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20 Apr 2014

Hints & Tips When Replacing Transformer Ignition Coils

The Bosch transformer ignition coil was introduced to replace the older oil filled coil in 1987 to cater for the rapid development in engine technology, which had led to a higher tension spark being required in new vehicles. Since it's introduction, difficulties and confusion have been experienced by some people when selecting and installing the correct transformer ignition coil to replace the oil filled coil.

Fitting an incorrect coil can result in costly consequential damage to other parts of the vehicle, such as the ignition module, the Electronic Control Unit (ECU), the new replacement coil and wiring.

When replacing the oil filled coil with transformer ignition coils, a number of basic steps need to be followed, and several important factors needs to be taken into account, before choosing and fitting the replacement transformer ignition coil.

Determine the reason for the original coil failure

Check that the failed coil was the correct fitment for the vehicle. Test the wiring and the ballast resistor, if appropriate and determine that the dwell control is working. The dwell test is essential to ensure that no damage is caused to the new replacement coil. As a guide, the dwell at idle speed should be approximately 18% and increase to 70% at increased engine speed. The transition should be smooth from idle to approximately 3500 rpm. If the dwell is high at idle and decreasing with increased engine speed, or remains the same, the ignition module must be replaced. If this fault is evident, coil primary current will be very high at low engine speeds and this will overheat the windings.

Always use the Bosch recommended replacement coil

Do not choose a replacement transformer coil just because it has the same DC resistance as the original oil filled coil. The electrical characteristics of the transformer coil are vastly different to the oil filled coil for the same application. The magnetic circuit and the special steel used in the core construction, along with the windings, determines the overall performance of the coil. These factors are all taken into consideration when Bosch determine a suitable replacement transformer coil. Bosch then back this up with performance testing, using the appropriate distributor and module combination. This process is followed to achieve the results required across the entire operating range of the engine

Only use Bosch recommended ignition coils with Bosch modules

Electronic ignition coils designed to be used in conjunction with Bosch modules must not be used with other manufacturers' modules. Bosch modules have current and dwell control appropriate to a specific Bosch coil. Where Bosch has undertaken testing on replacement coils for use with other manufacturers' modules, these will be listed in the Bosch coil catalogue. e.g. Ford Laser, Nissan Bluebird.

Bosch EK kits must not be used with HEC or MEC ignition coils

It is recommended that ignition coils designed for use with contact sets are not used with any electronic ignition module, as higher than normal back voltage may damage the switching transistor. Bosch EK kits are designed to operate with SU120R and GT40R ignition coils, along with the original ballast resistor. Only the original vehicle coil can be used, otherwise the switching transistor will be destroyed.

Take care when installing ancillary devices

Kill switches or similar devices must not be connected to the negative side of the coil, as the primary of the coil can be overheated. Anti-theft devices must open the positive side of the ignition circuit. If installing ancillary circuitry to the negative side of the coil, only high impedance circuits should be used. Items such as tachometers, cruise controls, speed sensors and warning devices must be investigated before fitment.

Always refer to the Bosch recommendations for each particular vehicle. For further information on Bosch Ignition Coils please refer to the Bosch website www.bosch.com.au or

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