Coil Resistance

[Manfred]

I've read Duane's advice about replacing the ignition coil in an old as a matter of preventive care. I've read Rick's notes here: http://www.motoelekt.com/ignition.htm and understand some of the problem with a coil with lower resistance than spec calls for. What is the risk with a coil that has higher resistance than called for? I have a coil rated at 3 ohms, my bike (1987 R80RT) calls for one at 1.5.

[Kurt in S.A.]

I think the general answer is that higher resistance will reduce spark energy or that a system designed for 1.5 ohms will not be able to produce the most energy possible if the primary coil resistance is 3 ohms. If you have a marginal ingnition system, this could result in a situation where the bike might not run.

Kurt in S.A.

[Manfred]

Thanks!

My Clymer's manual tells me the coil on my bike is a 0.7 ohm, showing the bikes with dual coils (rather a single coil with two connectors) at 1.5. I will call Rick and check with SnowBum.

[Ken in Oklahoma]

. . . What is the risk with a coil that has higher resistance than called for? I have a coil rated at 3 ohms, my bike (1987 R80RT) calls for one at 1.5.

Disclaimer: I've never tried what you're proposing to do, nor have I seen it done.

The risk would be that you will have less energy sparking the plugs, in effect you would be degrading the ignition system. The higher resistance would mean that the coil would need more time to saturate. Likely it would run fine at lower RPMs, but maybe not so well at higher rpms and loads. In a sense I would be concerned that you're taking away some of the 'headroom" from your ignition system.

Also, I don't know what is going on inside your electronic ignition module electrically, so there is the possibility of a problem manifesting itself in some other manner due to the mismatch.

So why are you considering a coil swap? Are you trying to solve a problem that you have? I think I can say for sure that you won't see a performance or gas mileage improvement. Or is it that you're trying to save a few bucks by using a coil that you already have instead of springing for a new one?

Editing: Oops! I see that Kurt posted much the same as I was trying to say as I was busily composing my masterpiece.

Ken

[Manfred]

Hey, Ken

As documented in another thread, my bike began to be hard to start when cold - this started after a low impact low-side a year or so ago. I thought it was the battery - replaced with an Odyssey ADM battery, which helped. I replaced the aged float and choke gaskets - which didn't help.

So I'm thinking it is likely the coil or the ignition module.

[Kurt in S.A.]

Editing: Oops! I see that Kurt posted much the same as I was trying to say as I was busily composing my masterpiece.

Some people have a masterpiece in them...some of us only have the Cliff notes!!

Kurt in S.A.

[Manfred]

Turns out I miss-read my Clymer's. Single pole coil is 0.7, dual pole coil is 1.5, so that agrees with Rick and Snowbum. I put some dielectric grease on the spark plug connector to coil and will see if that improves things.

Also - the ignition module on my bike is a Telefunken unit with BMW logo and P/N 12121244477.

[Duane Ausherman]

This coid resistance thing is not too accurate....... at best. We must measure it with DC at zero rpm. In actual running conditions it behaves far more like it is an AC circuit, not DC. The rpm has a great affect.

We use our ohmmeter with DC because very few of us can measure it accurately under operating conditions.

[Manfred]

Duane - do you agree that a 3 ohm coil in a bike spec'd for a 1.5 ohm coil is asking for trouble?

[Duane Ausherman]

Yes I do. I am just saying that there is more to the story. The pure DC resistance measured between coils could be small, but in operation could be far bigger, or smaller. It is not so simple as just the resistance measured by an Ohmmeter.

The rule may hold up and it is certainly the only resonable thing that we have to go on.