'75 GT750 weak charging

All to do with wiring, charging or just trying to figure out whats gone wrong.

Moderators: oldjapanesebikes, H2RICK, diamondj, Suzsmokeyallan

Post Reply
Madbuffalo
To the on ramp
Posts: 463
Joined: Thu Feb 17, 2011 8:55 pm
Country: USA
Suzuki 2-Strokes: GT750
Location: Grand Rapids, Michigan, USA

'75 GT750 weak charging

Post by Madbuffalo »

The MadBuffalo isn't charging as it should and I'm hoping you guys could lend a hand with trouble shooting.

Voltage coming from the regulator at 3500rpm is only 12.1V. Back in 2012 I had tested it at 13.6V. Spec should be 13.5V-14.5V. I do get good numbers above 4000rpm. While it might be fun to ride in that range I don't think it would be good for the old girl. :roll:

Alternator: I am not certain how to test mine but voltage at the green supply wire down at the alternator is 4.1V (KOEO). Voltage at each of the yellow wires near the regulator while idling is about 9.5V AC. Resistance between the alternator rings reads 6.2 ohms. Spec calls for 10-20 but my spare alternator is even lower at 4.2, and an older thread on here also seemed to confirm that might be normal.?

It has a good 1 year old solid state battery that held 12.9V for a few days on the bench before I put it back in the bike.
No really... it's supposed to smoke.

1974 Suzuki Nomad 340
1975 Suzuki GT750 Waterbuffalo
2018 Kawasaki Z900rs
User avatar
Alan H
Moto GP
Posts: 3160
Joined: Thu Feb 16, 2012 11:50 am
Country: England
Suzuki 2-Strokes: 4 x GT550s - J, M, A, B.
Location: The Republic of South Yorkshire

Re: '75 GT750 weak charging

Post by Alan H »

Sounds like either the regulator is acting up - you might be able to get the lid off and clean the contacts.
Or alternator brushes worn - get new (or better get some oversize ones from the local motor spares store and file to size.)
Check the voltage at the end of the incoming green connection to the brushes with it disconnected and see what voltage you get to the rotor. Too low, it can't charge, 12v ish and it may be the rotor, coils, or rectifier, or even just a bad connection.
Post again when you've done some checks.
Think of how stupid the average person is, then realise that half of them are more stupid than that.
Red Sloan
Around the block
Posts: 72
Joined: Sun Nov 30, 2008 6:21 pm
Location: Muskegom Mi.

Re: '75 GT750 weak charging

Post by Red Sloan »

Hey Josh , are you going to the Blessing Sunday ?
Red
Ride on even after it all over
Madbuffalo
To the on ramp
Posts: 463
Joined: Thu Feb 17, 2011 8:55 pm
Country: USA
Suzuki 2-Strokes: GT750
Location: Grand Rapids, Michigan, USA

Re: '75 GT750 weak charging

Post by Madbuffalo »

Red Sloan wrote:Hey Josh , are you going to the Blessing Sunday ?
Red
Yep, Want to stop by later on today and help me solve this charging issue?
No really... it's supposed to smoke.

1974 Suzuki Nomad 340
1975 Suzuki GT750 Waterbuffalo
2018 Kawasaki Z900rs
Madbuffalo
To the on ramp
Posts: 463
Joined: Thu Feb 17, 2011 8:55 pm
Country: USA
Suzuki 2-Strokes: GT750
Location: Grand Rapids, Michigan, USA

Re: '75 GT750 weak charging

Post by Madbuffalo »

Alan H wrote:Sounds like either the regulator is acting up - you might be able to get the lid off and clean the contacts.
Or alternator brushes worn - get new (or better get some oversize ones from the local motor spares store and file to size.)
Check the voltage at the end of the incoming green connection to the brushes with it disconnected and see what voltage you get to the rotor. Too low, it can't charge, 12v ish and it may be the rotor, coils, or rectifier, or even just a bad connection.
Post again when you've done some checks.
Thanks Alan. Dirty regulator contacts are a possibility: I had to break the seal back in 2012 while doing some electrical diagnosis. And I should have mentioned brush lengths are good; the shortest of the 2 being 8mm. Spec calls for 5-12mm. I did check the green wire voltage where it screws into the brushes: Key on engine off it was supplying 4.1V. Is there a better way to verify voltage to the brush? I did not disconnect it from the brush first so will try that and report back.
No really... it's supposed to smoke.

