Recently a BMW mechanic of many years told me the earlier K100 bikes had issues with the ignition coils.  He said the earliest examples failed frequently (something to do with the part no. being stamped on the coil body) and even the second generation being low quality.  He further recommended I fit coils from the K1100 series motors. Has anyone else heard of this?  As I was about to begin a long tour, I asked the mechanic if he thought this would pose a reliability problem (I was to ride though some very desolate country).  He replied I'd probably be ok, but my mpg would not be very good due to weak spark.
Well, I'm home again without any roadside repairs needed on my bike, but the mpg was not what I'd hoped for.  My buddy's R1100RS (oilhead) was consistently less thirsty than my 1985 K100RS.

Firstly the fuel consumption of a twin of similar capacity will always be better, a multi cylinder is not as efficient in that regard as a twin.
There are two types of coil for the K100 and can be distinguished by the lead mounts.  The early types that were not of low quality but didn't stand the test of time were all black where the later model coils had a red/orange mount where the spark plug leads plug into them.
You could use the K1100 coils but they are not any better than the later K100 coils and you will need to modify the wiring harness where the wires plug into the coil and put new plug leads as they aren't the same and the plugs you use are also different (D7EA for K100 and DR7EA for K1100)
There is no reason at all to use the K1100 coils over the later K100 coils.
If you have the early all black coils then replacing them is a good idea as they do frequently give trouble.
If your bike has been active all its life then there is a good chance it has the later coils already.

If I understand you correctly, the old coils need non-resistor plugs, the newer (K1100) require plugs with resistors?  The mechanic who advised me to change the coils never mentioned that.
Isn't it possible to use a resistor, if required, anywhere in the system?  For example, it could be in the spark plug itself, or in the "cap" that fits over the plug.

just for clarificaition on the older coils
if you have copper cables you could benefit from resistor type plugs .  or resistive plug tops,  but never both should be used
 
most cables these days have some resistance inherant to them ...such as silicon types  and the newer versions and variations of the same and stock oem bmw leads (about 8-10 k per metre)
 
the idea is to limit the resistance values from high voltage post of the coil to tip of spark plug electrode  to about 5 k total on each leg
this use of resistance is to comply with emi (rf noise ) regulations ...and is generated  by the ignition pulse of rf into the air which affects any radio receiver nearby . by limiting the spark current the rf is significantly reduced .
  
many of us remember the older 60s era vws that you could hear comming past on your radio receiver, which was verry common
 
further to that
if you happened to fit resistive plugs and resistive leads you could well damage the coil .
when the coil fires you are actually firing a pulse to each of the spark plugs that share the coil
that pulse is limited in its amplitude by the loading (resistance ) of the ignition lead to spark gap and earth .
so if you have higher resitance in the leads you will have less spark energy available at the plug .
 
if you happen to have more resistance than say 10k in each leg of the ignition coils , the terminal voltage produced by the coil is excessive , as  lighter loading (or higher resistance ) to let the pulse disapate will cause a higher terminal voltage across the coil.
this could cause cross firing of the coil secondary between layers internally, eventually causing failure of the coil pack (see pic below).
likewise if you have poorly connecting plugtops and or coil connections, this effect could likely cause a failure long term .
 
in each case above you will have lower available spark energy available at the spark gap ...causing lower power to the motor (or the effective ignition spark and air- fuel charge burn )
 
note if you have just one lead connection issue it will reduce the effectiveness of spark to both cylinders connected to the same coil
for some this might not be immediatly noticeable , especially on frequent short trips . but the results are self evident .
 
here are a couple of coils imported from uk ,,,still operational but no measureable resistance from one high volt tower to the other
 

could this explain why these coils are in a "sell off" (part out) of a k ....that performed poorly ?
 
in any case  it seems to me that these coils we remooved from a poorly maintained system , and possibly corrosion affected by the wet road conditions experienced over there .
 
i believe the red top coils measure differently to the all black
 
the red top coils about 12.6 k   whilst the black about 10.2k on the secondarys
 

secondary                                 primary
 
 
 
from the circuit above you can see that if you add any resistance to either of the leads it will affect the whole circuit involved  as there is no other earth reference .... in difference to a high number of conventional car type systems where the secondary of the coil is effectivly earthed and each lead (through distributor)  is treated as individual .
 
 
 
hope that helps

I agree, The upgraded coil with the orange towers fixed a myriad of issues on my 85.
Also something that may not have been mentioned earlier unless I missed it is: even if you did elect to change all the wiring ends needed to connect the 1100 coils to the early machine, they are the incorrect primary resistance for the design of the early ignition control unit.
Various opinions have been ventured as to what would occur by using them.......some say the excess draw of the lower resistance primaries would overload the circuit in the module that is not designed for it.

As to spark plug/Ignition wire resistance: The early bike has a 5000~ resister built into the metal cap end at the spark plug. It has non resistor wire and non resistor plugs.
 I have no knowledge of what the 1100 comes with but it's not prudent for your use anyhow.

I'm not sure what you currently have or have not installed at this time.

