Polo primary protection

Cause of a misfire is not as obvious as first thought

Published:  23 November, 2012

Case study by Andy Crook<

A VOLKSWAGEN Polo had entered the workshop with a constant misfire. Codes P0302 and P0303 identified the offending cylinders but not the cause.

Misfires can be attributed to fuel, compression or ignition faults. This car had registered misfires on cylinders two and three. The fault code made sense as this vehicle is fitted with a ‘wasted spark’ ignition system so failure of one side of the coil would result in a misfire on the paired cylinders (in this case, two and three). The coil is the amplified type as fitted to a number of VAG vehicles and is switched by a 5V square wave signal generated by the ECU.

Lost spark

The first step was to check the spark with a suitably gapped plug. This quickly proved there was no spark on the troublesome cylinders. The question was why?  

When testing electrical circuits I always split the circuit into supply, protection, load, control and ground elements. This makes testing the circuit much easier and it is important to understand how the circuits are protected and controlled before testing. The supply and ground circuits are common to both sides of the coil, so this eliminates them as causes of the misfire. However, there is no circuit protection, or so it appears. The load is the primary windings and the control is performed within the amplifier circuits switched on by the five volt square waveform from the ECU.

Testing the signal from the ECU with an oscilloscope proved both sides of the coil were receiving a good clean consistent square waveform. This is enough to condemn the coil pack.

Diagnosis

To prove the diagnosis, I used a current clamp to analyse the primary current. The data showed it was in the region of 28 amps. However, most primary circuits are in the region of 6-10 amps. This is due to the current limiting function of the amplifier. A low 0.5 ohm resistance coil is used to ensure full coil saturation even at high engine speeds. This rapid build-up of current would become a problem at low engine speeds, so the amplifier limits the current in the circuit to prevent damage. It also switches the circuit off when the ignition is on and the engine is stationary. This is often referred to as closed loop ignition control, image below.<

Placing a current clamp around the supply to the coil allows a technician to monitor both sides of the coil at once. The resulting waveform shows the difference between the two sides of the coil.<

The first trace is normal operation, the current ‘ramps’ up to around 8.5 amps and is held level by the current limiting function of the amplifier. The second trace shows an instant 8.5 amps or short circuit in the coil. The circuit is not damaged due to the current limiting function, this explains why many primary circuits do not have fuses.  

Want to know more?

Andy Crook runs a number of training courses on a wide range of subjects including ignition analysis. Contact Brian Thomas on 01443 405726 for further details.

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