'77 GS750 charging issue

[Joe Nardy]

I'm helping GBW try to figure out his charging issue on his '77 GS750. The symptom is that the charging voltage is 12.4 volts at idle, 12.6 at 2.5K rpm, and under 13 at 5K rpm. We have gone through the stator papers and the stator and R/R check out okay. The battery is new and measures 12.9 volts. He has the Duanage R/R mod and the coil relay mod. The sense wire from the R/R is connected to the hot wire of the brake light switch. If we check the output of the R/R with it unplugged we get 20-22 VDC so it seems the R/R is outputting enough voltage. If we unplug the sense wire and check the output voltage disconnected we get close to 80 volts. We're going to run the R/R wires directly to the battery and start disconnecting loads to see if we can get the voltage up. Anyone have any other thoughts?

Thanks,
Joe

 

[TheCafeKid]

Ive actually seen, on these old 750s, the rotor be bad. Yes, a rare occurrance for sure. But on my roomies old 79, we tried many different stators, many different R/R, all tested good, wired straight in line so there were not other things that could be at fault, and it still simply wouldnt charge. We pulled the rotor and replaced it with a different one and it charged...

Also, if GBW REPLACED the stator with new, his rotor is bound to be the wrong type. The early stuff used a 12 pole stator, with a 6 mag rotor. The new 18 pole stators wont work with that rotor. You need one with 9 mags, say, off an early 850 or 1000. The crank snouts SHOULD both be 25mm (i think) so it should go right on the 750 with the proper bearings

 

[Joe Nardy]

Ive actually seen, on these old 750s, the rotor be bad. Yes, a rare occurrance for sure. But on my roomies old 79, we tried many different stators, many different R/R, all tested good, wired straight in line so there were not other things that could be at fault, and it still simply wouldnt charge. We pulled the rotor and replaced it with a different one and it charged...

Also, if GBW REPLACED the stator with new, his rotor is bound to be the wrong type. The early stuff used a 12 pole stator, with a 6 mag rotor. The new 18 pole stators wont work with that rotor. You need one with 9 mags, say, off an early 850 or 1000. The crank snouts SHOULD both be 25mm (i think) so it should go right on the 750 with the proper bearings

Josh,

What were the symptoms with the bad rotor? I would think you wouldn't get good AC voltage if the rotor was bad or the wrong one. We get 80 volts AC on all three legs of the stator.

Thanks,
Joe

 

[TheCafeKid]

Josh,

What were the symptoms with the bad rotor? I would think you wouldn't get good AC voltage if the rotor was bad or the wrong one. We get 80 volts AC on all three legs of the stator.

Thanks,
Joe

NOt 80v..i missed that part. It was still throwing voltage, just not very much.

Have you tried eliminating the red wire from the harness and runnin one straight from the R/R to the battery?? Thats what i ended up having to do on my 1000...something in the harness between the RR and whatever was bad. So, i just bypassed it.

 

[Joe Nardy]

NOt 80v..i missed that part. It was still throwing voltage, just not very much.

Have you tried eliminating the red wire from the harness and runnin one straight from the R/R to the battery?? Thats what i ended up having to do on my 1000...something in the harness between the RR and whatever was bad. So, i just bypassed it.

That's our next step.......

Thanks,
Joe

 

[posplayr]

Joe,
I provided an update the the stator pages. From what you describe you have too much resistance in the positive R/R (+) lead and the battery which is likely in the fuse box.

http://www.thegsresources.com/_forum/showthread.php?t=143019

Depending on the type of fuse box you have, your approach to cleaning might vary. If there are wire crimps inside of the fuse box then what I have done is to dip the crinps in an acid like navel jelly to clean the corrosion and brighten the brasss. The n solder those crimps to block the corrion from coming back.

You can meansre the effects of the resistance by verifying the voltages from

r/r (+) to battery (+) adn R/r(-) to Battery (-) should be less than 0.25V at 5000 RPM.

Other suggestions are to move the Honda 6th sense wire to the Coil relay mod +12V output as that is closest to the battery voltage.

If you do hook the R/R (+) directly to the battery you should see improvement, but then you are not fused. If the fuse box can't be cleaned then do a new inline on the (+) leg to the battery.

Grounds are important, and the R/R (-) should be grounded to the frame as close to the R/R as practical as well as a wire running to the battrey (-) terminal.