Suggest you return to te stock configuration as mentioned above before attempting to address any thing else.
Regards, Ibj...

Lower impedance primary coils on the early series ignition systems would also have a greater back emf voltage spike that might just tip the ecu over  and short it out .

there was a big moove on second gen systems (industry wide) that  mooved to lower resistance primary coils ....for what reason still amazes me ...but might have something to do with economising on coil production and size ...the inherant higher currents meant that many took the switching devices offboard the ecu ...and meant there was an "ignition amplifier module" added to the system

"I'm not sure what you currently have or have not installed at this time."

My motorcycle's coils are entirely black, so they no doubt are the suspect items.

I would certainly be planning at least to put the newer type on in the very near future.
To change to the K1100 type is not something I would not even consider as there is no advantage to be gained over the newer type K100 coils.
The only reason that I can see BMW changed the coils from the K100 is so that they could not be easily used in place of the K1100 coils because of the different ICU and damage that could occur.

as long as the leads and connections are maintained well ...I don't see any issues other than environmental ..or locality differences that might affect build up of crud and thus some corrosion .  well perhaps including rough treatment

could be a good idea to do the tests every season before starting off ?

for us lot down here I suspect we have a slight advantage of no salt on the roads during the winter - cold months ...but its good to know the differences of what might be available no ?

There are 2 kinds of early K100 operators:

Those who have the original black tower coils.......and are destined to spend countless hours tearing their hair out trying to find the elusive problem with drive ability caused by their weak/failing Original coils........

 And there are those who have already installed the better orange topped ones and are blissfully motoring trouble free down the road of life!

Charlie, how is the damage caused with an unmatched set of leads is it in the secondaries that the issue arrises? being that we all should ensure as close a match as possible impeadence value to each plug.

yes indeed keith

the energy produced from the transfer of the pulse from the primary to the secondary (read change in state from no current to high current and visa versa )
the high voltage has to dissipate somehow ....if we restrict the current going from the output terminals to a different and now reasonably low rate (towards the earth ) a high potential still exists  across the secondary coil for a slightly longer time  . that high energy spike may jump the coil layers if this builds up too much without being released  ...in the long term causing an arc-carbon track to appear along that path .
now because that path is setup strange things occur .
multiple voltage spikes are produced while the coil is still ok but increasingly the effectively shorted turns starts to be counter productive to the energy output production
this has more to do with the dielectric between the windings on the coil former (and insulation of the wire )
there have been a number of ways to counteract this ...namely to install a better insulator between each  layer of coil winding at time of manufacture ...but for the most case "we have what we have" to deal with .
the later series of coils may have been designed better  ...especially after feedback of the failures noticed on earlier released product .
as with all electrickery as one end of the output pulses high the other end pulses low in relation ... draw a sine wave
and invert it for the opposite end of the coil
if we had a mismatch between resistance to earth (for want of a better description ) between  one end and the other the amount of pulse energy release is still restricted by the total resistance of the circuit ...but in a weird way
since we are now dealing with ac circuits (waveform changing from positive to negative as the spark field is created , (resonance of the coils, ringing of the coil , dwell etc etc ) the product of the current release becomes a different version of power factor   in this case power output  becomes "I" squared x ""R" so for every little difference of resistance the current disproportionally decreases  to the higher resistance end of the output  so you end up with a high going spike out one end matched by a restricted low negative going spark at the other end until the polarity changes in a half a millisecond or so...I better stop, its doing my brain in .....giggles
 
all too far complex to do anything about, but keep design specs and good working conditions in place for as long as you can, to ensure good reliability . which is remarkably easy to do ..if we take the time to look and fix anything that is abnormal . its called maintenance and is far too easy to ignore .
we change our oil every five to ten thousand ks ...but has anyone pulled the plugs and leads to check in the last year ?

Well Charlie, that certainly clears things up !  My "take away" from your very complete explanation is that, if I don't replace those coils, I'm apt to find myself standing on the side of a desolate Nevada highway yet again! (I've been trying to ride to New Mexico for two years running.  Both attempts ended in failure in the middle of nowhere!)
Those coils are none too cheap, but Euromotorelectrics seems to have the least painful prices for them.  However, I'm certainly open to other suggestions. 
Anyone?

why so brick ?
does your leads and coils not measure correctly ?

you could go the cheep way and buy the standard accessory leads from any car store ....but look at realm engineering they have great leads as a set for really good prices

I had a failure in one of my coils, but it was the casing that failed.
The spark was coming out the side and arcing on the metal band that runs top to bottom on the coils.
Replaced with the orange top model and not a problem since.

rosscorossco

Thanks Charlie, will check all of mine the 3 and the 4 cylinder bikes, i receintly replaced a coil pack on my hyundai Lantra 1998ish because it was sparking from the post to the retaining bolt, perhaps it was a resistence issue causing it to take another somewhat shorter path, because i hadn't done enough maintanence on it, wonder if the coils would work on a K100. they're both wasted spark systems. So from what you write its really a good idea to get the pathway from the coil post to the centre electrode of the spark plug as equal as possible for the 1,4 and 2,3 sets and as conductive as possible a connection of the sparke plug to the engine to ensure a long life of the coils or at least cull any easy way for the coil to take a break from its duties.

