Topic: 1976 CB750F Weak Charging SOLVED

[SigEpRider]

After much work here is how I solved my issue:
1. I went back to the stock wire harness. I had to fix the wire to the neutral light and replace the diode (neutral light stayed on unless clutch lever was pushed in or motorcycle was out in gear). I also made sure to clean as many connectors as possible on the charging circuits. Additionally, I replaced the bullet connectors of the sub wire harness by the sprocket. This ensured a tight connection from the stator and field coil.
2.  I played with switching the ignition switch from another project bike because I thought I was getting too much voltage drop to the regulator.
3.  I finally had the battery tested as the charging was still low, but increased with revs. The battery finally tested as bad (perhaps from me using it to much with the stater motor). I used a good MotoBatt battery from another bike and IT WORKED. now I get over 14 volts at 3,000 RPM.

In conclusion….always keep you original wire harness if you can and always make sure you have a strong/good battery for charging.

Thanks for all you help.

Im hoping to get some direction from the experts on the charging system for my 1976 CB750F.

Background:
1976 CB750F
Electrosport regulator/rectifier (PO had a cheap eBay reg/rect and that wasn’t supplying voltage to field coil so I replaced it with an Electrosport)
Using a good MotoBatt battery reading 12.33V
Voltage into key switch 12.33V
Voltage out of key switch (black wire) 11.55 V
Voltage at regulator rectifier (black wire) 11.45 V
Charging system check:
1,100 rpm 12.38 V
2,000 rpm 12.66 V
3,000 rpm - 13.0 V
4,000 rpm -13.3 V (Max maybe 13.5 volts)

Would you consider this to be weak charging since the voltage doesn’t reach 14 or 14.5 V?

If yes, then:
Field coil resistance: 8.2 ohms
Continuity between leads - yes
Continuity between leads and ground - no

Stator could resistance
1-2 - .4 ohms with continuity
1-3 - .4 ohms with continuity
2-3 - .4 ohms with continuity
No continuity between stator wires and ground

Does my field could need to be replaced? (Resistance should be 7.2 ohms)
Does my stator also need replacing? (Resistance should be .2 ohms/one of the tests fluctuates between .3 and .5 ohms)
I’ve read that some damage can occur to the charging system if the bike was dropped on the generator cover. I’m really thinking I need to replace the filed coil if the charging system is weak. Possibly the stator also.
Any other test I can do?

Thanks

[scottly]

Where are you taking the resistance measurements? For instance, if you are measuring the field coil from the white wire at the regulator to ground, there may be a poor connection at the bullet connectors hiding under the sprocket cover, and likewise for the three yellow stator leads. Since you mentioned getting fluctuating readings on one of the stator tests, I would check that out more closely. A poor connection on one phase would cause a loss of some charging, like what you have.

[SigEpRider]

I took the field coil measurements and the stator measurements at the bullet connectors by the sprocket cover. I even unplugged them from the harness to get a direct measurement.

[scottly]

Do the connectors need cleaning?

[SigEpRider]

I think they did. I cleaned them up and made sure they were tight. I also noticed the resistance readings were better after cleaning them and I have a new sub harness installed (precious harness was backed up).

In talking about it, cleaning the connections helped the charging but hasn’t helped to give full charging at the battery. I also forgot to mention that I checked the AC voltage from idle to 4,000 rpm. I got 10 VAC to 30 Vac. This is all leading to the stator I believe. Especially is it was damaged when the bike was dropped by the PO

[scottly]

Stators and field coils don't usually fail. 99% of the troubles are caused by poor connections or broken wires, and a resistance test will find those.
I don't go by the AC voltage. What is the voltage at the battery at 3000-4000 RPM? Does it slowly climb?

[SigEpRider]

I’ll clean the connections more thoroughly and see if that helps improve charging some more.

Yes, the volts rise slowly and are at 13 VDC at 3,000 RPM and 13.3 - 13.5 at 4,000 RPM.

