coil resistance readings

Hoping someone can shed some light on coil resistance values. The bike is a 2004 1150GS ( dual spark ) and the coils on the primary plugs were replaced a few years ago ( bike has just over 100,000 miles on it now ) I was doing some winter service and noticed it would not run on the secondary plugs alone, when the primary coils were replaced it would not run on just the primary plugs but ran OK on just the secondary ones. This leads me to believe that the main coil might be suspect. Checking the main coil primary resistance I get .4 ohms ( spec. is .5 ), secondary resistance I get 7220 ohms ( spec. is 7500 ) both reading a little low. I checked the primary plug coils and get .9 ohms ( spec. is .87 ) reading a little high. The bike was running well but the gas mileage dropped off a little, I'm wondering if the low readings warrant replacing the main coil?

That is 3.7% less than spec on a component that more than likely has a 5% tolerance. I would not worry about it.

Agreed, the measurements are fine, but that's just the "static" test; the dynamic test is do they intermittently fail when at rpm and trying to fire the plug.
If you have a timing light with an inductive clamp-on pickup, that can tell you if the high voltage is consistently reaching the lower plugs.
I don't know how the GS does it, but on my 04 RT, some of the clips that route the high-voltage cables to the lower plugs were waaaay too tight, to the point of damaging the insulation. New cables plus spreading the clips a little really seemed to help. I replaced the dual coil under the tank anyway; have to admit I "shotgunned" that part of the system.

You can have coils that measure to spec and still be faulty because of leakage on the High voltage side.
There comes a point when substitution or monitoring on a scope is the only true way to tell.

tomtietjen said:

Hoping someone can shed some light on coil resistance values. The bike is a 2004 1150GS ( dual spark ) and the coils on the primary plugs were replaced a few years ago ( bike has just over 100,000 miles on it now ) I was doing some winter service and noticed it would not run on the secondary plugs alone, when the primary coils were replaced it would not run on just the primary plugs but ran OK on just the secondary ones.

Your wording is unclear. If it will not run on primary plugs only after coil replacement then it is still broken.

This leads me to believe that the main coil might be suspect. Checking the main coil primary resistance I get .4 ohms ( spec. is .5 ), secondary resistance I get 7220 ohms ( spec. is 7500 ) both reading a little low. I checked the primary plug coils and get .9 ohms ( spec. is .87 ) reading a little high. The bike was running well but the gas mileage dropped off a little, I'm wondering if the low readings warrant replacing the main coil?

Copper changes in resistance 20% for every 100 degrees C so cold coils read low

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Be sure to short out secondary wires when testing primary coils, and never short a primary coil, just disconnect it and do not short the connector, it is low voltage.

Rod

What I meant was the bike would not run on the primary plugs alone and that is why the coils were replaced. It was also pretty obvious they had gone bad as they were split open and rusty. Surprisingly the bike still ran pretty well with them in that condition although it was a little hard to start when cold and gas mileage dropped to about 35 mpg, spark was still evident at the plugs.

tomtietjen said:

What I meant was the bike would not run on the primary plugs alone and that is why the coils were replaced. It was also pretty obvious they had gone bad as they were split open and rusty. Surprisingly the bike still ran pretty well with them in that condition although it was a little hard to start when cold and gas mileage dropped to about 35 mpg, spark was still evident at the plugs.

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Secondary coils do not fail as often as primary. They do fail, and so do the plug wires. A good shop with an inductive pickup scope in their diagnostics can test the coil. Given the expense of travel to/from a shop, shop rates and hours this may not be a viable solution. It could just be cheaper to buy new coil wires and plugs and if performance does not improve then you have spares.

While I love my dealer, if I was pretty sure it was the coil no way would I ride 75 miles to sit in the dealership for a couple of hours to find out. I would spend a couple of hundred on new ones to find out. for less than $100 I would try wires and plugs first if I did not have to have it running right away.

Rod

Are you sure the primary coils can just be disconnected without any damage if I want to see if the bike will run on the secondary plugs alone?

tomtietjen said:

Are you sure the primary coils can just be disconnected without any damage if I want to see if the bike will run on the secondary plugs alone?

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The connector is the stick coil input. Just unplug the connector from the stick coil, and do not short it. It is switched 12V DC, just like the injector drive, there is not stored energy. You can leave the stick coil on the plug in the head or take it off. You do not want to pull the sick coil off the plug, and leave the connector on it. As fragile as the stick coils are, I am sure that would kill one pretty quick.

On the secondary coils, the plug wires are the high voltage output. Leaving them disconnected forces the stored energy in the coil laminations to go somewhere, like arc over in the coil or overvoltage the transistors in the Motoronic, either way this is bad. Mixing the two types of ignitions on the bike has confused more than a few. Of course the bike and the plugs do not care. Now to check stick coils you could pull the tank, disconnect the low voltage input to the secondary coil, reassemble and test, but that is a LOT more work. Just short the terminals in the boots with a piece of wire to ground.

Rod

Thanks for your input