1977 GT750 Charging rotor.

[ryder2shae]

Hi guys,am a newby here and its my first topic.My 1977 GT 750 is not charging.I tried two stators and found out the voltage output is too low (around 15 Volts Ac).So,I figured out my rotor is bad.I ordered another used rotor fro ebay,which the seller said works perfect and tried it on and same thing.low voltage.
My big question is,Whats the resistance between the slip rings on a good working rotor? The manual says 10 ohms but both the rotor on the bike and the one i bought from ebay reads 4.5 and 4.7 ohms respectively.

Any help would be much appreciated.
Thanks

[jabcb]

The alternator specs are on pages 15 & 18 of: http://www.ozebook.com/compendium/suzi/suzidata1.htm

The GT750s only had Nippon Denso alternators, so the rotor should have a resistance of 10 to 20 ohms.
Some GT380s & GT550s had Kokusan alternators, which have a rotor resistance of 2 to 5 ohms.

Your situation does seem a bit odd -- perhaps your bike actually has a Kokusan alternator.
So as a first step could you post a pic of your rotor so we can determine which one you actually have?

Note on pics: the website server is not accepting pic uploads at this time.
You have to post it somewhere else, like photobucket, and include an image link in your posting.

What is the history of your bike?
Is it a recent acquisition that you are sorting out?
Or have you had it for a while & just recently started having electrical troubles?

[ryder2shae]

Thanks a lot for the response.Much appreciated.I recently bought the bike.had charging issue ever since.I will take pictures of both the rotors tomorrow and upload them and post the link

Cheers

[ryder2shae]

Hi guys,I took the pictures of the rotors and posted on photobucket.Here are the links.The first two pictures are the rotor that was on the bike and third and forth are the rotor that i bought from ebay.. Can you tell me whether they are Nippon Denso or kokusan rotor?

http://s1082.photobucket.com/user/ryder ... 8.jpg.html" onclick="window.open(this.href);return false;
http://s1082.photobucket.com/user/ryder ... 9.jpg.html" onclick="window.open(this.href);return false;

http://s1082.photobucket.com/user/ryder ... 1.jpg.html" onclick="window.open(this.href);return false;
http://s1082.photobucket.com/user/ryder ... 2.jpg.html" onclick="window.open(this.href);return false;

Thanks again

[tz375]

The 72 shop manual says 10 ohms, but yours is probably OK. I'd need to go and find one in the shop to test to confirm that.

More likely to be a dead regulator or poor connection to the field coil. Try a different regulator and see if that does the job.

[pull the wire]

I tested my known good rotor and it was 5 ohms. My manual says it should not be less than 10 ohms but it was working fine. With age, time and miles, wiring connections are always an issue. Be sure all of your grounds are clean and tight, check the connections and terminals behind the left side cover for the alternator wiring and make sure the ground wire for the electrical panel, where the regulator and wiring are located, is clean and tight.

[ryder2shae]

I took the regulator out and supplied 12v to the rotor from the battery direct and started the bike but I was only getting like 15v ac between the 3 yellow wires from the stator.It should be making like 50 or 60v ac,since the rotor is getting 12v direct,right? Correct me if am wrong,please..
I also wired up an electronic regulator/rectifier from a 1996 Gsxr and voltage output is like 5 volts dc.So,I believe its not the regulator thats the problem.Again please correct me if am wrong.
Cheers

[tz375]

That sounds correct to me. It should be pushing >30 AC Volts at a few thousand RPM.

Makes me suspect the stator. I can't get to a rotor right now but I'm sure that the ones I tested were 5 ohms or less and they charged fine.

[jabcb]

The rotor looks correct.
From the parts diagram it should look like this:


The Kokusan alternator has the brushes running parallel with the crank.

Perhaps the spec of 2 to 5 ohms & 10 to 20 ohms applies to the alternators in the GT380/GT550.

I agree with tz375 in that the stator is suspect.

To check the stator:
1) check the resistance between each combination of yellow wires.
The spec says "nearly 0", but that's from the perspective of the analog multimeters we had back in the 1970s.
2) check the resistance between each of the yellow wires and ground.

[ryder2shae]

I just checked the resistance on both the stators I have.Both stators read 0.8 ohms between the yellow wires and infinite resistance between the ground and yellow wires.
If I supply 12 volts to the rotor directly and start the bike,the stator should give me the maximum ac voltage output,right?(since the voltage regulator on these bikes works by cutting the voltage supply to the rotor).
Does the rotor needs the load resistance across the connectors?Thats the only thing thats left for me to try.

Thanks guys for the help.Much appreciated.

[jabcb]

The brushes may be worn out or cracked.

The wiring may be damaged & limiting the current supplied to the rotor.
To check for this check the voltage drop between the wiring connecter at the voltage regulator & the connection to the brush holder.

[ryder2shae]

I tested the voltage drop..battery is 11.5 v (not fully charged)
10.8 v at the orange wire coming back from the ignition switch.
9.1 v at the terminals were the carbon brushes for the rotor
8.1 v at the slip rings on the rotor.

Looks like a lot of voltage drop.Is it normal?
Thanks

[Alan H]

It's very low and your battery is almost flat at 11.5v.
Should be 12.5 - 12.8 and 13.5 - 13.8 when running a charge. Check all connections and especially the earth.
You will get more volt drop on that circuit as it is quite a high current, so you must have good connections.

[jabcb]

Charge your battery up. No point in damaging your battery while your figure this out.

Depending on the specifics, the 15V AC you got might have been ok.


With a fully charged battery, repeat you no load alternator test.
Bypass the voltage regulator & supply the rotor with full battery output.

With the rectifier disconnected, what are no the no-load AC voltages at 1.5k & 2.5k rpm?

With the rectifier connected to the alternator and the rectifier DC+ output disconnected, what are the no-load DC voltages at 1.5k & 2.5k rpm?
The spec calls for >16V & >27V respectively.



I've had a few bad rectifiers over the years, so that may be the problem.

[ryder2shae]

Thanks for the response.That what i did.I knew my regulator was bad.So I wired 12 volts directly to the rotor and tested the ac voltage at the 3 wires coming from the stator. It was only like 15 volts ac at 1500 rpm.
I wired up an electronic regulator/rectifier from a 1996 gsxr 750 for the gt.I know the regulator/rectifier is good..Does this electronic regulator/rectifier works for the olg gt or am just doing a stupid mistake?
Thanks guys