1974 Suzuki Nomad 340
1975 Suzuki GT750 Waterbuffalo
2018 Kawasaki Z900rs
User avatar
Alan H
Moto GP
Posts: 3160
Joined: Thu Feb 16, 2012 11:50 am
Country: England
Suzuki 2-Strokes: 4 x GT550s - J, M, A, B.
Location: The Republic of South Yorkshire

Re: '75 GT750 weak charging

Post by Alan H »

You have to disconnect as the circuit goes into brush 1, through the excitation coil (rotor) out on brush 2 then to earth. You'll pick up all sorts of effects that may give strange readings if the wire to the rotor isn't disconnected, then with one test lead on the green and t'other on earth with ignition on, you should get near as dammit 12v (just a bit less than battery voltage as the rotor excitation power goes through a resistor in the regulator.)
When the revs rise and the regulator contacts change over, the resistance value changes to give more (or less) power to the rotor to give more (or less) magnetism inside the generator and affects the output to the rectifier. This depends on the battery voltage so the alternator gives more output as the battery voltage is lower, and less output to the rectifier as the battery voltage approaches fully charged.
The voltage on the 3 yellow wires to the rectifier should be so that any two tested on AC volts should give the same as any two others - e.g.
1-2 = 19vAC (might be more or less depending on engine revs)
1-3 = 19vAC
2-3 = 19vAC

If one pair gives significantly different, then there is a problem with some of the coils in the stator or perhaps a poor contact in the plug.
Voltage from alternator to rectifier will be AC, and less than the voltage from rectifier to battery DC, due to losses in the rectifier. This is normal, unless one or more diodes in the rectifier have failed.
Don't be worried about the technical parts, just follow the tips, post the results and I (or others) will interpret what you have and tell you where to go (politely of course!) next.

PM me your email address and I'll email a service manual with lots of details regards charging (and other stuff that's useful.)
It's for ALL MODELS to 1973ish so if anyone else would like a copy FOC, just PM me your email addy.
It's also useful for later models as regards electrical and some mechanical, but not so much carbs as they changed.

If you have to clean the reg contacts, do it with either rough paper/card or VERY fine emery paper.
Think of how stupid the average person is, then realise that half of them are more stupid than that.
Madbuffalo
To the on ramp
Posts: 463
Joined: Thu Feb 17, 2011 8:55 pm
Country: USA
Suzuki 2-Strokes: GT750
Location: Grand Rapids, Michigan, USA

Re: '75 GT750 weak charging

Post by Madbuffalo »

Green wire when disconnected from brush gives 10.4V idling and 11.4V at 3,500rpm.

AC voltage between any 2 of the 3 yellow wires was always consistent within about .3V AC. I checked them 3 different times and got 9.6V, 7.4V, & 12.4V respectively. Always below the 19V you noted.

I cleaned the rings and after that voltage at the orange wire at 3500rpm was up from this morning's 12.1V to 12.8V. Better but still not the 13.5-14.5V it should be.

Then I cleaned the VR point which yielded no change to any of the previously tested numbers.

I'll check back in a couple hours, then put things back together for tomorrow's "Holland Blessing of the Bikes" event, and hopefully hit this again tomorrow afternoon.

Thanks again for your help thus far.
No really... it's supposed to smoke.

1974 Suzuki Nomad 340
1975 Suzuki GT750 Waterbuffalo
2018 Kawasaki Z900rs
User avatar
jabcb
Moto GP
Posts: 4240
Joined: Mon Dec 13, 2010 4:32 pm
Country: USA
Suzuki 2-Strokes: 69 T350 thru 75 GT750
Location: southwestern Pennsylvania

Re: '75 GT750 weak charging

Post by jabcb »

I usually upgrade to modern rectifier + voltage regulator or rectifier/regulator when I’ve had electrical problems like this.

