The old charging issue ..

[OzJavelin]

I'm HOPING this isn't a stator/rectifier issue .. My 78 GS750 keeps flattening the battery within riding for about 30min. It will start cold from the starter button fine, but after about 30min riding (with headlamp on) it will not restart from the starter button .. Starter won't turn over .. But will start fine from the kickstarter. When I get home I check the battery and it's about 11V. if I recharge the battery it holds about 13V .. And will go up to about 14.5V when running above idle with the headlight off. With the headlight on it drops below 12V.
I've been told previously to always ride with the headlight on to prevent charging issues with the GS ..

[allojohn]

It's time to do the wiring alteration eliminating the wire to the headlamp and upgrade your R/R - hopefully the stator is still OK.

[tom203]

New year, little hope,so why not test your charging system before you get stranded. With headlight on, 30 minutes is about all you'd get with decent battery that ISN'T being charged. Get a multimeter, review the charging system tests at Basscliff's and pinpoint the problem.

[OzJavelin]

Following the diagnosis chart .. With the headlight OFF I get 13.5V at 2500 and max of about 14.5V at at 5000rpm blat ..
This is an analogue multimeter, so I might be a fraction one way or another on the readings.
This seems OK to me so I didn't proceed any further.
Am I meant to test with the headlight on or off?
The battery seems to hold charge ok if I just leave it attached to the bike .. It only after vie ridden it seems to go flat?
I'll read thru the stator papers more ..

[Redman]

.... And will go up to about 14.5V when running above idle with the headlight off. With the headlight on it drops below 12V.
.. ..

On a 78 having headligh on/off switch, with original wiring, with the headlkight off 1/3rd of the stator is swtiched out of the circuit so only have 2/3rd of the charging system capacity. But then again with headlight off that is a lot less load on the system so less of a need of the charging. With the head light on, then the headlight switch also sitches in the other 1/3rd of the stator, so then should have full capaicity of the charging system. The head light swtiches the headlight off and on, and also swtiches out and in 1 phase of the stator.

You say that when headlight off you seem to have good charging voltage, but with headlight on you have somewhat poor charging voltage.
Soulds like a problem with that 1/3rd of the stator, or associated wiring.
Yes, Investgigatge further with the stator papers. Good that you have a volt meter.

Yes, test with headlight on because thqt is when you have the problem.

Only reason I think someone would say to leave headlight on to Avoid charging system problems is if someone had bypassed the headlight switch so that charging system using all three legs of stator even if headlight is off.

[tom203]

Since you get decent output with headlight off, what Redman says makes sense. Could be that switched out phase wire is grounding out after it passes thru headlight switch on its way back to rectifier or might be defective diodes. If the problem was internal to stator, I'd think that you'd get poor output at all times no matter whether lights were on or off.

[BassCliff]

Hi,

I once had a stator that would test good in the garage but the battery would still not get charged. I tested everything, cleaned and checked all connections, installed a new r/r unit, etc, to no avail. After replacing the stator all was well. I can only speculate that the insulation had melted from the windings but they would not short out until the engine oil and stator got hot and the wires started moving around.

I'm not necessarily saying your stator is bad. But, in my case, after eliminating everything else a new stator turned out to be the fix. Sometimes it's a process of elimination.

Thank you for your indulgence,

BassCliff