# 750i no spark on one side...please help



## Ironlung (May 15, 2013)

Finally needed to join and post a question. I can usually find what I need but just a simple search...Congrats on such a fine site.

So before replacing my Brutes fuel pump and trying to diagnose the thing I lost my left side coil. The coil over heated, popped and blew oil. I believe that had to with a fuel regulation issue....fast forward....Fuel issues resolved.

New coil installed and still nothing.
I check the voltage/and resistance on the two leads and compared it to the working side and they are identical. Im not really sure what im testing here but they seem to check out...
I ran jumpers from the working leads over to my "new coil" and was able to get spark. This proves my plug/wire and coil are good. Ive checked grounds, fuses, etc. 

This tells me there is a problem upstream towards the computer. Just for skits and wiggles I put my uninstalled dynatech ignition system back on and tried the same test with the same results...Thought maybe i fried a factory component..Nope. 

Is there "something" between the coil leads going to one side and the computer that could be at fault? Maybe a relay? Can the computer only operate 1 cylinder? 

Any help or better test ideas? Im open to any and all suggestions...

Thanks in advance for ANY help and the help Ive got here before.
Todd:bigeyes:


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## NMKawierider (Apr 11, 2009)

What year and model? Whether a CDI or ECU, they only determine the crank position from it's sensor, then follow a fixed map for advances and provides the ground to the coil during charging, then severed when it needs to discharge(firing). Coils won't charge if they can't get the voltage required or amps so even if your tester shows the correct voltage, it may not be charging. You did say it had a problem. An internal short will to at things up line so check connectors ,fuses,and all connections on the CDI or ECU for discoloration from pulling too much power through it. You can also switch coils to test.


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## Ironlung (May 15, 2013)

Thanks for the reply.

I should re-clarify I can get it to run fairly easily. Its just running on a single cylinder.

Its an 09 Brute Force 750I

I swapped in two known good coils at this point.
Battery is a lil weak from all the cranking but have been doing my testing with a charger nearby. 

Ill recheck for discolored wires etc. Ive been studying the wiring diagram from the service manual and should be able to test the ecu with a meter? Also might try a buddy's ECU if he would be so kind. 

The crank position sensor is my next thought. I know I had to unplug/replug into that when I installed/uninstalled the dynatec.

Thanks again.


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## filthyredneck (Feb 10, 2010)

Weak battery will definitely contribute to poor running conditions. I had a throttle position sensor go bad on me on my 2008 BF 750 I used to own, sounded like it was running on one cylinder and was running rich. Are you positive that one cylinder isnt getting spark? Its possible to test your tps with a digital multimeter.


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## Ironlung (May 15, 2013)

So I used a buddy's machine and was able to replicate my issue on his machine by plugging in my CDI. So I have a bad CDI and need one asap. Ebay isnt showing anything other than new...415$ Anyone point me in the right direction...Looking for a used 08 plus CDI box for a Brute 750.


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## dman66 (Feb 20, 2011)

post it up in the wanted sections or 4 sale sections of the brute sites....


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## daniel26 (Sep 17, 2021)

Hi everyone. I am having the same problem on my Brute 650 2010. Today i check coil resistance and they are good. Also check Crankshaftsensor and its good too. Only back cilynder its working. I am really thinking that my CDI is passing out… 
Edit. One terminal on the left coil was disconnected. Problem solved


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