TOPIC:

Hey All,

When I try to read voltage at the battery with the motor running it jumps all over the place and is unreadable. Reads 12.56 not running. Is this normal? I was trying to check charging system.
1979 1000 LTD
Thanks Dave,

If its a digital meter try holding it as far away from the motor as possible .... mine did that till I moved it to the tale section.

Tried moving it as far away as possible, still no change

You may have a bad diode in your R/R.
Steve

I would look for a bad battery ground connection and or loose engine to battery ground. Etc.
Of course you could just have a loose connection in the electrics box.

To expand on what the others have said, your Regulator-Rectifier (R/R) is what sets the approximate charging voltage. For an alternator, it's an alternating current (surprise surprise), and it basically cuts off when the voltage get too high, and cuts back in when it gets too low. But since it's always cutting in and out, it will jump around.

Your battery, however, acts as a capacitor and will smooth out the voltage. This is why you should never run an engine without a battery. You could start it with the battery and then take the battery out, but the voltage spikes will likely damage the electronic components. Battery eliminators are designed to fix this by providing a capacitor to act as if the battery was there, even if they don't provide any power.

So if your R/R is going, it might let too much voltage though and it can spike. If some components (battery, R/R, or the engine) aren't grounded properly, your battery will not act as a capacitor very well, and could give the same symptoms.

Digi multimeters tend to do that, a lot of RFI 'noise' gets picked up. Old school analouge moving needle is best here for a running motor. Some of the later GPz have a push button analogue volt meter on the tacho. ..also it works when running and on the move too.

12.5 seems low to me, maybe 12.7 battery or 13.5 to 14.5 ish Volts running more acceptable. Your battery maybe aging a bit, this is the time of year you tend to notice it.

My problem is I can't read voltage with the bike running and a new battery is slowly loosing charge. I have 50 v AC at all three yellow wires coming from the alt at 4000 rpm 13 - 14 volts at idle. The ground tests good in that plug but the body of the rectifier isn't grounded. I'm thinking bad rectifier? BTW my meter acts normal when I read voltage at the alt plug, disconnected from the rectifier bike running

When the weather changes, weak batteries take a dump.
Steve

...and a new battery is slowly loosing charge. I have 50 v AC at all three yellow wires coming from the alt at 4000 rpm 13 - 14 volts at idle. T

The 13-14 V at idle seems all good.

Not so common on bikes, but one can have a 'parasitic' battery drain, ie from as simple as a conductive deposit on the battery casing , to a car and the interior light stays on, Get a meter and set it to milliamps. (1/1000th amp)

Then disconnect either terminal of the battery and use the ammeter in line and see if there is any current flowing . Use the meter between either battery terminal and the battery lead (disconnected) Any current floing here is a problem. If not, clean the battery casing with bicarbonate of soda or similar mild alkaline.

You might just have a duff 'new' battery. You can' drop test' it with the right tool. or just crank the motor and see the voltage doesn't go below 10 volts ish.

Do not put the ammeter 'across' the battery put it in line

ie battery terminal (meter) load...if any = good

terminal (meter) terminal = bang!

POOF?
Steve

Yes 'poof' been there done it, usually meters have an fuse.

I remember as a keen but naive teenager bridging (across terminals) a Renault 5 battery with a thin jumper, within a second the wire was glowing red hot and I had a straight line burn across both palms of my hands. Maybe more appropriate on a body modification forum.

Than as a rookie auto electrican, trying to be 'competent' , a borrowed 'snap on' spanner got burnt doh!

Every occassional nugget of wisdom here came from failures, learn by my stupidity, trust me.