1-4 Coil bad, but what was the cause?

I have a 81 XJ650 Maxim that I picked up cheap and is in non-running condition. I the electrical was a mess on this thing. The PO had the main fuse bypassed with a wirenut! I put a aftermarket fuse box on it and found another jumper soldered in the headlight assembly that always kept the headlight on, so I removed the jumper and the headlight, turn signals and horn all seem to be good now. I tried my first attempt to start the bike after rebuilding and cleaning the carbs, and it's a no go. Plugs 1 and 4 would randomly spark, with just the ignition on after trying to start it and you can hear sparks arching somewhere else on the bike. I have narrowed it down to a bad 1-4 coil, the ohms on the sparkplug wires is an open so I guess thats why it was sparking randomly. My question to the experts here is, what causes a coil to go bad and ohm as an open? Its like it was a fuse and is now blown. Could a bad TCI cause this, or is this a result of the PO's fix of bypassing the fuses with wirenuts? Anything else I should check or be aware of?

Thanks guys and gals.

Dave

81 XJ650 Maxim - It will be a runner if I have anything to do with it.

 

First of all, did you measure the resistance of the coil secondary with the caps off or on? The caps are 5K resistors and sometimes they go bad. If you measured with them on, take them off (they unscrew from the wire ends), and check them and the coil separately just to be sure it is a bad coil.

If it is, most likely it's a broken solder joint where the wires connect to the coil windings, although it could be a broken coil wire itself. Coil wires are very fine and can break spontaneously after 30 years of heating, cooling, and vibration. The potting compound inside sometimes reacts with the wire and solder as well. Also, fine cracks develop in the coil casings over time, which let in moisture that corrodes the windings.

 

Thanks for the reply SQLguy.

Yes, I tested them with and without the caps and I still measure an open circuit. The other coil, without the caps is 11k Ohms, So I know that one is ok.

I just want to make sure that if I replace the coil, I'm not gonna fry that one too.

 

A bad TCI could burn out the coil primary, but generally not the secondary.

One thing you can check, though, just to be safe, is to disconnect the two-pin plugs at the coils (with the TCI connected, and ignition off), and measure resistance between the Orange and chassis on the one, and then between the Gray and chassis on the other. Both should be high resistance.

 

Good question, but I think they just burn out. I've had one bad coil on my 650 Seca and one bad coil on my 650 turbo.

 

Had a secondary on my 750J jump from 11K to 80, so something happened. Ended up swapping in a set of Dynas. Made a writeup out of it, too.

 

Thanks everyone for there posts and help. I ended up getting a used coil off ebay and it ohms good. I was still having a problem with it getting spark. I checked all of the electrical connections in the electronic ignition. This is what I found:

With a sparkplug out to watch for spark, I noticed it sparked once when I turned the key on and again when I turned the key off, no spark when i hit the start button, but I can hear a spark or some electrical jump somewhere. I hit the forums for answers. Many problems similar to mine pointed towards the TCI. I really didn't want to replace the TCI so I double checked the connectors to the TCI, cleaned them, etc... If I put my ear near the TCI while the ignition was on i herd sparking. Not good. So I pulled the TCi removed the cover and saw all kinds of poor solder joints. I touched up every solder joint on the board and reassembled. I put the TCI back on the bike turned on the switch and i no longer heard the sparking from the TCI. I hit the switch and she roared to life. I am so happy it was an easy fix. I don't know if this would help anyone else who may have problems with their TCI and can be fixed with reflowing the solder joints on the TCI circuit board.

 

Excellent! Thanx for posting wha fixed your problem!