Ignition Coil: Description and Operation

Ignition Coils





The ignition coils (T1/1 and T1/2) have two high-voltage connections (4a, 4b). Each connection supplies one spark plug with high-voltage.

The engine control module limits the primary current to approximately 7 A. The maximum ignition voltage is approximately 32 kV.

WARNING: The primary connections carry a voltage up to 400 V. The core bracket must always be connected to vehicle ground.





The ignition coil is constructed much like a transformer, where the primary and secondary windings are physically separated from one another. Both ends of the secondary winding are equipped with a high-voltage connection which form, together with two spark plugs, a closed circuit. The secondary winding produces high-voltage during ignition. The ignition spark occurs at both spark plugs at the same time, whereby one spark jumps from the center electrode to the ground electrode and the other from the ground electrode to the center electrode. As a result, constant positive and negative high-voltage to ground is present at the secondary connections. The ignition spark on the exhaust stroke (waste spark) requires very little energy (voltage), so that nearly all the energy stored in the coil is available for the power stroke.





Since the secondary winding is switched in series with both spark plugs, it has the effect of an open circuit (e.g. on the spark plug connector) on both ignition sparks.

The resistance of the secondary winding must be checked between both high-voltage connections: Nominal value: 5.2 - 8.5 kOhms

Resistance of primary winding between connection 1 and 15: Nominal value: 0.3 - 0.4 Ohms

Advantages of static high-voltage distribution:

- Reduced electromagnetic noise level (no open sparks),
- No rotating parts,
- Noise reduction,
- Fewer high-voltage connections.

Location of Ignition Coils and Distribution of High-Voltage





The ignition coils are located between the intake manifolds. Each ignition coil provides high-voltage simultaneously to two spark plugs.

Ignition coil Cylinders
T1/1 1 and 4
T1/2 2and3

Note: If during testing with the engine analyzer no picture appears on the oscilloscope, the high-voltage pickup must be switched from ignition wire 1 to ignition wire 4 on ignition coil T1/1, or from ignition wire 2 to ignition wire 3 on ignition coil T1/2.





Each ignition coil is connected to battery voltage, and the engine control module (HFM SFI) switches each coil to ground. The high-voltage output of each coil is sent simultaneously to two spark plugs, each assigned to a different cylinder. When fired, one cylinder is in its power stroke and the other is in its exhaust stroke, i.e. 360° crank angle (CKA) apart. The cylinder which is in its exhaust stroke has no combustible mixture, therefore the spark has no effect (waste spark). One crankshaft rotation later, the cylinders reverse their roles.

The second ignition coil operates in the same manner, however ignition firing is offset by 180° crank angle (CKA).