[Freddy]

Hello! New to the forum, this is my first post.
My son (14 years old) work through the summer and decided to blow his money on a project bike, and he picked up this 76 CB500T. Bike appears complete, low miles (about 9000) and appears to be parked sometime in the 80s. PO tried to start it up, but he couldn't so here we came and picked it up.
Long story short, after rebuilding the carbs, fiddling a bit with the spark plugs, she same to life, starts right away, idles and revs beautifully.. Of course, everything needs to be gone over, but figured we start to make sure the engine is in good order. Having had a lot of trouble with my 81 CB900C with the charging system, first thing I tested was that, and to no surprise, no voltage variation at the battery, even revving over 6k rpm.

Next step is to troubleshoot the charging system.
Does anyone have the values that I should get by testing R/R, stator and rotor?
Is there a site where I can get good parts for this bike (I can't seem to fin much available)?
Anyone went to the permanent magnet system (I got the Vape from revival cycles on my 81 CB900C)?

Thank you!!

 

[WintrSol]

You didn't post what the battery Voltage was - less than 12.6V I presume?
The round connector at the back of the transmission is where the generator output connects to the harness, and from there to the rectifier. The neutral light switch wire also goes through is; probable green with red stripe. The solid color wires (3) come from the stator, and should not measure Ohms to ground; not less than a very high resistance leakage from grime. They should also read very low resistance from each to the others, on the order of a couple Ohms or less - depends on how good your meter is at less than 3 Ohms. The harness side of those 3 wires should measure near 0 Ohms to the rectifier. With all that checked, there should be fairly high ACV between them when the engine is running and that round connector open; when it is connected, there should be some ACV at the rectifier; how much depends on the state of the battery, and whether the regulator is controlling it. The regulator is supposed to short the stator to ground when the battery approaches 14.5V.

It's at this point I would recommend replacing the stock rectifier and regulator with a newer design. A search of this forum will reveal a list of possible alternatives.

 

[Freddy]

You didn't post what the battery Voltage was - less than 12.6V I presume?
The round connector at the back of the transmission is where the generator output connects to the harness, and from there to the rectifier. The neutral light switch wire also goes through is; probable green with red stripe. The solid color wires (3) come from the stator, and should not measure Ohms to ground; not less than a very high resistance leakage from grime. They should also read very low resistance from each to the others, on the order of a couple Ohms or less - depends on how good your meter is at less than 3 Ohms. The harness side of those 3 wires should measure near 0 Ohms to the rectifier. With all that checked, there should be fairly high ACV between them when the engine is running and that round connector open; when it is connected, there should be some ACV at the rectifier; how much depends on the state of the battery, and whether the regulator is controlling it. The regulator is supposed to short the stator to ground when the battery approaches 14.5V.

It's at this point I would recommend replacing the stock rectifier and regulator with a newer design. A search of this forum will reveal a list of possible alternatives.

You are correct - meter was reading 11.3V.

• 1 The solid color wires (3) come from the stator, and should not measure Ohms to ground - This is with the stator connected I assume?
• 2 They should also read very low resistance from each to the others, on the order of a couple Ohms or less - Going to check this one
• 3 . The harness side of those 3 wires should measure near 0 Ohms to the rectifier. - Not sure I get this
• 4 there should be fairly high ACV between them when the engine is running and that round connector open - Between the 3 wires of the RR I assume? Any combination since this is ACV, correct?

This is what the RR looks like:

All the wires go into this bundle underneath the carbs, not sure this is how is supposed to look:

Thank you so much!!! Really appreciate the help!!

 

[Freddy]

Going down the rabbit hole here.

So I learned that this bike has separate rectifier and regulator.. The PO gave me a modern unit all in one, and it sounds like a common item to swap, but I just can't find a thread that clearly explains how to wire a new combo unit on the bike.

Don't want to waste anyone's time, can you point me in the right direction?
Thank you

 

[WintrSol]

1. The stator wires are read to ground when the connector is open, measuring the stator side of the connector. Do check that - a relatively low resistance to ground would upset the whole system
2. The stator is basically coils of relatively large wire, so should be very low resistance
3. Harness side with the connector is open, measured to the two (of 4) rectifier connections. The other two rectifier contacts go to ground and directly to the battery (+). Note that two of the three stator wires are connected together in the harness; they were switched together by the headlamp off/low/high switch in the 450 model to increase power when the headlamp is on.
4. You are measuring the stator output here, so with the connector open and measuring the stator side.

The part labeled 'RS21' is the regulator. It has three wires - ground, +12V from the key switch, and an output to short one of the stator wires to ground - the yellow wire visible.

As far as the bundle at the back of the transmission, perhaps Honda upgraded the connector between the stator and harness with that rectangular one. I don't know.
Most after-market combined rectifier/regulators are plug&play, and have a connector for the one that went to the rectifier (4 contacts) with an additional wire for the +12 from the key switch (maybe another for the old regulator's ground). I have the wiring diagrams for the 450 in pdf form, which should be very much like the 500t in this section; I can send it, if you PM an email address.

 

[WintrSol]

BTW, if you remove the front sprocket cover, you will see the stator wire bundle, combined with the neutral switch wire, and may follow that bundle to the connector at the back of the transmission (round on the 450)

 

[Freddy]

I finally got around to work on the 500 a bit.
So I took out the thing below (not sure if it's regulator or rectifier)

And found that the colors matched the r/r unit, like someone said, so I plugged it in.

I taped off the extra black wire that comes out of the new r/r unit.

The bike started up and revved fine.
When I measured at the battery terminals, before I got below 12 (like 11.6 or around there) and absolutely no change, with slow draining of the battery.
I now get variation, but no matter how much I Rev it it won't go passed 12.8, which I think it might be too low.

What should I look at next?
What voltage should I get at the battery?

Thank you.

 

[mike in idaho]

Connect that black wire to one of the black wires in the bike's wiring(switched power, through the key switch), brake light, turn signal flasher or coil connection, whichever is handiest. See what you get for voltage with that wire connected

 

[Freddy]

Connect that black wire to one of the black wires in the bike's wiring(switched power, through the key switch), brake light, turn signal flasher or coil connection, whichever is handiest. See what you get for voltage with that wire connected

ALright.

I connected the black wire to the taillight.. No change, still hitting around 12.7/12.8

What should I look for next?

 

[mike in idaho]

Have you unplugged the original voltage regulator? wire diagram here: http://oldmanhonda.com/MC/WiringDiagrams/CB500T.jpg You should also connect the white and yellow wires together,in the headlight, to get maximum charging rate.

 

[Freddy]

Have you unplugged the original voltage regulator? wire diagram here: http://oldmanhonda.com/MC/WiringDiagrams/CB500T.jpg You should also connect the white and yellow wires together,in the headlight, to get maximum charging rate.

Have you unplugged the original voltage regulator? wire diagram here: http://oldmanhonda.com/MC/WiringDiagrams/CB500T.jpg You should also connect the white and yellow wires together,in the headlight, to get maximum charging rate.

I got rid of this guy

I haven't opened the headlight. I'll check it out and try to connect white and yellow together.