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No spark. are my coils bad?

  • Thread starter Thread starter Nickel
  • Start date Start date
N

Nickel

Guest
So I recently replaced a chopped up harness on my 78 GS750E with an unmolested one from ebay. I THINK i put everything in the right place based on trying to learn how to read a wiring diagram. I have no starter switch or kill switch as the previous owner hacked them off. Previously, I was able to get spark. Now, I cannot. The only way I can currently get the motor to turn over is jumping the starter relay.

I've read around and watched some videos and learned about the primary and secondary tests for the coils. Primary on both give me a 3.5Ohm. When I test plugs #1 & #4 I get nothing. When I test #2 & #3 I get nothing. Does this mean both are bad? How plausible is that? Would jumping the relay prior to the harness swap somehow have cooked them?

Can anyone help? Not sure what other info you might need to help diagnose, but I have the knowledge of a 5 year old here.
 
You're probably doing the secondary test wrong, now way both the secondary's on two coils went, most stock coils will outlast the bike for sure. You can't cook them by jumping the relay. Maybe you didn't get something wired in correctly from the harness? recheck the secondary with the coils, there's a great youtube video that describes it and the proper use of a volt meter (I'm at work so I can't post the link), check that you're getting at least 11 volts to both coils via the orange/white wires.
 
azr said:
You're probably doing the secondary test wrong, now way both the secondary's on two coils went, most stock coils will outlast the bike for sure. You can't cook them by jumping the relay. Maybe you didn't get something wired in correctly from the harness? recheck the secondary with the coils, there's a great youtube video that describes it and the proper use of a volt meter (I'm at work so I can't post the link), check that you're getting at least 11 volts to both coils via the orange/white wires.
Click to expand...

I saw a forum member here say to take both plug wires off their plugs for each relay and put the voltmeter on 20k under Ohms and you should get a reading in the 12 range. Wouldn't that bypass harness completely?
 
Nickel said:
I saw a forum member here say to take both plug wires off their plugs for each relay and put the voltmeter on 20k under Ohms and you should get a reading in the 12 range. Wouldn't that bypass harness completely?
Click to expand...

The coil is a transformer. The low and high sides are isolated from each other electrically, not magnetically. You test the high side by looking at the resistance between the two spark plug wires. You test the low side doing the same with the supply and trigger but on a lower meter range. 1000:1 or more.

There should be a test procedure and coil specs in the service manual here http://members.dslextreme.com/users/bikecliff/
 
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I would test for voltage first. Start at the battery and see where you loose it, then start at the coils and see where you find it. Whatever is inbetween will most likely be your issue. The coils should always be hot and then the signal generator is what makes them fire, right?
 
Oldman99 said:
I would test for voltage first. Start at the battery and see where you loose it, then start at the coils and see where you find it. Whatever is inbetween will most likely be your issue. The coils should always be hot and then the signal generator is what makes them fire, right?
Click to expand...

Yes and more than likely that's where the problem lies. As Azr pointed out the coils themselves are reliable, orders of magnitude more than the wiring on the low side.
 
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