Jim

 

[Joe Nardy]

Joe,
I provided an update the the stator pages. From what you describe you have too much resistance in the positive R/R (+) lead and the battery which is likely in the fuse box.

http://www.thegsresources.com/_forum/showthread.php?t=143019

Depending on the type of fuse box you have, your approach to cleaning might vary. If there are wire crimps inside of the fuse box then what I have done is to dip the crinps in an acid like navel jelly to clean the corrosion and brighten the brasss. The n solder those crimps to block the corrion from coming back.

You can meansre the effects of the resistance by verifying the voltages from

r/r (+) to battery (+) adn R/r(-) to Battery (-) should be less than 0.25V at 5000 RPM.

Other suggestions are to move the Honda 6th sense wire to the Coil relay mod +12V output as that is closest to the battery voltage.

If you do hook the R/R (+) directly to the battery you should see improvement, but then you are not fused. If the fuse box can't be cleaned then do a new inline on the (+) leg to the battery.

Grounds are important, and the R/R (-) should be grounded to the frame as close to the R/R as practical as well as a wire running to the battrey (-) terminal.

Jim

Jim,

Thanks for the tips. The '77 GS750 does not have a fuse box, just one main fuse. We have moved the R/R + and - directly to the battery, added a fuse to the + so that eliminates the check from R/R to battery voltage loss, right?

If we disconnect the sense wire the voltage at the battery is still the same, 12.9 volts at 5K rpm. Shouldn't disconnecting the sense wire make the R/R output a lot more voltage?

Thanks,
Joe

 

[gbw]

The only load I can think of that we haven't eliminated is the starter solenoid. Any chance that could be dropping voltage to ground? Maybe I'll just disconnect it and kick start to test.

Thanks Joe for all the help today. We started at 9 and worked until 4:30 on the *%&*# thing. Even though its exactly the same now as it was this morning, I had a blast with you and Dom.

You should have seen Joe bouncing back and forth between my electrical problems and Dom's leaking gasket/clutch cable. Dom would say, "hmmm, my clutch cable is bad", and I would say, "hmmm, I need some connector, some wire, a soldering iron, and an inline fuse." Joe would step away for a second and show up with a new clutch cable (still in the package) and everything I asked for as well. And on top of that he simultaneous polished Dom's cases and half-moon covers!

No wonder they call him The Shaman! Plus, he knows where to get "The Best Gyro in DuPage County"

Thanks again!
Greg

 

[Joe Nardy]

The only load I can think of that we haven't eliminated is the starter solenoid. Any chance that could be dropping voltage to ground? Maybe I'll just disconnect it and kick start to test.

Thanks Joe for all the help today. We started at 9 and worked until 4:30 on the *%&*# thing. Even though its exactly the same now as it was this morning, I had a blast with you and Dom.

You should have seen Joe bouncing back and forth between my electrical problems and Dom's leaking gasket/clutch cable. Dom would say, "hmmm, my clutch cable is bad", and I would say, "hmmm, I need some connector, some wire, a soldering iron, and an inline fuse." Joe would step away for a second and show up with a new clutch cable (still in the package) and everything I asked for as well. And on top of that he simultaneous polished Dom's cases and half-moon covers!

No wonder they call him The Shaman! Plus, he knows where to get "The Best Gyro in DuPage County"

Thanks again!
Greg

Actually, it was a tachometer cable (I had two), although I did have a clutch cable if needed.

I'm bummed we couldn't figure the problem out. Hopefully we'll get some good tips here and get it resolved once and for all. Dom's still here, polishing up headlight rings and buffing scratches out of his tank.....

It's always fun working on bikes and a tasty Gyro is never a bad thing. Sorry about the after effects.......

Thanks,
Joe

 

[posplayr]

Jim,

Thanks for the tips. The '77 GS750 does not have a fuse box, just one main fuse. We have moved the R/R + and - directly to the battery, added a fuse to the + so that eliminates the check from R/R to battery voltage loss, right?

I would still check it ; there may be a surprise. I know it is just a piece of wire.

If we disconnect the sense wire the voltage at the battery is still the same, 12.9 volts at 5K rpm. Shouldn't disconnecting the sense wire make the R/R output a lot more voltage?

Thanks,
Joe

From your earlier description, you are charging (i.e. 80 volts). Also the R/R is not bust as you had 20V coming out of it.

If you lift the sense wire the R/R measures 0v and should not regulate at all allowing max voltage (voltage goes up as you asked above). If it stays at 12.9v volts check those voltages as mentioned above before concluding the R/R is bust.