Ignition coils. Part 3

In my work, I stick to the principle, that the replacement of the component should be reasonable. Accordingly – before performing the replacement, it has to be checked, that the current element/knot is damaged. Otherwise, unjustified expenses are supplemented with questions – what exactly repaired the defect. And is it repaired at all and just disappeared for a while (in case of sporadic problems)?


Below you will find the description regarding most popular causes for damages of ignition coils.

For example, the contact loss of the primary winding is a very rare defect – for this reason, it is not included in this list.


Most popular defects:

1. Power (current) leakage via the inside of the “neck”. Visual confirmation: light, long lines inside the “neck”;

2. Power (current) leakage via insulation material of the ignition coil. Visual confirmation – grey/darker (usually 2 .. 3 pcs, evenly arranged on the body of the ignition coil) lines, around 1cm long. Both defects are described here.


In case of both defects, the number of misfires rises. The problem is, that even perfect (slightly used) ignition coils will have visible lines inside the “neck” and also on the body of the coil. It is very hard to define, in which moment the influence of these artifacts becomes very significant.


3. A defect with similar manifestations as defects, mentioned before – power (current) leakage inside the coil: between secondary winding layers. Unfortunately, this defect has no visual manifestations.


4. Now let’s turn to the more exotic defect. This defect’s characteristic: no visible signs of power (current) leakage, the spark is of the same strength (and length) as for completely “healthy” ignition coil.


In the image – BMW AG connection diagram for the ignition coil. With one exception: (6) the diode, which is included in the construction of the ignition coil, is marked. Unfortunately, this element is not marked in BMW AG materials.

In a normal situation, the high voltage, required for ignition, is generated, when the management transistor (4) closes. Then on the transistor’s collector, the voltage impulse till 250 V is created, and in the coil, it is transformed to 25.000 V high voltage (transformation coefficient 1:100).

But there is one more moment when the high voltage is generated: the moment when the transistor opens. At this moment, to primary winding 12 V are supplied, but at the secondary winding – voltage of 1200 V appears. This 1200 V voltage is generated approximately 2 ms before planned (“true”) impulse, which means, that by 600 RPM there are around 7 degrees “hurry-up” against the “true” impulse. By 1200 RPM this error is already some 15 degrees (it is proportional to RPM).

Moreover, you have to take in the account, that with such high outpost angle before DTC, the compression in the cylinder is lowered, and that makes it easier to ignite the fuel with lower voltage of the spark.

To fix this problem, the diode (6) is integrated into the ignition coil – at the moment, when transistor opens, in the secondary winding the impulse has “wrong” polarity – the diode closes and doesn’t allow this impulse to reach the ignition coil. If this diode is damaged (short circuit), the abnormal performance of cylinder is possible – detonation, error messages regarding misfire.

Moreover, in this situation also the faulty performance of misfire algorithm is possible because the fuel mixture is getting ignited, but – at the inappropriate moment.


Unfortunately, it’s hard to detect this problem – even when measuring the diode with a multimeter, it can fail, because it has a series connection with the secondary winding of the coil and impedance resistor (2).


If misfires, which can not be solved with replacing the spark plug, ignition coil, even the injector of the exact cylinder, are detected, it makes sense to change (even for some time) the ignition coil of the cylinder, which is BEFORE the damaged one (in firing order).


For example, the firing order of 6 cylinder engine is 1/5/3/6/2/4.

If the larger misfire problems are for cylinder No.3, the ignition coil of cylinder No.5 also has to be replaced (also cylinder No.5 should have identified misfires but, possibly, with less number of events).


One of the simplest ways to identify the damaged ignition coil – switching ignition coil between the damaged and one of the normally performing cylinders. Which one of the normally working cylinders to choose?

BMW AG recommends – adjoining one. And that is very important! The adjoining cylinder will never be after/before the damaged one (in firing order).

Why is it so important? In this post – description, how the misfire detection algorithm of MSV/MSD management units can be mistaken.


The idea of this entry – unfortunately, the ignition coils have some specific defects, which can not be detected with simple methods. No matter how sad it sounds – the suspicious ignition coil has to be replaced with new one.