Losing power at idle when turn lights on

[Guest]

I have a 1979 suzuki gs750l that loses power at idle (at a stop light) when the lights are turned on. What would be a good approach to try to determine what is at fault??

[tom203]

Go here.do Quick Test and report your 1 thru 6 results. note that you must start with decent battery charge

http://www.thegsresources.com/_forum/showthread.php?p=1272192#post1272192

[ATM]

TOM - Here are the results from the battery analysis.

My multimeter crashed and i had to buy a new one!
That is why i did not respond to you right away.

Battery: Yuasa high performance 12 volt ytx14ah(l)-bs
motorcycle: 1979 suzuki gs750l
problem: Loses power at idle when the lights are turned on

1) key off: 12.62 volts
2) key on and lights on: 11.70 volts
3) at idle: 13.1-13.3 volts (erratic)
4) at 2500 rpm: 13.8 volts
5) at 5000 rpm: 14.7 volts
6) key off: 12.75 volts

one thing that i have done recently because of
the stalling at stop lights with lights on was
to adjust the idle by turning the throttle stop
screw. I was running previously at an idle speed
of 1000 rpm. I am now running at an idle speed of
1500 rpm.

This did not help however. Here is what happened
next after i obtained the results shown above....

With the idle speed at 1500 rpm, the lights were
turned on:

Initially the voltage increased to 13.8 volts.
But after a minute or two with a hot engine,
the voltage slowly decreased down to 11.6 volts
and the engine was just barely hanging on (right
on the cusp of stalling).

With the lights off, this engine will not stall!
Perhaps this is a tribute to how tough these engines
really are. (in spite of cylinder compression readings
that i measured about 2 weeks ago of 80 psi, 80 psi, 30 psi,
and 80 psi). Sad to say, the guy who owned this bike
before me did not take care of this bike at all.

What do you think of all this??

[posplayr]

My multimeter crashed and i had to buy a new one!
That is why i did not respond to you right away.

Battery: Yuasa high performance 12 volt ytx14ah(l)-bs
motorcycle: 1979 suzuki gs750l
problem: Loses power at idle when the lights are turned on

1) key off: 12.62 volts
2) key on and lights on: 11.70 volts
3) at idle: 13.1-13.3 volts (erratic)
4) at 2500 rpm: 13.8 volts
5) at 5000 rpm: 14.7 volts
6) key off: 12.75 volts

one thing that i have done recently because of
the stalling at stop lights with lights on was
to adjust the idle by turning the throttle stop
screw. I was running previously at an idle speed
of 1000 rpm. I am now running at an idle speed of
1500 rpm.

This did not help however. Here is what happened
next after i obtained the results shown above....

With the idle speed at 1500 rpm, the lights were
turned on:

Initially the voltage increased to 13.8 volts.
But after a minute or two with a hot engine,
the voltage slowly decreased down to 11.6 volts
and the engine was just barely hanging on (right
on the cusp of stalling).

With the lights off, this engine will not stall!
Perhaps this is a tribute to how tough these engines
really are. (in spite of cylinder compression readings
that i measured about 2 weeks ago of 80 psi, 80 psi, 30 psi,
and 80 psi). Sad to say, the guy who owned this bike
before me did not take care of this bike at all.

What do you think of all this??

So the QT results are with lights off?
If so what are they with lights on?

[tom203]

Yes,repeat quick test with lights on- even though we can sorta anticipate results with what you said.
Odd that this extra load (headlight draws about 60 watts) causes a rapid falloff in output.
The stock setup had one phase of alternator controlled by headlight on/off switch- charging output was reduced when headlight was off. But this connection can be a problem even if alternator and R/R (rectifier/regulator) are OK.
While you're retesting,take a look at your R/R and see if wiring appears to have been messed with.
Those compression readings are low- did you test with throttle held open? Has bike been sitting idle for years?

