85 k100RT 102,000 mi, Very rough running when started, I kept it running while I pulled off plug caps 1 and 4, there was no change in engine. Plugs were wet, with new plugs I get weak or often no spark. Number 1 plug wire measures 3.5 k ohms and the others are 5.5-5.7 k ohm.

No signs of any external arcing when running at night. The coils are both open (> 2 meg ohms) between the high volt sides.
I have seen on this site where this should be 12.6 k ohm, I wonder why it runs at all. Also they are open between the high volt and ground, is this normal?
Do I need new coils and wire(s) or is the problem somewhere else?

New Hall sensor a few years back but I will try checking it with a LED when I get home.

Nice weather today, getting desperate, I am about ready to leave out 2 plugs and take it for a ride anyway.

Hi there,I had the same problem by december. Looks like the coil serving cil 1 4 aint working, if the engine is running and both coils measure the same, guess failure is due to dirty contacts. Clean all the low and hi conections, hig ones may need some fine grinding paste to clean de cup inside, dirty primaries are also known to cause this failure. Hope this helps
Best regards
JAP

Change the coils around and see if the problem shifts with it, then you have your answer.

I did clean all the low voltage leads on the coil (several times) the high voltage looks shiney and clean. I also swapped wires several times and sometimes get spark and sometimes not so something seems intermittent. I will have to try again this evening. I think I will pull all the plugs, short them all with some bare copper wire to a good ground and then try switching the neg low volt wires.
I always seem to have constant 12V to the + side but do not know how to see if the neg side is pulsing as it cranks over.
The spark always seems thin and yellow, not the fat blue spark I am used to on other older antique engines around the farm, but the dealer said this is "normal" for BMWs with electronic ignition?

Not so - you should get a good fat blue spark!

hi gardell ....
yes it is correct that the primary and the secondary are not connected at all .
and yes it has been confirmed that good coils secondary measures in the region of 12 k between the outputs
did you use paper clips to probe the high tension stack/terminals ?

its important that both plugs be fitted and be good when looking at this test .
because the output of the high tension is double ended ....meaning that when the spark initiates it needs to have the same gap to earth at both ends .
it seems a wierd setup to me, but thats the way its designed and nowhere near a strange setup in motor bikes at all.

if you were to only look for 1 of the plugs spark ....(leaving the other plug off or fouled ) might contribute to not seeing spark at all .

? could it be time to change the plugs anyhow ?


it would be highly adviseable to open up under the tank and clean up the ignition computer (ignitor) terminals /connections and the under seat computer connections with deoxit .

whilst under the tank clean the earth points on the frame nearby and confirm that the main earth (just near the batery ) is also clean and not corroded .
i beleive that the negative of the pair feeding the ignition coils is the "switched " side of this circuit , so it would be important that all these paths have a good ability to switch high currents without losses.

hope this helps

Thanks for added info on how the ignition works, it now makes more sence some of the inconsistency I was seeing the other day. I will check with new BMW plugs (10$ each). I am also planning on pulling the tank off and cleaning everything with deoxit, I found it for sale at a local radio shack.
My multimeter has a long and narrow enough tip to probe to the bottom of the primary sockets on the coils and no sign of corrosion in there, it still measures open, so I am thinking of getting some used coils.

good idea mate ...i think og replaced his recently ....with good results

dontcha wish you could get inside and fix the the problem ?

there is a spark plugs thread currently running gardall ....ngk plugs have a positive result .... and dont cost the earth ..d8ea or d7ea for the standard ones ...dont forget to ask for the ferules if not fitted

I saw the info on plugs, bought the NGK (about 2.50$ at autozone) but they did not have and did not know where I could get the ferules, I could only find 2 in the junk bin here so I picked up the official BMW plugs on the way home. I may make a few of the ferules on the lathe.

Ferrules? Every spark plug known to modern man has one screwed to the end of it if it is removable, same thread, same diametre, all of them. Kids. They hire kids to staff the shops and unless they grew up with it or someone took the time to be patient with them they often don't know a bloody thing. Get a ferrule from a pile of used sparkies down the junkyard. I have twenty in the corner of one of my tool boxes.

So I put the new plugs in the wires (with all other wires connected) and measured between the plug inner posts they are open between #1 and 4 they are open between plugs 2 and 3, supporting what I measured the other day that coils are both open.

Next I connected the threaded area of each plug to each other and to ground (12 ga solid copper wire) and cranked her over. First I see a nice bright spark on all 4 cylinders . After several seconds cylinder 1 starts missing and after about 10 seconds cranking there is no spark on cyl 1 or 4. Numbers 2 and 3 still look good.

Actually it seems you only need to connect the threaded part of plugs 2 and 3 togather to get a nice spark, they do not need to be connected to engine ground. Makes sence from what charlie99 posted.

I switched the low voltage wires and get same thing. Switching high voltage leads and I still get no spark from the one coil. Sure seems like one bad coil, a little strange both coils read open between the high voltage sides.
I am going to go ahead and clean everything under the tank while I try to source some new or used coils.

You do have your multimeter at the right setting don't you?? Don't ask why I'm asking

Yes I have checked coil output resistance multiple times, I was a little worried that it is only a 15$ multimeter so I checked it with another. I cleaned all the connectors under the tank and same thing, good spark for 5 or 10 seconds.
Question on checking the hall sensor with a LED, can this be done by just cranking the engine and looking for the LED to go on and off or do I need to remove the sensor and put something metalic in the gap? I have checked most of the other voltages and resistances on the ignition and injection computers. Guess I have to just wait for parts now, I should go cut some firewood anyway.

There is a bit of info somewhere to build a simple tester where you test the signal from the sensor. Not to difficult. However, if your plugs keep on sparking the Hall sensor is OK as that one triggers it. If the spark is there, but changes, then the park producing parts are suspect; could be the injection module, the coils, the plug wires or the plugs themselves or the wiring between the module and the coils.

I got the new (used) coils and wires in the mail yesterday, the coils measure 10 k ohm across the high tension side. By mixing old and new I found 4 wires around 5.4 k ohm. Getting a very nice and stable blue spark on all cylinders (it was thinner and white befor). I am pretty sure the problem is solved but still need to reassemble everything. Thanks guys, What an excellent fouum.

At this point I am not sure I am going to test the hall sensor because I cannot find a connector that will allow me to hook up the diode tester without the risk of shorting something out.

give it time gardell .....you will find a reason and the enchouragement , and advice if you need it......... from your new friends .

good result on the spark mate .......i hope that this is a turning point for you and the k .......slap it together and go for a ride mate ...sometimes ...just the thrill of acheiving a repair, can put all things in a different spotlight ,,,,,enjoy brother !!!!

Well done Gardell.
Regards,
K-BIKE

Thanks again everyone for the help, Bike is running great with the new (used) coils and wires. It feels great to have the information available to troubleshoot this myself instead of dropping another 1000$ at the dealer. Sometimes it is hard to believe I have had this bike for 26 years and 103000 miles.