Topic: CB750K No voltage change with RPM -- no magnet at stator cover

[Plzhalpimlost]

Hello folks - let me start by saying that I have read every single existing thread that I could find on this site about charging issues, so know that I wouldn't waste your time unless I was truly, deeply stumped

Bike: 1975 Honda CB750K

Problem: When the engine is on, voltage idles at 12.3v -- voltage does not move with rpm, even for sustained periods of time like highway or cruising. It may be wobbling between 12.2 and 12.4 depending on how charged the battery was when I started the ride. The bike can go surprisingly far -- over 100 miles without issue -- so the problem is less that it "keeps dying" but more so that I KNOW it isn't charging properly. I don't feel comfortable going on a longer cruise as I can see my voltage isn't moving into a charging range. It died on me only once, but I could start it back up. While I'm not often stranded, it doesn't change the fact that something is clearly wrong and that the bike can't be trusted at the moment.

Specs:
-New wiring harness
-New Motobatt AGM Battery
-Cycle X balanced (but not lightened) alternator rotor
-Oregon Motorcycle Parts regulator and rectifier (still separate parts in stock locations)
-Hondaman Fuse Box
-Hondaman ignition
-New starter relay
-New handlebar controls
-New ignition switch (yet to install Hondaman relay, but will once I figure out this current problem)
-Starter motor safety unit bypass

What I've checked so far:
-Battery has been load tested and passed with high marks
-3 yellow wires at stator had .5ohms resistance between them
-Yellow wires to ground - infinite resistance (no beep)
-Field Coil - green and white wires (when disconnected from harness) - infinite resistance to ground (no beep)
-Field coil - green and white wires measure 7.2ohms of resistance between them BUT from the white wire that is about to connect to the regulator, and the neg battery terminal, is 8.2ohms. SHOULDN'T THIS BE 7.2ohms ALSO? PROBLEM?
-Green wire going to field coil - no resistance to ground (beep)
-Voltage regulator - Black wire measures about 1v less than at battery (11.6v compared to 12.6 at the battery). I have made a fused jumper to go from POS battery terminal to black on regulator - this made no difference when the bike was running. Still constant voltage throughout RPMs. Also, shouldn't a lower voltage at this black wire cause the regulator to overcharge the battery? It certainly isn't...
-Tried adjusting voltage regulator - no change
-No magnetism at all on alternator cover (or the bolts) when ignition on (engine still off)

Ask me anything and I'll go out, test it, and report back.

Oh, and here is the bike in question. Three years - handbuilt by yours truly. Engine, paint, seat, you name it.

Thank you in advance!

[scottly]

Jump the white field coil wire to the battery + terminal.

[scottly]

-Field coil - green and white wires measure 7.2ohms of resistance between them BUT from the white wire that is about to connect to the regulator, and the neg battery terminal, is 8.2ohms. SHOULDN'T THIS BE 7.2ohms ALSO? PROBLEM?

The green field wire should be connected to ground (bat-), so you should read the same 7.2 ohms. The green wire on the reg also needs to be grounded. It's a common mistake to just connect the reg and field greens to each other, when they both need to be grounded.

[TwoTired]

Don’t do a jumper, as that sometimes kills a good regulator. Disconnect the white wire from the reg before connecting to bat +.  This should make the electromagnet active, and the alternator charge at full capability commensurate with the rpm.

[Plzhalpimlost]

Don’t do a jumper, as that sometimes kills a good regulator. Disconnect the white wire from the reg before connecting to bat +.  This should make the electromagnet active, and the alternator charge at full capability commensurate with the rpm.

I'll do this tomorrow and let you know -- thanks! If it works, what does that mean?

[scottly]

-Field coil - green and white wires measure 7.2ohms of resistance between them BUT from the white wire that is about to connect to the regulator, and the neg battery terminal, is 8.2ohms. SHOULDN'T THIS BE 7.2ohms ALSO? PROBLEM?

The green field wire should be connected to ground (bat-), so you should read the same 7.2 ohms. The green wire on the reg also needs to be grounded. It's a common mistake to just connect the reg and field greens to each other, when they both need to be grounded.

[Plzhalpimlost]

Don’t do a jumper, as that sometimes kills a good regulator. Disconnect the white wire from the reg before connecting to bat +.  This should make the electromagnet active, and the alternator charge at full capability commensurate with the rpm.

I'll do this tomorrow and let you know -- thanks! If it works, what does that mean?

