, every connector you can find.
thought I needed the rotor thing also,
3 of the 4 or 5 spades in the big one from the charging side,
the white one under the side covers, were burnt.
ripped it apart, installed individual spades one at a time,
havn't needed the charger in weeks

[badunkafunk]

Thank for the responses guys!  I haven't had time to revisit this yet but when I do I will check all my connectors and run through this trouble shooting guide.

LukeM - at 5000 rpm it never reached 13 volts.  As for the rotor, 4.5 is a good sign when it's off (says my Clymer), but it doesn't rule out a short or something else when it's running/hot.  Is this a common problem?  It is a used rotor...

So if the cables and connections and everything else check out, I'm thinking about trying a new battery again (could possibly have been damaged while testing driving the bike every time I switched out a part).  The only other thing I can think of is trying out another brand new rotor.



Some additional electrical stuff that could be relevant:  

-  My front left turn signal light does not blink. It just stays on.  Possible symptom?

-  I have a sidecar and three compartments, all with brake lights.  I have tested the voltage without the compartments attached.  They work fine and shouldn't be an issue but they draw power so I thought it was worth mentioning.  I have left the bike and come back a few days later, and the battery is not dead, so I am assuming that if there is a parasitic draw somewhere it is not the main problem.  The battery has only gone dead while driving for awhile.

[sgtslag]

An AGM is an excellent, and expensive battery.  Running them dead in the bike is not good.  Take it out, charge it fully, then take it to any auto parts store for a free load test -- do NOT throw parts at your bike hoping it will get better!

If you have the cash, spend it by taking your bike to a qualified mechanic, and get it running so you can ride it.  Do yourself a favor and order this book:  Motorcycle Electrical Systems: Troubleshooting and Repair (Motorbooks Workshop).  The author is very good at explaining electrical systems, how they work, what tools you need and some you can make yourself, as well as how to troubleshoot electrical systems on old, and new bikes alike.

Luke is almost correct on how to use a DVM.  However, using the lowest resistance setting (Ohms), may, or may not, give you a valid reading.  Diodes have what is called a "break-over voltage" -- after you reach this voltage, they will conduct, but not until you apply the minimum voltage.  For Silicon Diodes, the break-over voltage is 0.6 Volts, at which point they turn on and conduct, in one direction, or polarity.  Only if your DVM's resistance mode applies 0.6 Volts, or higher, in the proper polarity, will it give a reading other than infinity.  The Diode Test mode applies 0.3-0.6 Volts (Germanium Diodes conduct at 0.3 Volts), to check for conduction, to see if the diode is working properly.  Cheers!