trouble shooting ;-)

[mrclubike]

Hi Guys. Many thanks for the answer. i checked with a voltmeter yesterday evening from one cap of coil wire to the other and had 21,28 kOhms. so obviously, looks correct (should be between ~17000 to ~30000 ohms). So perhaps the ICU module.. but i do not know how to test it.

That doesn't mean it is definitely good 

It can still be bad
Check continuity between the primary and secondary winding's
It should be infinity
I have a bad Brown Dyna coil that will only run one cylinder at a time
When checked  with a VOM   the primary and secondary winding's  show OK by themselves
But it is shorted between the primary and secondary winding's it has 20k ohms between them instead of infinity

[guyom]

HI all, so here is the last update : i confirm and reconfirm with Euromotoelectric: (by the way really friendly). :

Hello,


It looks like models from 1980- 1984 had a coil with a resistance of 0.7 ohms.

Then for a short time, BMW used a Bosch coil with 1.5 ohms resistance, (gray bosch coil known to crack) then back to 0.7 ohms with the newer Bosch 426 coil, which is the one we sell, and the latest design from Bosch.  This coil works best with our ignition module 12 14 2 325 284 (with heat sink)

So, since your vehicle was made in 1981, it would seem as though your stock coil should be showing 0.7 ohms and should work well with the BMW ignition module ending in 284.

So want to confirm that Blue Dyna combined with 12 14 2 325 284 (with heat sink) and the answer is : YES.

Good to know i will test that. Perhaps the erratic file come from the match between the Harley Davidson Coils and the stock ICU.

Final question : what is the difference between a 0,7 ohm, 1,5 ohms or 3 or 5 ohms coils ??

[Barry]

Final question : what is the difference between a 0,7 ohm, 1,5 ohms or 3 or 5 ohms coils ??

The primary resistance determines what current will flow to energise the coil.

Using ohms law the nominal primary current will be:

5 ohms    2.4 A
3 ohms     4 A
1.5 ohms  8 A
0.7 ohms   16 A *

* By the time resistance gets this low the actual current will not be as simple as saying 16 A because the wiring resistance becomes relatively significant and also the ICU is likely to have a built in current limiter.

The theoretical reasons why lower primary resistance was pursued are two fold:

Lower primary resistance brings with it lower inductance and that makes the coil reach saturation quicker with benefits at high revs. For example with 3 ohms the coils didn't reach full saturation at mid to high revs.

The 2nd reason is to increase spark energy  which =  the primary current squared  times 1/2 the inductance.  Thus a low resistance low inductance coil looses a little because of low inductance but gains much more from the increased current.

Note I said "theoretical"  above. In practice all versions of the airhead ignition seem to work fine.  While there is no doubt that lower resistance coils do produce more spark energy the question is whether or not it was needed.

[guyom]

thanks Barry for the electricity lessons.. !! i like to understand the "why".

Is it why the recommend to have a 16Ah battery on the bike (if 0,7ohms give 16A ?) By the way i bought a Battery in BMW shop and they sell me for this model a 24Ah....

Well...

[Barry]

Battery capacity is not really related to ignition coil current.

You can use any battery that fits the space. There are a some generic AGM batteries that can be  18, 20 or 22Ah but all the same physical size. 

[mrclubike]

Just to warn you If your original ICU mounting bracket  looks like mine

You will have to make your own mounting bracket to make the newest heavy duty  ICU   fit

The adapter bracket that BMW offers will not work because there is no where to bolt it to

[guyom]

thanks for news... ok for the battery and thanks for the warning for the heavy Duty ignition module. Technically i redo the wiring for the bike with a M-Unit from Motogadget and install a RR45 regulator so nothing except the ICU is stock ;-) So why not finish the job with you ignition system.

Thanks for time !

PS: i found a ground wire loosen... so perhaps that explain the situation...