Not "tomorrow" yet but I just went to the garage and tried the jumper wire from + bat terminal to white wire on the field coil, and it did indeed cause magnetism on the stator cover bolts -- progress! Too late at night to start it up and rev it  but I will give it a go tomorrow and report back. Still, I'm curious what this means in the way of what's wrong with the current wiring...

[Plzhalpimlost]

-Field coil - green and white wires measure 7.2ohms of resistance between them BUT from the white wire that is about to connect to the regulator, and the neg battery terminal, is 8.2ohms. SHOULDN'T THIS BE 7.2ohms ALSO? PROBLEM?

The green field wire should be connected to ground (bat-), so you should read the same 7.2 ohms. The green wire on the reg also needs to be grounded. It's a common mistake to just connect the reg and field greens to each other, when they both need to be grounded.

Will check soon. I've heard people mention this when referring to combo reg/recs, but not for current setups. This would be strange as it would mean that the wiring harness I bought was wrong in some way... I installed the wiring harness and "plugged it in". So I really don't know where EXACTLY the green wire goes from the regulator once it shoots into the wrapped harness. I'd have to strip it all back and really change something, which I'm not opposed to doing, but it would surprise me if it WASN'T an error on my part, but instead a part I bought.

[scottly]

So this is an aftermarket wiring harness? The green wires in the harness are all connected to frame ground, and the battery - is also connected to ground. You should read zero ohms, or as close to zero as your meter will read, from the battery- terminal to both the reg green and the field green wires, while they are connected. If yes, proceed with jumping the white field wire to battery +. This bypasses the reg and runs the alternator WFO. The stator cover screws should be magnetized, and when the engine is running the voltage should increase.

[grcamna2]

subscribed

[scottly]

.

[TwoTired]

If applying 12v direct to the white wire makes the magnet activate.  Then likely the vreg isn't putting a voltage on the white wire.  Verify the green wire to the vreg indeed connects to battery minus.  You can do this with the battery disconnected and use an ohmmeter.
If you get low to no ohms, that checks ok.  If not, fix that connection.
Next would be to check the black wire terminal for voltage with it all connected and key switch on.  If you get battery Plus terminal voltage, you are good there.  And then if you get no voltage at the white wire connection, your vreg is faulty.  If you do get voltage on the white wire, your charge system works.

[Plzhalpimlost]

If applying 12v direct to the white wire makes the magnet activate.  Then likely the vreg isn't putting a voltage on the white wire.  Verify the green wire to the vreg indeed connects to battery minus.  You can do this with the battery disconnected and use an ohmmeter.
If you get low to no ohms, that checks ok.  If not, fix that connection.
Next would be to check the black wire terminal for voltage with it all connected and key switch on.  If you get battery Plus terminal voltage, you are good there.  And then if you get no voltage at the white wire connection, your vreg is faulty.  If you do get voltage on the white wire, your charge system works.

Ok, I went down today, and jumped a wire from the positive battery terminal to the white wire where it would normally connect to the regulator. Engine on and at idle I was getting over 13v, but it shot up to nearly 18v at only 3,000rpm. Overcharging. I shut off the bike quickly.

Green wire going to vreg measures no resistance to battery negative. All good there. No issue.

Here's where it gets strange:
-Black wire at vreg when connected and ignition on measures .4v less than at battery terminals. (12.1v compared to 12.5v).
-When I disconnect the white wire and measure the vreg side, it also measures 12.1v -- same as the black wire.
-But when I recconect the white wires from harness to vreg, and stick the probe in to measure voltage, the voltage goes away!

So, it seems like there are two problems that I don't have solved:
1. Why is the black wire at vreg a lower voltage than the battery?
2. Why would the voltage at white vreg wire go away when connected to the harness?

If I've figured out anything I guess it would be that the vreg DOES work properly, but there is something in the harness or connections that are not right...

Thoughts?

[Plzhalpimlost]

Ok, a slight "correction" if you will from above.

I was staring at the wiring diagram looking that the white wire in the harness that runs from the vreg to the field coil thinking "it's just one uninterrupted white wire - what could be wrong with it?". It turns out nothing. No short in it either before anyone asks.

While connected to the vreg, I unplugged the white wire where it connects to the field coil, and at the end of the white wire, we still have 12.1v. So, it's actually when the white wire is plugged into the field coil that it loses its voltage. I ran a jumper acting as the white wire with the same result.

Field coil wires arent grounding, so no suspected short there either. I have a spare field coil that I could do some tests on if anyone would like me to confirm anything.

I hope you guys are having as much fun as I am. 

