Technical Service Bulletin CDI Transformer (Coil)

Coil failures

Ignition CDI transformer failures in 911 models:

In researching CDI transformer failures in 911 and 930 models, both aftermarket brands and Bosch, a few patterns have occurred. Usually a car that is suffering coil failures will suffer multiple failures over a period of time. The failures will occur on all models of CDI coils. This applies to all currently used in these cars including but not limited to Partsklassik, Bosch, Permatune, and MSD coils. Most of the cars in which these failures are occurring are 911SC, 930. However, it happens as well in Custom Built Twin Plug applications. People reported a small amount of failures in 3 pin CDI cars. However, not people have not collected enough data n these models due to the very low failure rate.

All the ignition coils that we have inspected have failed in the same manor no matter the brand. In all cases the failure has been the same. A high voltage short to the coil case resulting in spark energy flowing through the housing to ground. This is instead of going thru the spark plug. Coil “flash over” often induces this problem. When energy moves through the transformer too quickly, coil flash occurs. It can also happen if the energy put into the transformer cannot make it out of the transformer. (Coil wire disconnected and not grounded.) This results in energy trying to find a path to ground.

When the transformer flashes over, the insulation is no longer able to contain the energy. This results in a small carbon path to ground. Once a current track has established then the current will tend to flow down the same track. This eventually results in a hard failure. We have seen the current leak through the exterior coil tower. It goes to the ground terminal on the coil, also through the bottom of the transformer to the case. As well as through the primary/secondary windings to the case, too.

Secondary ignition resistance is key to the correct operation of the transformer. This is because it is responsible to shape the spark event. Also, the location and amount of resistance is extremely important to the correct operation.

Three pin CDI cars require 6000 Ohms of resistance per spark plug. 5000 of those ohms must be in front of the transformer by the resistor in the ignition rotor.  Six pin CDI cars require 10,000 ohms resistance per spark plug. There, 6000 ohms in front of the transformer by the 5000 ohm resistor in the rotor. Then, a 1000 ohm resistor in the coil wire. If these values are not correct they will place stress on the transformers insulation. That can result in a transformer or CDI box failure.  Also, when performing a compression test make sure that the CDI box is dis-connected. Or one can make sure that the coil wire has disconnected. One must also ground it. You must complete this before cranking the engine. If you allow the coil wire to float, it will result in damage to the transformer.

Twin plug 911 models are highly likely to suffer from both CDI and transformer failures. The ignition rotors used in most twin plug distributors are a nonresistance rotor. This means the only resistance in front of the transformer is the air gap. The gap is between either the rotor and the cap. (Bosch Style) Or it is between the rotor and the cap in Marelli style distributors. Also, the ignition wires used in twin plug applications are often resistance per foot wire. This results in varying levels of resistance depending on length.

Bosch has discontinued the correct ignition rotor for the 911SC and 930 models as of February 2016. Bosch is now supplying the 930.602.902.01 rotor as a replacement rotor for these cars.

The issue is that the replacement rotor is the one used in the 1984-89 3.2L Carrera. That no longer uses a CDI ignition system. The rotor used in that system only has a 1000 ohm resistor installed in it.  

Another issue with the 911SC and 930 models is the current replacement ignition wires supplied by Beru. They suffer from a fitment issue. The 1K resistor on the wire set supposed to plug into the cap has a nominal size of 8 mm. This is now too big to fit into the distributor cap. This results in the wire not contacting the brass connector in the cap. Then, the spark energy has to jump from the cap to the plug wire. This results in energy ringing back through the transformer and into the CDI box. Both of these items if not corrected can and will result in transformer and or CDI box failure.

To avoid ignition system failures:

  1. Maintain factory correct resistances including correct location of resistance
  2. Check for correct contact of secondary wiring so as to prevent spark jumps within the system
  3. Make sure that replacement components comply with factory specifications and fit for secondary resistance
  4. When building/servicing twin plug vehicles check for correct resistance in the secondary system
  5. When using Non-resistance rotors make sure to add resistance to the coil wire


All the information included in this bulletin is designed for the automotive professional. It is designed to supplement the factory service manual. These are issues we have encountered in both our product development and the regular course of servicing vehicles. Your specific vehicle may or may not have any of the problems listed in this document. This document also assumes that the mechanic is familiar with the safe operating and testing of high energy ignition systems. Failure to adhere to minimum safe workshop practices can result in serious injury and or Death. Trying to test the system without the specific diagnostic equipment can result in serious injury. Damage to the system is also a possibility.

From time to time we will up-date this information as required.

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