For the triples, Oregon Motorcycle Parts has good modern rectifiers & regulators that are direct replacements for the stock parts.
http://www.oregonmotorcycleparts.com

You might also see some benefit from inspecting & cleaning the associated electrical connectors.
BAS (Bike Acquisition Syndrome) - too many bikes but have room for more

Suzuki:
GT750 2x75
GT550 72 & 75
GT380 72
T500 69 project & 73 project
T350 69 & 71
Honda 85 CB650SC & 86 CB700SC
09 Triumph Bonneville SE
rngdng
AMA Superbike
Posts: 1769
Joined: Tue Oct 14, 2008 6:47 pm
Location: Blythewood, SC, USA

Re: '75 GT750 weak charging

Post by rngdng »

You realize that the stock GT750 regulator is adjustable, right?




Lane
If you stroke it more than twice; you're playing with it.

Too many bikes, too much time, ENOUGH SPACE, FINALLY! Never enough money.........
Madbuffalo
To the on ramp
Posts: 463
Joined: Thu Feb 17, 2011 8:55 pm
Country: USA
Suzuki 2-Strokes: GT750
Location: Grand Rapids, Michigan, USA

Re: '75 GT750 weak charging

Post by Madbuffalo »

Yep, Just want to be sure we're fixing the problem not the symptom.

I'm allowing Alan some time to come back with the 'next steps' as he's been very helpful thus far. Otherwise I'll likely go Jabcb's route and purchase modern regulator & rectifier. Probably a good idea in either case. I suppose it wouldn't hurt anything though to bend the VR tab and see if output improves; if no more direction comes from you guys in the next few days.

Thanks,
Josh
No really... it's supposed to smoke.

1974 Suzuki Nomad 340
1975 Suzuki GT750 Waterbuffalo
2018 Kawasaki Z900rs
rngdng
AMA Superbike
Posts: 1769
Joined: Tue Oct 14, 2008 6:47 pm
Location: Blythewood, SC, USA

Re: '75 GT750 weak charging

Post by rngdng »

I had to adjust mine when I first got it. If the output from the alternator is not sufficient, you won't be able to get the voltage high enough. I'm sure you checked the brushes....I can't remember.


Lane
If you stroke it more than twice; you're playing with it.

Too many bikes, too much time, ENOUGH SPACE, FINALLY! Never enough money.........
User avatar
Alan H
Moto GP
Posts: 3160
Joined: Thu Feb 16, 2012 11:50 am
Country: England
Suzuki 2-Strokes: 4 x GT550s - J, M, A, B.
Location: The Republic of South Yorkshire

Re: '75 GT750 weak charging

Post by Alan H »

Have a look and think about the details in the manual.
Remember that you HAVE to have a good excitation circuit from battery to reg to rotor before the alternator will have any chance of working.
There must be a good source of power to and through the rotor before anything else.

The rectifier is a 6 diode 3 phase unit and ALL diodes must be OK to work properly.
I still reckon that the favourite is poor connections.
The rectifiers can fail, but they are overrated so rarely do so.
I've done a few of these now, trust me, I know a bit about these units!
We'll sort it one way or another.
Think of how stupid the average person is, then realise that half of them are more stupid than that.
Madbuffalo
To the on ramp
Posts: 463
Joined: Thu Feb 17, 2011 8:55 pm
Country: USA
Suzuki 2-Strokes: GT750
Location: Grand Rapids, Michigan, USA

Re: '75 GT750 weak charging

Post by Madbuffalo »

Fixed.

Testing the orange wire periodically yielded different voltages every time which led me to suspect a faulty voltage regulator. I installed my spare VR and am now getting 14.1V, well within the 13.5-14.5 range spec'd. Then I re-tested between yellow wires and got 13V AC at 3500 rpm. Not quite the 19V AC specified but it's much better than the earlier 7.4V AC and the battery is now maintaining a strong charge.

Thanks guys. These little issues are always a good learning experience.
No really... it's supposed to smoke.

1974 Suzuki Nomad 340
1975 Suzuki GT750 Waterbuffalo
2018 Kawasaki Z900rs
Post Reply