Battery failing to charge

[Chicago Ted]

Hey everyone. My '81 CB750F left me high and dry beside the highway the other day. My friends and were riding about 30 miles to a festival. Along the way, I smelled something like burning plastic, but, I could not identify what was causing it, so, I didn't stress about it. As we left the festival, I hopped back on, thumbed the starter, but it wouldn't turn over. Without other options, I push started it, and got rolling. About 20 miles down the road, I lose all power, and I stop getting sparks to the coils, I believe.

I trailered the bike home, and hooked it up to my battery tender. After charging overnight, it will start up without issue. After determining that it was a charging issue, I opened up my alternator.

I tested the stator, and got good readings on all of its wires. I put my two ohmmeter leads onto the two rotor rings, and those showed no connectivity. This leads me to understand that my rotor needs replacement, yes?

I then moved onto the Regulator and Rectifier. I unplugged the unit, and first tested the ground(green) to diode(yellow) connections. With the ohmmeter leads in one direction, with my meter set to 2k ohms, with the diode symbol next to it, I read ".795" for all three yellow leads. In the other direction, it appears to have no connectivity. The same was true when I tested the connection of the yellow leads to the white lead. .795 in one direction, no connectivity in the other direction.

My Clymer's manual tells me that there should be 2k+ ohms in one direction, and low resistance in the other direction. If this is the case, are my readings indicative of a faulty rectifier?

Lastly, I tested my regulator. I started up the bike, and put my voltmeter leads on the battery. At idle, I see about 13 volts. When I rev it, it gets up to about 14.7 volts, and then no higher. I think this indicates a functioning regulator.

However, that makes me curious. If my rotor has an open, and/or my rectifier is fried, how am I getting this higher voltage to my battery? Is my charging system busted, or not?

Thanks as always for your help here, guys.

[rich]

Never seen 14.7 at the battery with a bad rotor. Recheck the rotor and take the battery into an auto parts store for testing.

[Chicago Ted]

Well that is reassuring, I suppose. The battery seemed happy to take a charge off of my battery tender. I suppose I could bring it into the auto shop, but is there a way I could test it myself?

What about my reading off my rotor? Is it possible a functioning rotor would not show connectivity between the two rings?

[rich]

On the rotor you should be checking for resistance not continuity. 4 ohms resistance between the two rings the brushes ride on is what you want to see.

[shinyribs]

Did you find the source of the burning plastic smell? Since you're getting 14.7 volts your bike is charging just fine. Since it's not passing 14.7 volts your regulator is fine.

Probably fried a connector. The parts are all working, but the old wiring just isn't carrying the charge from your charging system to your battery. Diagnosis via internet,of course. I'd check your connections. They're famous for dying on these bikes. Especially the charging ones.

[Chicago Ted]

When I say I was checking for connectivity between those rotor rings, I was using my ohmmeter. I know I am supposed to get just a few ohms, but it read as "infinite", same as if the leads from the meter weren't touching anything at all. I figured this meant no connectivity, and therefore a bad rotor. Must my reading be wrong, if I am getting 14.7 volts at the battery when I rev it? Perhaps I need a different meter. Any recommendations?

[TOOLS1]

If you are getting 14.7 volts, you are charging. Have you checked for a short that might have drained the battery?
TOOLS

[Chicago Ted]

I have looked at my wire harness, and did not see anything obvious. That would explain the burnt plastic smell, however. Are there any usual culprits that I should check? Might it be behind my headlight?

[TOOLS1]

Could be anywhere. The way to check is to disconnect the negative battery cable, and hook your Ohm meter, or even a test light (I use a test light) between the negative wire, and the battery terminal. If the light comes on with the switch off, you have a short. Start disconnecting things until the light goes out. That will be the circuit that has the short.
TOOLS

[Chicago Ted]

Alright. I will check for that shortly. As an additional reference point,
after topping off my battery on the battery tender, and then leaving it
disconnected for 24 hours, what should I expect the voltage across the
battery terminals to be?

[LukeM]

If you have 13.5v on your battery at idle, and 14.7v at 3000 rpm or higher, your rotor and stator are working OK. Also, I presume your regulator is working OK, as your rotor/stator puts out almost 16-18v at high idle.  As with most electrical systems, check your connections and grounds, making sure everything is corrosion free and ground connections are good and clean.

+1 for burnt connectors.  Both my CB750 and my current Shadow had burnt connectors on the yellow wires. Once corrosion starts building up on those, it's just a matter of time before they fail.  More corrosion = more heat buildup, which means more corrosion.  

Standard disclaimers apply for the following information (for reference only, based on my personal experience, no warranty expressed or implied, etc):

If you are SURE the rotor is good, and the reg/rect is good, nothing prevents you from cutting out the yellow wires' connector, and soldering the wires directly.  You lose the easy in/out replacement function, but you gain a much more stable connection between the two pieces.  Make sure you splice the wires properly, solder them with good rosin core solder and a good hot iron, and heat shrink over the new connections.  

For reference, CHECK THIS LINK.

Luke M

[Chicago Ted]

At idle, I want to say that I was reading something closer to 12.5 across the terminals. Is that problematic? This morning, with the bike off, I read something similar across the terminals.

[shinyribs]

A battery sitting at an idle state that reads 12- 12.5 is a healthy battery. Not meaning while the bike is idling, but the battery just sitting there.

[Chicago Ted]

Oh, good then. I check it again this morning, and it still told me 12.5, with the ignition off. If I did have a short in the wiring, would I expect to see the battery draining while the bike is off?

I will have time in the next few days, I expect, to get poking around my wiring harness to check for shorts.