Blowing coils, here are my ohms readings

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zebugman

New Member
Sep 17, 2013
10
0
0
Amurica
I'm blowing coils. The first went out because most likely it got wet. Bought a replacement, and within three miles, the motor flamed out, and the coil tested open.

My spark plug gap is 0.027.

I took out the coil and unplugged everything.

Here are all my readings.

Coil:
Red on black, black on white: 2.4
Red on blue, black on white: open
Vice versa: open

Interestingly, when I touch the large metal sides of the coil holder that wraps the magnet, I get 0.6 ohms.


CDI:
On 200k, black wire to boot, 6.9
Blue wire to boot: open
Blue wire to black wire: open one way, 185k the other way.

Not sure what they need to be. I'm going to RMA the coil though. Could something else be causing the problem?
 
Last edited:

crassius

Well-Known Member
Sep 30, 2012
4,032
158
63
USA
you could have a bad solder joint at the top of the coil where the winds ground to the armature - try reading from blue to the wire right there (scrape a bit of enamel off the wire to get a good connection)

I have seen a lot of coil failures caused by a bad left main bearing on the crankshaft that allows the rotor to smack into the armature and loosen that soldered ground connection. Check this by prying up a bit on the end of the crank with a screwdriver to see if there is any movement there.
 

crassius

Well-Known Member
Sep 30, 2012
4,032
158
63
USA
if the armature is properly seated on the circular area of the case casting, there will be about 8 thousandths clearance between rotor & armature

you would pry up on the end of the crank where the rotor nut is to check for a sloppy bearing
 

maniac57

Old, Fat, and still faster than you
Oct 8, 2011
4,484
22
0
memphis Tn
ANY up and down movement in your mains is bad. I would replace it if there is slop before you get a rude surprise at 30mph....
Shaking piston chunks out of your pipe is no fun.
Not to mention all your tender bits at risk from a motor going bang!
 

zebugman

New Member
Sep 17, 2013
10
0
0
Amurica
Well I tried prying it and o didn't really get movement. All I got was where it was flexing the crankshaft. But no space or gaps or anything.
 

maniac57

Old, Fat, and still faster than you
Oct 8, 2011
4,484
22
0
memphis Tn
You should be good then. I would think if the mains got that bad, there would be other signs as well.
 

maniac57

Old, Fat, and still faster than you
Oct 8, 2011
4,484
22
0
memphis Tn
Possibly water killed it. SEAL THAT COVER!
Every single customer bike I have seen with a blown coil was due to water.
Every. single. one.
Never seen the solder defect yet, but I know it happens.
 

maniac57

Old, Fat, and still faster than you
Oct 8, 2011
4,484
22
0
memphis Tn
Light coating of RTV silicone on both sides of the gasket, and a glop around the wires where they enter the case.