Ignition coil module failure – causes and diagnosis

The driver module (power stage) switches the ignition coil's supply and most often fails when it exceeds its permitted operating temperature. Causes include excessive load from worn spark plugs and ignition leads, supply voltage drops or negative voltage in the trigger signal, so the root cause must be found before replacing a coil with an integrated module.

Why driver modules fail

A common cause of faults diagnosed as ignition system errors is failure of the driver module (power stage). This is a special module that, based on pulses from the engine control unit, switches the coil's supply on and off, producing a spark between the spark plug electrodes.

The module usually fails because the electronic components inside it exceed their permitted operating temperature. This can happen because of:

• excessive load on the module,
• problems with the power supply and the signals fed to the module.

Fig. 1. Overheated module

Fig. 2. Burnt-out module control component

The module usually has 3 wires: supply (terminal 15), earth (terminal 1) and the coil trigger pulse, usually from the engine control unit (check the terminal designation in the technical documentation – often pins 21 or 24). Any disturbance in any of these signals can damage the module.

If the module in the ignition system gives off excessive heat (gets hot), there is probably a problem with the wiring and the module will soon fail. If you notice the module heating up before it fails, find out what is causing it. If the coil has failed because its module burnt out, you also need to find the root cause, as a coil with an integrated module is likely to fail again after replacement.

What to check to prevent a repeat failure

First, rule out wear of ignition system components such as spark plugs and ignition leads. Their excessive resistance can increase the energy demanded from the ignition coil, which is controlled and supplied by the module.

Second, make sure the power supply is correct. It is not just a matter of checking that the voltage is right (11.5–14.4 V) with the ignition on, but also what it is under load – mainly whether there are voltage drops caused by damaged connections and supply wires. An additional coil earth wire that bypasses the module is sometimes fitted precisely to stop the module overheating from the current flowing through it. Such a wire should be connected to the engine block.

Fig. 3. Additional earth wire for a coil with a module

The last point is the trigger signal. A correct signal should be a square wave between 0 V and 12–14 V. Unfortunately, this is not always the case, because in many designs the signal comes from inductive sensors. These sensors work much like a dynamo: as the teeth approach, the electromagnetic field changes and a voltage is induced in the sensor coil, while in the gaps between the teeth a negative voltage is produced. It is this negative voltage that causes problems – it should be filtered out.

Filtering is usually done in the engine control unit, but sometimes this fails too and the negative voltage reaches the control pin as the coil trigger pulse. If, when checking the voltage or waveform at the coil control pin, you see a voltage below 0 V, this potential difference will overload the module thermally and eventually destroy it. Unless the control unit itself is faulty, checking its earth connection can sometimes solve the problem.

Also check whether only the module has failed, or the coil as well.

Diagnosing a faulty ignition coil

Diagnosis of a faulty coil should cover:

• coil connections – correct connection, clean connectors, no current leakage,
• correct connections to vehicle earth,
• correct operation of the sensors and filtering of their signal,
• correct operation of the voltage regulator and correct voltage in the vehicle's electrical system.