Could my coil drain my battery, why?

Rook32432

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Hello, while I’m new to posting I am a veteran lurker.
I have previously owned a 79 but unfortunately had to sell. I now own a rather troublesome 81.
When I first bought the bike it was completely stock apart from the missing battery box.
The original ignition black box was shot so I fiddled with the gonzo swap, before that too failed.

I opted to swap to the more familiar points setup and run an aftermarket dual coil.
Shortly after getting it running I “simplified” the wiring down to accept a ganged fuse plate. From which I run power to lights and ignition etc. I tested the stock charging system before and after making my changes. I used the tcbros “chopper wiring” diagram as reference. The bike ran well for a few months but I would occasionally get a hiccup in the spark it seemed, so I revisited the points and timing and carbs and wiring and drink and will to live and so on and so forth.

Finally it seemed to run smoothly(as smooth as you would expect). Taken on many 100 miles trips and I trusted it enough that I’d ride almost every day any chance I got. Start up no issue 1-3 kicks(3 on rarer occasion). Advanced well enough for me and would reach speeds of 90+ (I dared not push my luck), you get the gist.

Well about two months ago I was out and about when it began to bog at higher rpm, from 1/2 to full throttle. So I checked my advance and adjusted the springs as they seemed to not be returning consistently , I cleaned the carbs and checked timing etc which all seemed to fix the issue for some time.

Then a few weeks ago the problem came again, this time a complete loss of ignition. I let it rest and I limped it home with erratic spark. Days later I thought of the easy stuff first and cleaned the carbs checked the floats and checked spark replaced the battery and coil. It kicked over as easy as before and ran well as before. Until today..

I rode about 5 miles and the problem began again, i thought “new coil, new battery? Surely the gods would see my hard work with pride and spare me this time..” no, no they did not, the bike kicked one good time only to stumble and die.

Suspecting the coil I tested it and it tested 3.1 ohms on the primary and 15k at the wires without the plug cups. I reattached the cups and tested the secondary again with 30k ohms.

Another puzzling bit is that with the key on I noticed the battery would go from 12.60+ v to 5-7 v. I tested the fuse plate until I found that the wire to the coil seemed to be drawing too much voltage and the battery was draining. I reconnected everything except the wire to the coil and the battery tests at 12+v.

Is it normal for a bad coil to drain a battery, if so why? Is it possible there’s some gremlin hiding in the tubage of my frame causing a bad ground?

So the bike sits, sweaty and silent. Any help at all would be greatly appreciated.
 

arcticXS

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Your coil should draw around 4 amps, as it has 3 ohm primary resistance. Any more than 4 amps must be caused by a wire shorted to ground. However, a weak battery may be pulled down to lower voltage by a 4 amp draw. A healthy battery of 5-6 AH or more should not even drop below 12 V with only the ignition on.
 

jetmechmarty

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Are you sure your charging rotor is OK? A big voltage drop when turning the key on is a sign of a shorted rotor. It will run like crap when the rotor is going away.
@rickthefix lives very near to you. Among other things, he can fix your TCI box.
 

Rook32432

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Are you sure your charging rotor is OK? A big voltage drop when turning the key on is a sign of a shorted rotor. It will run like crap when the rotor is going away.
@rickthefix lives very near to you. Among other things, he can fix your TCI box.
Well I had thought of the charging system, perhaps worn brushes and planned on testing the harness. But I was thrown off once I saw the voltage drop and just rabbit holed it to the key then to the fuse plate, then to each wire. I did however, possibly regretfully, disconnect the charging system completely attempting to isolate the draw to the coil. No change with the rotor and reg disconnected. Once I hooked everything back up and removed the wire to the coil, battery voltage climbed up from 4-7v to 12.5+ with key on.
 

mike in idaho

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You might check for voltage drop in the ignition switch and the wiring, between the battery and the coil. Set your meter to "volts", connect one lead to battery positive and the other lead to the 12 volt coil connection. With the switch on and one set of points closed the meter should read one volt or less if things are in good shape.
 

