Huge voltage drop! Need diagnosis help


XS650 Enthusiast
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New Jersey (South)
Hey guys back again. Bike started acting funny and cut out on me while I was cruising lucky enough I was only about 1/2 mile from home.

I added quite a bit. H4 headlight mini tach and speedo from Mike's both backlit. Also added in a mini volt meter and 3d printed a mount for it to my tach. Replaced my reg/rec and got a 3.3 ohm dual output coil.

Started by checking my battery when I got home and it was still at 12 volts so reg/rec is working (bike was off)

* As a note I believe I cut out because old gas and shit got in my carbs. I've since cleaned them.

I think when I added the voltmeter I may have killed my battery somehow because now when I turn on the ignition I drop from 12+ volts to about 5 volts. Also as a note I have everything other than my backlit tach on a switch which stays off during starting. So far I only kickstart but I do have an electric still which doesn't fire up at all after I turn the ignition.

I've heard that a battery can still read 12+ volts with a dead cell but as soon as it's put under load it will drastically drop.

So far I've only got it fired on idle once since and it's run really rough...barely alive.

I did a compression test with a harbor freight gauge and got 90 psi after 10 kicks wide open, so with their error it's should be about 125 each cylinder.

Anyone have any ideas? Or any more info you'd like me to gather?
I would charge the battery
Start it and measure charging Voltage across battery
12 V Off is on the low side
I would charge the battery
Start it and measure charging Voltage across battery
12 V Off is on the low side
12.0 V without any load is pretty much 100% discharged. A healthy, fully charged battery should have 12.6 V when measured with a decent meter (i.e. a meter with a high internal resistance) This basically means that battery voltage has limited usefulness as a measure of charge status.
And the charging voltage needs to be close to 14.0 V for a battery to charge to 100%
There is a routine to follow doing it if one should do it according to best practice

google translation from Swedish a company that sells car parts
There are nowdays these closed Battery thar can behave differently
But on these bikes there is the Voltage OFF and Voltage ON
Especially at the coils and ignition.

I had a Weak charging on my first bike and never saw 12.5 V and with lights on the battery would go down.
12 V at battery with the old Acid Battery ca 11.5 at the oils and it will start.

Charge the battery fully or as much as possible.
Then let the battery rest for 24 hours to allow the battery to fall asleep. (Nothing must be connected to the poles during rest)
Then check the voltage with the multimeter.

If the voltage exceeds 13 Volts, the battery has been overcharged on occasion and then lost capacity. (The battery has been destroyed prematurely)

If the voltage falls below 12.6 Volts but not below 11 Volts, the battery is old and exhausted or sulfated due to poor charge levels on several occasions. (Sulphation is a chemical reaction that degrades capacity)

If the voltage falls below 11 Volts, the battery has severe sulphation or one or more short-circuited cells. (Short circuit may be due to manufacturing defects or the battery being exposed to shocks or vibrations)

A fully charged healthy battery at rest has a voltage between 12.7 to 12.8 Volts.

ATTENTION! Just using a multimeter does not give complete information about how much capacity is left in the battery, but can give an indication of what the battery has been exposed to.
Thanks @arcticXS I just charged it a bit put my meter on it and it's reading 13.35? ( Disconnected and off the charger)

Started at 13.35 and slowly dropping now it's below 12.6 after a couple minutes

Depending on charger you you need to have it connected for some time Mine chargers have LED s hat come on Green
This winter I have charged about once a month and it takes up to two hours before green light comes on
This is with good batteries. More or less full starting
I Used an Autocharger for many years On the acid type batteries from Discharged .It is to high current but I got away with it.
What is the history of this bike? How long have you had it? Stock alternator. Is the rotor original? If so, it may have failed (shorted). That will drain the battery right now. Check that first.
I've had the bike about a year did a top end on it. It has the pma kit on it with pamco. I have had the pma off before to get my crank fixed. My other thread details everything I did to it. 1983 xs650 and the infamous whirring noise

Yes I have a trickle charger with an LED indicator and have left it on all day before and it never went full.
Some context. Symptoms you describe in post #1 and subsequent findings revolving around the battery and charging of the battery, would constitute the hypothesis, the battery is bad and needs to be replaced.

Symptoms described, (missing, lack of power) can be caused by a low voltage battery. Reread post #9.

Could be by chance the battery problem showed up after adding the volt meter or could be you have a short that ha caused to battery to fail due to age. Straw on the camels back and such.

Again, start with a new battery and go from there and then go through a maintenance check of the charging system including the Regulator and rectifier and it wouldn't hurt to go through the whole loom and switches checking connectors, (bullet, couplings), and all grounds. Clean or repair or replace, (if needed),

And to paraphrase Grizd1, (RIP), Electrical problems can look like fuel problems. Check electrics first. I think his phrase describing how he was tort was a little more colourfull
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Started mine up yesterday after it sitting for a while, 79 degrees f, had to. I've got an analog voltmeter wired in between the clocks. She read around 10 volts with the key on. Yeah, there may be a little disparity between what the volt meter reads and what the battery reads direct. The electric starter wouldn't fire her off after cranking for 10 seconds or so, so I kicked her. Fired right off.
Check that battery.
Going through my electrical connections as everyone suggested while I wait on a new battery.

Old battery after charging and sitting twelve hours dropped to 11ish volts. Dead.

Alternator and windings look good. Reading all 0.8 ohms and connections look good. Still a nice copper color to the windings too. No burnout.

Ground between the battery and multiple points on my frame and engine looks solid. No drops read same everywhere (although with the 11ish volts)

Time for the tedious process of looking at all my other connections. Led front and rear turn signals, tail light, headlight, horn, control switches, backlit speedo, backlit tach, volt meter, and everything in my fake oil can ( fuses, ignition, starter button, and the shutoff switch which all the above accessories are connected to)

Btw do you guys think that's a good enough area for my reg behind the engine like that. It's the way it was when I bought the bike but I wonder now


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While running at 2500 to 3000 rpm you need to see 14.1-14.5 v on the battery. That equates to 13.6 to 14.0 v after your ignition switch. There are detailed checks outlined in the tech section but a weak battery will cause you to chase your tail searching electrical problems. First verify the health of the battery...
So i just did a little short circuit test using the old battery (didnt want to immediately ruin another one) removing fuses i found that i have one wire running up my tube to my headlight turn signal controls cluster thats got a short on it. fine removed it from the fuse.

still another short somewhere....

found that removing any one of the pamco primaries eliminated the short. The power from my fuse box going to the pamco and coil is on the same fuse as my solenoid (removed the solenoid from the circuit and its still lit up)

removed my coil and tested it...all good

I think i have a short somewhere either in my pamco wire leads or on the board itself. considering any time i could get the bike running it was really rough could this be intermitent or some other kind of short on my pamco?

i think the first short on my headlight wiring was the cause of the battery death but im unsure if im being dumb about the pamco primaries causing a short.

Maybe its time to replace my pamco...i have no idea how old it is but it was on the bike when i bought it.

any thoughts? (before i rewire my whole system lol)


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