[posplayr]

Yes,repeat quick test with lights on- even though we can sorta anticipate results with what you said.
Odd that this extra load (headlight draws about 60 watts) causes a rapid falloff in output.
The stock setup had one phase of alternator controlled by headlight on/off switch- charging output was reduced when headlight was off. But this connection can be a problem even if alternator and R/R (rectifier/regulator) are OK.
While you're retesting,take a look at your R/R and see if wiring appears to have been messed with.
Those compression readings are low- did you test with throttle held open? Has bike been sitting idle for years?

I'm guessing the stator starts out ok then starts to short. If we get away results maybe he can heat it up and do phase to ground tests in revised phase b to confirm.

[tom203]

Yes, or maybe stator "return" line from headlight switch is shorting out somewhere. He's got to do some digging! Probably this is a good time to run stator direct to R/R to eliminate that possibility.

[posplayr]

Yes, or maybe stator "return" line from headlight switch is shorting out somewhere. He's got to do some digging! Probably this is a good time to run stator direct to R/R to eliminate that possibility.

Yea, cut it off, test it (phase B) and then reconnect if it seems good.

[ATM]

Help please. A manual question for you.

To do some of the things you guys are talking about, I will probably have to obtain a repair manual if I want to try to do this the right way. Where can I get an excellent repair manual for a fair price that will give me good visuals of some of these electrical components, their respective locations, and how to get to them?? PDF version?? Hard Copy, as from a book?? Off of a CD?? Off of a DVD??

[tkent02]

Should be one in here somewhere:

http://members.dslextreme.com/users/bikecliff/

[posplayr]

To do some of the things you guys are talking about, I will probably have to obtain a repair manual if I want to try to do this the right way. Where can I get an excellent repair manual for a fair price that will give me good visuals of some of these electrical components, their respective locations, and how to get to them?? PDF version?? Hard Copy, as from a book?? Off of a CD?? Off of a DVD??

If you want to learn about your bike that is fine, just don't assume you know what we are talking about because you have a manual. The world has moved on since 1983 when the manual was probably published. It is dated.

[tkent02]

I don't think any manuals cover charging system testing very well for the average DIY guy with only a multimeter and kindergarten level electrical knowledge.

Posplayer's tests are what you need.

[tom203]

Yeah, a manual is pretty useless on charging issues. This funhouse has got the goods,updated for this century! Remove side covers and take some pics and post here, so we can point out the key stuff. Or head to a site that shows part drawings, like this... Pick year,model and get intimate

http://www.alpha-sports.com/suzuki_parts.htm

Repeat the quick test (lights on) ,post results,and let's go from there. Charging issues are ridiculously common. Avoid buying stuff till you've narrowed down the problem.

[ATM]

Update: Losing power at idle when lights turned on

Thank you guys for the different links you have provided. They have been helpful. Using the workshop/repair manual wiring diagram as a reference, I took out the battery, the battery holder, the starter solenoid, a main fuse box, a smaller fuse box with a single fuse, the turn signal relay, etc. I analyzed the wiring diagram to get a better understanding of how things are supposed to work. Most of the wires were color coded correctly except for those orange wires coming out of the main fuse box. Upon inspection, there was evidence of arcing as well as corrosion at the conductor connection between the stator and the rectifier for the phase of the curcuit that runs when the lights are turned on.

Other things:

1) I tested the rectifier with the multimeter. The diodes for all three phases seem to be working properly.

2) I checked the resistance for the stator phases. They were all about the same.

Gotta leave now. Will write more later.

Aaron

[ATM]

HELP. Three black lead wires near battery. What do they connect to??

After taking a closer look at the electrical wiring, I am now putting things back in place. There are three black lead wires. What does the third wire connect to?? One goes to the negative battery post. One goes to one of the locations that keeps the battery holder in place. What does the third lead wire connect to?? It appears it may go to the negative battery post (which means two of the three leads connect to the negative battery post). What do you think??