I also read a lot that the stator and field coil rarely fail. I just wanted to cover all my bases. Do you think the field coil reading 8.2 ohms between the green and white wires could be so far out of spec that it may be the field coil? I’ve read conflicting info that it has to be at 7.2 ohms and some people even say that 8.2 ohms is okay. Thoughts

[scottly]

Did you also clean the field coil connections? What do you measure from the white wire at the regulator (disconnected from the reg) to the battery - terminal? Also, with everything connected normally, key on engine not running, what voltage do you measure on the black wire at the reg, and the white wire?

[newday777]

I'm suspicious of the aftermarket reg/rectifier. Lots of posts of them failing and doing low output as you are having. They are not all they are purported to be.
Go back to the stock regulator and rectifier. The stock regulator is adjustable.

[newday777]

What is the ambient temperature when you are getting your readings?

[SigEpRider]

I'm suspicious of the aftermarket reg/rectifier. Lots of posts of them failing and doing low output as you are having. They are not all they are purported to be.
Go back to the stock regulator and rectifier. The stock regulator is adjustable.

I would go back to stock if I could, but I got the bike with a cheap eBay reg/rect already installed. The regulator side was fried and not sending power to the field coil and thus not charging at all. When I installed the electrosport reg/rect the field coils began to get powered and the system is now charging. Just not hitting full 14 V. I’d have to gamble on a used regulator and rectifier off eBay to return to stock.

Ambient temperature was Phoenix hot as hell, lol. Probably 105 - 110

[bryanj]

You really need to do the full spectrum of voltage checks on red and black wires with ignition off.

A quick test would be to run a jumper wire from the battery positive to the white wire to field and see if you get a higher output

[SigEpRider]

Did you also clean the field coil connections? What do you measure from the white wire at the regulator (disconnected from the reg) to the battery - terminal? Also, with everything connected normally, key on engine not running, what voltage do you measure on the black wire at the reg, and the white wire?

Yes. I also cleaned the green and white connections at the stator cover. I measure 8.0 ohms of resistance between the white and green wire.

I measure 11.45 V at the white wire at the regulator when disconnected from the regulator.
Black wire at the regulator measures 11.45 V also
Black wire are the key switch measures 11.55 V
Red wire at the key switch measures 12.5 V

All those measurements were taken with the key on. I see there is a 1V drop at the key switch. Would that mean the switch is failing or needs ckeannng? (I’ve had to clean the switch on a Yamaha xs650 and that helped raise the voltage going to the alternator)

[SigEpRider]

There must be something to do when it comes to ambient temperature. I came out in the morning and took new measurements while it was cooler. Here are the new readings.

Field coil - green to white 7.4 ohms

Stator
1-2 - .4 ohms
1-3 - .4 ohms
2-3 - .4 ohms.

The connectors at the sprocket cover are super clean, but now I noticed that the bullet connectors for the stator are connecting but not as tight as I would like. I had to purchase a new sub harness because it was chewed up. So I’m going to fix the bullet connectors and report back after ensuring a tight connection.

[newday777]

There must be something to do when it comes to ambient temperature. I came out in the morning and took new measurements while it was cooler. Here are the new readings.

Field coil - green to white 7.4 ohms

Stator
1-2 - .4 ohms
1-3 - .4 ohms
2-3 - .4 ohms.

The connectors at the sprocket cover are super clean, but now I noticed that the bullet connectors for the stator are connecting but not as tight as I would like. I had to purchase a new sub harness because it was chewed up. So I’m going to fix the bullet connectors and report back after ensuring a tight connection.

Yes the high ambient temps play on these combo units. That is why I asked.

[HondaMan]

Does the bike have points and condensors, or some sort of electronic ignition system aboard?

[SigEpRider]

Does the bike have points and condensors, or some sort of electronic ignition system aboard?

Points and condensers. I had to take a coil out of the springs to help with the mechanical advance and timing.