Rook32432

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IMG_5648.jpeg
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Here is my wiring diagram and the tcbros instructions I yoinked for reference. Forgive the sloppiness, I’m drawing from memory at work. Unplugging the wire from the ignition to the fuse plate then key on, the battery held its voltage steadily increasing from the drained amount. Plugged the ignition wire back into the fuse plate then keyed on and the voltage immediately dropped to roughly 6v.

Removed the charging wiring only, and still with ignition wired up and key on the voltage came back down to roughly 6v.

I will go through it once more when I’m with the bike and see if I’m forgetting something.
 

Rook32432

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It was mentioned above the battery could be weak. Have you taken it to a parts store and had it load tested?
I did not. But the battery in question was a replacement from the parts store just days before. I figured it’d be awfully unlucky for two batteries to be bad within a couple months span. Then again I am the guy with broken bike so maybe. I just thought it unlikely and more likely to be something with the bike either I did wrong or a faulty part. But so goes the theory on assumption.
It was mentioned above the battery could be weak. Have you taken it to a parts store and had it load tested?
 

5twins

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For points, you want a coil with about 4 to 5 ohms on the primary. A 3 ohm coil will draw too much power, burn the points up quicker, and possibly damage the coil. Maybe that's your issue. It wouldn't occur immediately but would happen eventually, kinda like your problems.
 

Jim

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I did test the new battery with a multimeter and out of the box it read 13v and some change.
If you have the stock headlight, a quick and dirty check is to turn it on high beam and start the timer. If it's a standard size (for the XS) battery, you should be down about a half a volt at 30 min. In your case, disconnect the coil first.
 

Jim

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For points, you want a coil with about 4 to 5 ohms on the primary. A 3 ohm coil will draw too much power, burn the points up quicker, and possibly damage the coil. Maybe that's your issue. It wouldn't occur immediately but would happen eventually, kinda like your problems.
Well first off, the TCI bikes use a 3Ω coil and they don't pull the battery down like yours is. Same with most other electronic systems that use a 3Ω coil... so I doubt that's the problem. My TCI bike with a 3Ω coil drops about a half a volt when I turn the key on.

5twins is correct though in that your points and condensers won't be happy running a 3Ω coil. That's gonna shorten the life of both the points and condensers.
 

GLJ

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I have a question. You talk about your coil as singular (dual output). But you are running points. How are you doing that?
Normally a points bike has 2 coils.
 

Rook32432

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Well first off, the TCI bikes use a 3Ω coil and they don't pull the battery down like yours is. Same with most other electronic systems that use a 3Ω coil... so I doubt that's the problem. My TCI bike with a 3Ω coil drops about a half a volt when I turn the key on.

5twins is correct though in that your points and condensers won't be happy running a 3Ω coil. That's gonna shorten the life of both the points and condensers.
This makes sense and feels like a step in the right direction. Thanks to both of you.
I will going back to the parking lot where my bike is shorty to toy and test.

Any recommendation for a dual output coil with points?
 

Rook32432

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I have a question. You talk about your coil as singular (dual output). But you are running points. How are you doing that?
Normally a points bike has 2 coils.
My neanderthal cave painting of a wiring diagram is above. But there are many more threads discussing dual output coils and points. In short, both points + single condenser to one side of the coil and 12v to the other side.
 

Jan_P

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I am thinking weak charging
I would start measure charging Across battery --simple to do. .. and you might consider stall a Voltmeter At least temporarily .. So you can se what is going on..
Shorting yes that happens but can be at many places.
 

Rook32432

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JFI sake and some clarity.

When you changed back to points did you change the stater, (from the 80-83 type for a 70-79 type), or the wiring at the brushes and associated wiring from the Factory Reg/rect.

Did you change the factory Reg/rect to an aftermarket 70-79 points SS Reg/Rect, (if yes where was it purchased from), or to an Automotive Regulator with bridge Rectifier.
I kept the stock charging system as it tested good. That is to say it was charging the battery.
 
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