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quixotic_1 06-14-2019 08:18 PM

2004 Brutale S Ignition Coil Testing Help

Please understand that electrics are not what I would call a strong part of my skill set.

I have had a coil failure on my 2004 Brutale 750 S and hoping to figure out their condition with them off of the bike using a multimeter. The readings I am getting seem a bit odd, so I am looking for help from someone else who has done this on these models with the Champion stick coils. I have also consulted several versions of the service manual - and in the other languages I have as well - looking for the proper specs, but this information is absent in every version I have. My readings are fairly simple to explain with these as my assumptions:
  • I am using the two outer pins at the top of the stick coil as a channel as I get consistent readings there.
  • I am using the center pin and the spark plug connector as another channel.

So, all coils show 0.7 Ohm across the first channel above. However, only one coil registers anything other than an open line on the second channel - and it reads around 423K Ohm. The other coils - from cylinders 2 to 4 - read only an open line. (I am using a Fluke Automotive Digital Multimeter model 78.)

Thanks to a certain member here, I will have a new-to-me set very shortly as another point of comparison. However, in the mean time, I am about certain that I am doing something wrong. If anyone has the time, please educate me.

Thank you!

esq'z me 06-15-2019 08:38 AM

An ignition coil....stick or standard....has 2 sets of copper windings around an iron core (don't try to figure out construction, just accept it).

The smaller coil (fewer windings) is the "primary"... the larger coil (many more windings) is the secondary. The primary coil has a voltage supply (battery on non-CDI systems) and a ground. The voltage induces a magnetic field in the windings. To create ignition spark the ground circuit is interrupted (points or transistor opens) and the collapsing magnetic field induces a voltage in the secondary coil.

The secondary coil, being a much larger winding, has a much higher voltage than the primary (it is a type of amplifier). Much less current, but much higher voltage.

The secondary coil is connected to ground on one side and the spark plug on the other. Since there is an air gap at the spark plug terminal the large induced voltage must jump that gap to create a complete circuit and discharge the induced voltage. That is your ignition spark.

Typically, the primary circuit will have a low resistance, and the secondary a much higher resistance....which includes the spark plug in a stick coil. The resistance of the spark plug air gap is in the mega-ohms range and requires a special meter to measure.

Most conventional spark plug caps also contain a resistor (for radio noise suppression), but it is in the Kilo-ohm range.

On the MV stick coil, there is the primary circuit which is battery voltage (from the "Service Power Unit" or Service Supply Unit" on Brutales) to the ECU (ground). A second circuit is from the spark plug to the bikes primary ground circuit (shared by everything that needs a ground path)

So, to check your stick coil's secondary resistance you probe the spark plug connection with one of your meter leads to that chassis ground circuit. You will need to look at the schematic to figure out which pin in the plug is what.

The one where you are getting a resistance reading is likely shorted.....

I hope that helps.

quixotic_1 06-24-2019 01:53 PM

Thank you!
Thank you for the education on the coils. I do understand the function part - we used to build them in shop class back in middle school. Laying it out so nicely, though - you should teach this!

It was the testing results really sending me out to wander in the darkness. I guess I expected there to be some sort of control on the spark-inducing secondary, thus looking for some resistance. Obviously, that was misguided. I was perusing videos on testing stick coils and none of the ones used as examples were like the Brutale's. I wonder if keeping the plug on the end would have made a difference in the readings?

In short, what you wrote did help! Thank you for that!

Unfortunately, back when I purchased the bike and I replaced the fuel line fittings - replacing the OEM plastic with metal - I neglected to switch out the connectors on the tank \ fuel pump side. So, when assembling things this weekend, I managed to catch a fuel hose on something under the tank and broke the OEM plastic tank fuel coupler. I have a replacement set on order, so I should get this all sorted by this weekend if I miss some sleep.

Again, I greatly appreciate your help!

theknurl 06-25-2019 02:09 AM

He's talking about an '04 Brutale.......
It has a 1.6M ECU and 2 dual outlet coils.......

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