good idea keith
the spark from the coils is truly a brutal thing .....what im mostly concerned about and try hopefully to remind folks  that, firstly ...to have a look ......if they see a marked difference ...do something about It ....4k on one and 5k on the other is not a significant a difference when you consider that there is 10 to 20 something thousand volts trying ...as you say ...to look for the shortest route to earth .
I saw in here recently someone with 8 k on one and 4 k on the other ....of the pair  which is certainly is an alarm bell for me .
and others with resistive leads in addition to the resistive plug tops or resistive plugs .....at least one noticed some significant change after doing a bit of a change ...
just a heads up again for some  ol mate ....sparks jumping is not a good sign .

I took the suggestion of JAP (post #15 of this thread) and bought new coils to replace the original old style fitted to my 1985 K100RS.  During a recent tour of some 2000 miles there were times when the engine seemed to be firing on two cylinders momentarily.  I believe it was an electrical issue, as there was at least one gallon of fuel in the tank at those times of poor running, and the fuel pump and filter are quite new.   On the tour I changed out the spark plugs for another set of NGK non-resister types, but the intermittent miss recurred.  Back home, I measured the secondary ignition wires and the resistance in the coils.  Nothing out of spec there. I examined the electrical connections to the pump, both inside the tank and at the under tank connector block. Nothing suspect there.  I then measured the original coils and discovered that there is no measureable resistance in the secondaries however, the primary resistance is very close to spec.  Those coils preformed flawlessly when they were fitted to the motorcycle.  I changed them out just as a precaution against future problems, as I understood they were not the most reliable. 
At this point I'm puzzled that the secondary windings' resistance could be nil, yet the coils worked well in service.  Can someone offers some insights?
As to the miss, I've changed out the fuel filter as a cheap precaution and will continue to ride the bike in the off chance the poor running can be put down to fueling as opposed to ignition.

Hi there, I've experienced intermittent ignition failures due to a broken wire connection to the coils.
Best regards
JAP

brickrider wrote:I took the suggestion of JAP (post #15 of this thread) and bought new coils to replace the original old style fitted to my 1985 K100RS.  During a recent tour of some 2000 miles there were times when the engine seemed to be firing on two cylinders momentarily.  I believe it was an electrical issue, as there was at least one gallon of fuel in the tank at those times of poor running, and the fuel pump and filter are quite new.   On the tour I changed out the spark plugs for another set of NGK non-resister types, but the intermittent miss recurred.  Back home, I measured the secondary ignition wires and the resistance in the coils.  Nothing out of spec there. I examined the electrical connections to the pump, both inside the tank and at the under tank connector block. Nothing suspect there.  I then measured the original coils and discovered that there is no measureable resistance in the secondaries however, the primary resistance is very close to spec.  Those coils preformed flawlessly when they were fitted to the motorcycle.  I changed them out just as a precaution against future problems, as I understood they were not the most reliable. 
At this point I'm puzzled that the secondary windings' resistance could be nil, yet the coils worked well in service.  Can someone offers some insights?
As to the miss, I've changed out the fuel filter as a cheap precaution and will continue to ride the bike in the off chance the poor running can be put down to fueling as opposed to ignition.

have bought coils like that also brick

and they actually work

the voltage will still jump the gap that is internal to the coil secondaries

could jap be onto it ?

a slightly high resistance connector- lead at the coil primary might be a reason ...road conditions may affect that connection periodically ?


maybe a test of the breather system to the tank could prove worthwhile also ...no need to have a high vacuum inside there as the fuel load decreases

Thanks for your thoughts guys. 
Charlie, I understood JAP's comments to refer to the 12v. wire that feeds the coils having a short.  Perhaps I've misunderstood....
You say you've also bought the same coils I am now using and they work, but seem to discount that in your following comments  
I'd thought about vacuum in the fuel tank, as that has been an issue in the past with bike with carburettors, but I guess I dismissed it because the Ks have FI.  Do you think that could still be an issue?  In any case, when I changed the fuel filter I had a good look at the vent hole under the cap and all looked to be in order.
I'm unable to ride the bike at this time, but I hope (ever optimistic!) that cleaning various connectors and changing the filter will have resolved the issue.  Once I regain my health I shall try to prove that.

meant that I had bought the old black type coils ....that measured open circuit ...but actually sparked well ....currently not using them (they were for spares,  but I got stung by an invisible supplier )

not just the vent that catches water around the lid ....there is usually a pipe that travels from the filter basket up to another metal tube which exits the tank at the back .= vent ( depends on year model though ? after 85 ) 

no was meaning that if the wires that go to the coils had become frayed ...less copper conductor making the connection  or those spade terminals corroded ...would let less current get to and from the coils ....making for an intermittent - erm low spark energy out the secondarys

hope that helps