Aftermarket Ignition Wire Resistance on a K75

[TimTyler]

According to many sources BMW K75 ignition wires have 5k + 1k ohms of resistance built-in.

Concerned about compatibility of the Magnecor 3508 wires I installed a few weeks ago, I call Magnecor and asked about their cable's resistance. The tech stated that the 7.5 and 8mm wires provide about 2k ohms resistance per foot by design. They do not incorporate separate resistor elements.  I measured the Magnecor cables with an ohmmeter and sure enough the longest provides 2.8k ohms and the shortest 1.7k ohms of resistance. The Magnecor tech suggested I did not need to worry about switching to resistor plugs.

However at http://hrsbstaff.ednet.ns.ca/bvogel/K100/download/bike-wont-start1.htm

If you replace the wires with after market non resistor wires and caps and keep the original spark plugs, you will run into troubles and take a chance to damage the output transistors of the ignition module (resistance of the wire/spark plug will be close to 0).

So what's better for the bike? 2k ohms with the standard plugs or 7k ohms with resistor plugs?

I'm not an electrical engineer but it if the coils might be damaged by low resistance it would seem that using resistor plugs is a safe bet.

[Inge K.]

It's correct that you're in the risk of damaging the outputs on ignition module if you using cables/plugs
with much lower resistance than the ignition system is designed for.

Don't know that much about the resistance in the OEM wires for the K75's, but the 5K+1K ohm is for the
K100 models. The cables for the 75's did have a "spark gap" built in to the caps and because of that it's
impossible to measure the resistance in them.
When the models came on the market one was told that these two types of cables must not be mixed.
I don't know if this "spark gaps" was instead of the resistances or in addition to.....but someone with OEM
wires would be able to find out as the value is marked on the cap and on the coil connector.

But on the other side it should be a Whole lot of 75's running around With OEM wires for years without any
problem, haven't heard about any specific breakdown that have been traced back to low resistance at coil
outputs.

To be on the safe side I would have choosen 7K ohm road.

[TimTyler]

Thanks, Inge.

I saved my OEM cables and they are marked "5k ohms".

I installed NGK resistor plugs this afternoon.

[Inge K.]

How about the coil connector?

[TimTyler]

Oh, right.

It's tiny but clear: 1kΩ

[TimTyler]

Wondering if I've done any damage to the coils in the last 600 miles since the new wires have been installed  :dunno

[Inge K.]

Thanks, then the resistance in is the same as the 100/1100,s....was just very curious.
Could you also try to measure the resistance, as been said that the "spark gap" was only on the early 75's.

[Inge K.]

Wondering if I've done any damage to the coils in the last 600 miles since the new wires have been installed  :dunno

If the bike still function....non......it's a go or no go.
It's not the coils that can get damaged, but the output transistors in the ignition control unit.

[TimTyler]

The wires do not show continuity.

These OEM wires are relatively new. The previous owner brought the non-running bike into Ride West in Seattle in '07. The tech "checked the wires and found them all bad"  :eek:  Obviously he was not aware of the gap.  Previous owner bought new plugs and wires. That was 75k miles ago.

[Elipten]

What plugs did you use Tim.  I'm in the same situation as I just changed wires.

[Inge K.]

Thanks, then we know that the "spark gap" is still used, as the wires is relatively new.

[Inge K.]

What plugs did you use Tim.  I'm in the same situation as I just changed wires.

Most likely NGK DR7EA.

[TimTyler]

Most likely NGK DR7EA.

Affirmative.

Surprisingly, the plugs were gapped perfectly right out of the box. (But save the screw-on caps from your existing plugs as the NGK's come with threaded pins, not screw-on caps.)

[Elipten]

Would the NGK Iridium DR7EIX also be correct plugs to use if running the Magnecor wires?

[Inge K.]

That is OK, since these have a 5K ohm resistor.

[Motorhobo]

I got new wires from beemerboneyard and don't know if they're Magnecor ones Tim referenced. All I know is that I didn't replace the plugs that were in the bike before replacing the OEM wires.

I was aware of the spark gap issue with the OEM wires -- I remember asking about this a couple years ago, and the response as I remember it was basically, don't mix OEM and aftermarket wires but beyond that you don't need the use different plugs with non-OEM wires. So I replaced the plugs with whatever NGK were in there before. The bike has probably close to 15k since then.

So what's the final scoop in this issue...which plugs are most suitable for use with non-OEM wires of the type sold by BB and euromotoelectrics and what damage can ensure if same plugs are used with OEM and non-OEM wires?