Topic: Electrical surprise

[cuz]

Went to ride my CB750 to work this morning and was surprised to find an electrical issue.  When I turn on the ignition I get a really dim headlight and indicator lights on the dash and there is not enough voltage to crank the engine.  The battery reads 12 volts.  Thought I would ask for some advice before I start trouble shooting.  I didn't have any time to check fuses so it could be as simple as that.

[flybox1]

Just 12v exactly?

Thats a battery at 50% charge...or less.

[tlbranth]

Easiest thing right off would be to take the battery to Napa and have it load tested.

[cuz]

Ah, I wasn't aware of that.  What should it read at full charge?  I should also ask what it should read while running at higher RPM so I can check to see if I am having charging issues. 

[flybox1]

any 12v battery should read better than 12.70v
trickle charge it overnight.  after 2+ hours off the charger, take the reading again.
If you dont get a reading over 12.7v, do as tlbranth suggested and have it tested.

[Gene]

Yup - battery.  If it's a wet-cell make sure you have liquid in it.

[magnum56]

+1 for trickle charging the battery ... and testing. 

As to charging level voltage at higher RPM s ... I would expect to see no more than about 14.5 V
If the battery is not fully charged (or bad) ... you will may not even see close to that.

[cuz]

And it was the battery.  New battery installed, she fired right up again. Thanks for the suggestions fellas. I did a voltage check at 4000 rpm.  It was reading around 15 volts, peaking at 15.2 volts at times.  According to the manual, 14.5 volts should be read at this rpm.  I'll try to adjust the voltage regulator to bring the peak voltage down to 14.5. If unsuccessful, I guess it's time to update to a regulator/rectifier combo. Any other things I should check while I'm looking at this system?

[TwoTired]

And it was the battery.  New battery installed, she fired right up again. Thanks for the suggestions fellas. I did a voltage check at 4000 rpm.  It was reading around 15 volts, peaking at 15.2 volts at times.  According to the manual, 14.5 volts should be read at this rpm.  I'll try to adjust the voltage regulator to bring the peak voltage down to 14.5. If unsuccessful, I guess it's time to update to a regulator/rectifier combo. Any other things I should check while I'm looking at this system?

Don't touch the regulator adjustment.  It doesn't work like you think it does.  And I'd venture to guess a replacement will not help your situation.


Compare the voltage at the black wire of the regulator with actual battery voltage.  The regulator uses the black wire to sense battery voltage and to supply the alternator field with power.

If the black wire feeds lies to the regulator, it will think the battery is low and keep the alternator pumping power to the battery .  Overvoltage is the result, and through no fault of the regulator.

Muck with the factory adjustment, and you'll have a difficult time making it right again.  The regulator only turns the alternator on and off in relation to the battery status that is reported to it.

[cuz]

With the bike warm and revving at a constant 4000rpm I was measuring 14.92 volts at the battery and 14.14 volts from the black wire on the regulator.  What do you suggest?

[evinrude7]

i suspect something is off.  like resistance from bad connections.  i just went through this.  mine was the vreg.  calj or tt should pipe in soon with sound advice. 

[cuz]

I looked through my tickle trunk and found another regulator.  With it installed the battery measures 14.45 volts at 4000rpm. Now with the new battery I should be good to go. Interesting observation... The cover on the voltage regulator I removed was spun 180 degrees compared to the cover on the one I replaced it with. Someone must have tried adjusting it at some point.

[TwoTired]

I looked through my tickle trunk and found another regulator.  With it installed the battery measures 14.45 volts at 4000rpm. Now with the new battery I should be good to go. Interesting observation... The cover on the voltage regulator I removed was spun 180 degrees compared to the cover on the one I replaced it with. Someone must have tried adjusting it at some point.

That makes sense.
Mucking with the regulator to try and compensate for system voltage loss typically makes for battery overvoltage conditions after the system loss issues have been corrected or improved.

The battery voltage is something of a moving target, which is why it becomes challenging to adjust the regulator.  What you want is a known fully charged battery, an alernator capable of max output (revving), minimal system loads, and known acurate sense voltage to the regulator.  This way the vreg can protect the battery from overvoltage loss of electrolyte through electrolysis (looks like boiling).  This limit is typically 14.5v for a lead acid starting battery.

Under full system load, the black wire distribution of 12v from the key switch should not be more than 0.5v less than that found at the battery posts.  Equally important are voltage losses between the battery NEG terminal and the green wire distribution buss.  Losses in both these paths can result in good vregs powering the alternator into errant levels.

I think I've said before that the vreg is really more of an alternator limiter which keeps the battery from cooking.  The system voltage follows the battery state. "Regulation" follows battery state which the vreg tries to maintain at full levels when the rpm allows the alternator to overcome system loads.

Cheers,