[bryanj]

There is a poor connection somewhere on the black wire to the vreg, a 0.5v loss is about the max alowable and what is happening is that with very low load you can measure a voltage but when you try to draw amps the connection gives up, you could try running a seperate wire direct from battery pos to the white wire and see if it charges then, oh and the field coil does ground via the green wire just not via the fixing screws

[Don R]

 Someone recently had an aftermarket DOHC V-reg on his SOHC charging system. Just a thought, they are different types.

[Plzhalpimlost]

Someone recently had an aftermarket DOHC V-reg on his SOHC charging system. Just a thought, they are different types.

Oregon Motorcycle Parts doesn't sell a vreg for the DOHC, at least not on their website. And pics confirm its the right one (unless they are identical).

[Plzhalpimlost]

There is a poor connection somewhere on the black wire to the vreg, a 0.5v loss is about the max alowable and what is happening is that with very low load you can measure a voltage but when you try to draw amps the connection gives up, you could try running a seperate wire direct from battery pos to the white wire and see if it charges then, oh and the field coil does ground via the green wire just not via the fixing screws

Check Reply #12. I ran the jumper wire that you're talking about. It turned on the field coil, but then began to overcharge. Not sure where to go from here.

[Plzhalpimlost]

While connected to the vreg, I unplugged the white wire where it connects to the field coil, and at the end of the white wire, we still have 12.1v. So, it's actually when the white wire is plugged into the field coil that it loses its voltage. I ran a jumper acting as the white wire with the same result.

Field coil wires arent grounding, so no suspected short there either. I have a spare field coil that I could do some tests on if anyone would like me to confirm anything.

Ok, I just tried hooking up the spare field coil with the exact same results. Very strange -- it seems like we've ruled out every charging component on the bike - haha

Regulator works, but it's not getting the right voltage
Field coil seems to act "as normal" I guess??

As BryanJ mentioned, maybe when plugged in, the field coil is actually trying to USE the voltage in the white wires and it just gives up. Also, it seems to always drop to a very specific 13.5mV, so not nothing I guess.

Is it really time for me to just start ripping apart the wiring harness? 

[bryanj]

It went to overcharge as you were giving the field full battery all the time, the vreg cuts the power back as battery charges and in answer to the other question yes its time to start chasing the black wire from vreg into loom and see where the bad connection, or break, is

[Plzhalpimlost]

I'm not sure if I'm solving anything, or just finding more problems at this point.

Before tearing apart the wiring, I was testing some stuff out and found another possible problematic pattern.

With engine off -- when I was turning the ignition on, I actually had the coils still on. When I turned the coils off with the kill switch, the voltage measured at the black wire near the voltage regulator was actually very close to the battery itself. So I thought, oh, maybe the coils are the problem, but it turns out if I turn ANYTHING on it lowers the voltage at the black wire. Look here

Both tests were with ignition on, engine off. I then slowly turned things on and watched the power draw.

When measured from the battery:
Everything off except ignition: 12.7v
Turn on the lights: 12.65v
Turn on the coils: 12.45v
Turn on the heated grips: 12.3v

When measured at the black wire that would connect to the voltage regulator:
Everything off except the ignition: 12.65v
Turn on the lights: 12.4v
Turn on the coils: 11.9v
Turn on the heated grips: 11.4v

I was looking for another power wire to test, so I used the rear brake running light and measured again, except I obviously had to start with lights on -- everything followed the voltages measured at the black wire, maybe another .1v or .2v lower.

At the battery, this seems like very normal power draw since the engine is not running and recharging the battery -- but within the actual harness, these numbers are concerning me.

Why the 1 volt difference?

[bryanj]

Every connector has a resistance, every resistance will cause a voltage drop. Even the wire itself has a resitance depending on length and cross sectional area.
From your reports you have a problem somewhere on the black wire going to the vreg, you could bypass this wire with one extrnal to the loom from the black at the ignition switch to the black at the vreg but if one wire in the loom is damaged that is normally caused by physical damage so it is likely more will go.

[TwoTired]

If the white wire goes to near zero when connected, and the black terminal is still near 12 v,  I would believe the output driver in the vreg is blown and can no longer provide current.

New vreg time.  Or, find and original to install.

The v loss from battery to vreg, is due to less than ideal connections, switch contacts and fuse clips.  0.4 v loss is acceptable, assuming a standard lead acid battery. Even sustained high rpm shouldn’t make the batt voltage higher than 15v.

[Plzhalpimlost]

CONCLUSION:
It was indeed the voltage regulator that was broken. Oregon Motorcycle Parts sent a new one and the problem was solved. Thank you all for the help!!