BTW….i also have an ATC fuse box that someone made for me also. I wonder who that was lol

[SigEpRider]

There must be something to do when it comes to ambient temperature. I came out in the morning and took new measurements while it was cooler. Here are the new readings.

Field coil - green to white 7.4 ohms

Stator
1-2 - .4 ohms
1-3 - .4 ohms
2-3 - .4 ohms.

The connectors at the sprocket cover are super clean, but now I noticed that the bullet connectors for the stator are connecting but not as tight as I would like. I had to purchase a new sub harness because it was chewed up. So I’m going to fix the bullet connectors and report back after ensuring a tight connection.

Yes the high ambient temps play on these combo units. That is why I asked.

Wait a second. Could that be why I’m not seeing full charging. The ambient temperature is high causing the resistance in the field coils to go up and thus the charging to not reach full potential?

[scottly]

There must be something to do when it comes to ambient temperature. I came out in the morning and took new measurements while it was cooler. Here are the new readings.

Field coil - green to white 7.4 ohms

Stator
1-2 - .4 ohms
1-3 - .4 ohms
2-3 - .4 ohms.

The connectors at the sprocket cover are super clean, but now I noticed that the bullet connectors for the stator are connecting but not as tight as I would like. I had to purchase a new sub harness because it was chewed up. So I’m going to fix the bullet connectors and report back after ensuring a tight connection.

Yes the high ambient temps play on these combo units. That is why I asked.

Wait a second. Could that be why I’m not seeing full charging. The ambient temperature is high causing the resistance in the field coils to go up and thus the charging to not reach full potential?

No, the ambient temps have nothing to do with it! The stator and field coils are inside the engine, and get a hell of a lot hotter than Phoenix with no problems.
Since doing all this, what voltage are you seeing at the battery with the engine running? Remember, the voltage won't go up immediately, as the battery needs to be recharged, especially if you are using the electric starter and the engine doesn't start at the first push of the button.
Regarding the voltage drop at the key switch, connect one meter probe to the positive battery terminal, and check the voltage on the red wire, with the key on, and then the black wire. This will measure the drop directly: if the meter reads 1 volt on the red, and 1 volt on the black, it means the switch is ok and the drop is between the switch and the battery, including the fuse block and it's connections into the harness. On the other hand, if the red wire reads zero, well, a very small voltage, and the voltage reads higher, like 1 volt, it means the switch is causing the drop. Best case is about a .5volt drop at the black reg wire, but that might be hard to accomplish, so don't dwell on it too much.   

[SigEpRider]

Here are some measurements I’ve taken at the key switch.

First I used my volt meter ti get the battery voltage with the key on. It was at 12.33 V

I took the red lead from my multimeter and inserted in the the harness where the red wire is. I then connected the black lead to a ground. Turned the key on and the voltage was 12.33 V. I then repeated the same process for the black wire at the key switch. Voltage dropped to 11.55 V. That’s less than a 1 V drop.

I’m a little confused and want to make sure I follow your suggestion and get the proper readings. Now I will take the red lead and attached it to the positive battery terminal and the black lead to the red wire at the key switch. Then repeat the process for the black wire. Should I see 1 V or 12 V?

I really appreciate all the help your giving me.

[scottly]

When you are measuring from the battery +, you are looking for a very low voltage.

[bryanj]

1 v is too big a volt drop

[rotortiller]

A quick test would be to run a jumper wire from the battery positive to the white wire to field and see if you get a higher output

Like what bryanj said, saves a bunch of guess work and lets you know the alternator has the muscle needed to do the job.

[SigEpRider]

Positive terminal to red wire key switch on: .72V
Positive terminal to black wire key switch on: .97V

Did I do this correctly?
Am I looking at a voltage drop between the battery and the key switch?

[SigEpRider]

Oh wait. I think I understand now. Since there is a difference and the black wire is reading close to 1, then the key switch is causing the voltage drop. Meaning k need to clean the switch or replace. Correct?