CDI system Technical Service Bulletin
Known current issues with 3 and 6 pin systems
3 Pin CDI system
Painted Alternator housings:
Engines that use painted Alternator housings must be grounded to the engine case. One can do this by sanding the bottom of the engine case. The housing must also have a connection with the ground strap from the engine case.
Failures related to painted fan housings resulting in poor electrical ground between the engine and the housing are secondary systems ringing primary voltages due to a poor path to the ground. Alternator voltage spikes are due to poor path to ground for the regulator/rectifier. Both issues will result in eventual CDI failure no matter the CDI brand.
Secondary Resistance:
Secondary resistance specifications for 3 Pin CDI systems is 6000 Ohms per spark plug. One can achieve this with 5000 Ohms at the ignition rotor and 1000 Ohms with the spark plug. This is the optimum resistance to ensure maximum spark energy without the system ringing. Increasing resistance will not harm the system however it will reduce secondary energy available at the spark plug. When building a system, the 5000-ohm resistor must be in front of the coil wire. It is ok whether in the rotor or in the form of an external resistor placed on the coil wire.
After market Ignition wires:
Do not use ignition wires that are resistance per foot wires with the Porsche ignition system. Using resistance per foot wires will result in a different amount of energy reaching each spark plug. Cylinder number one will receive the most energy and cylinder number 6 the least. This will also result in a slight timing shift between all the cylinders due to different resistance values.
Most aftermarket wires do not have enough dielectric strength for use on the Porsche CDI system. When choosing a secondary wire, it must be at least 50,000-volt dielectric strength. Using a spark plug cable with less than that will result in eventual spark leakage.
Using resistance per foot spark plug wires can result in secondary electrical ringing back through the CDI system. This can result in eventual CDI damage as well as possibly Ignition transformer, (Coil), failure.
Non-Resistance ignition rotors:
Using nonresistance ignition rotors, like those found in most all twin plug applications and some aftermarket sales, will require a 5000-ohm resistor in coil wire.
Failure to use a 5000-ohm resistor in front of the ignition transformer will result in energy flashing over the ignition transformer. This causes the secondary current to move through the system too quickly. This results in secondary energy leaking through transformer insulation. This can result in energy leaking to ground, (Misfire), or to the “A” terminal and traveling back into the CDI. It will also result in secondary system ringing that will eventually damage the CDI and ignition transformer.
Spark Plugs:
Spark plugs should be non-resistor plugs. However, using resistor plugs will not harm the system. It will only reduce the secondary energy at the plug. When installing spark plugs on twin plug cars make sure the upper and lower plugs match in resistance. Ideally, you are using non-resistor plugs. When using different spark plugs, the plugs will receive a different timing shift and different amounts of spark energy.
Trigger systems:
The current availability of Bosch Ignition points has changed greatly. And most of the aftermarket points are unable to keep up with higher revving engines, (911E and 911S models). They cannot do it without bouncing. We suggest that you remove the distributor when replacing the ignition points. Then, using a distributor machine check for point bounce in the expected RPM range. Excessive point bounce results in the CDI receiving multiple firing signals. In Factory Bosch CDI system this results in eventual failure of the charging system, due to overload. In a Partsklassik CDI, it will not overload the charging system. However, it can result in electrical system ringing as the rotor may not be facing a post in the distributor cap.
When using aftermarket triggers it is important to be sure that the trigger works with solid core ignition wires. Do not use Pertronix Ignitor II trigger systems. These are not compatible with the CDI system and will result in system damage and or misfires. Using the standard Pertronix ignitor is permissible, however, it is important to periodically check its operation. It does not do well with heat. As the trigger gets hot, its signal quality degrades and can result in false firing signals. We have found that the most reliable aftermarket systems are optical systems like the crane cams system.
6 Pin CDI Systems:
Secondary Resistance:
Secondary resistance specifications for 6 Pin CDI systems is 10,000 Ohms per spark plug. One can achieve this by 5000 Ohms at the ignition rotor and 1000 ohms at the coil wire connector. And then 3000-ohms at the spark plug connector and 1000-ohms at the spark plug wire to the distributor cap. This is the optimum resistance to ensure maximum spark energy without the system ringing. Increasing resistance will not harm the system however it will reduce secondary energy available at the spark plug. When building a system, a total of 6000-ohm resistance must be in front of the coil wire. Whether in the rotor or in the form of an external resistor placed on the coil wire.
After market Ignition wires:
Do not use ignition wires that are resistance per foot wires with the Porsche ignition system. Using resistance per foot wires will result in a different amount of energy reaching each spark plug. Cylinder number one will receive the most energy and cylinder number 6 the least. This will also result in a slight timing shift between all the cylinders due to different resistance values.
Most aftermarket wires do not have enough dielectric strength for use on the Porsche CDI system. When choosing a secondary wire, it must be at least 50,000-volt dielectric strength. Using a spark plug cable with less than that will result in eventual spark leakage.
Using resistance per foot spark plug wires can result in secondary electrical ringing back through the CDI system. This can result in eventual CDI damage as well as possibly Ignition transformer, (Coil), failure.
One should also shield 6 Pin CDI system Wires to prevent escaping electrical noise. The noise comes from inducing a firing event due to the inductive trigger system.
Warning: The Current replacement Beru Ignition wire set sold both on the aftermarket and thru the Porsche dealer network contains a fault. The 1000-ohm connector that pushes into the distributor cap has been made with a nominal dimension. This is too big to fit into the distributor cap. This can result in the copper connector not contacting the distributor cap conductor. This results in an air gap in the system. That means that the spark energy must jump from the cap to the wire. This will result in the eventual failure of the CDI due to secondary ringing.
Non-Resistance ignition rotors:
Using nonresistance ignition rotors, like those found in most all twin plug applications and some aftermarket ignition rotors, will require a 6000-ohm resistor in coil wire.
Failure to use a 6000-ohm resistor in front of the ignition transformer will result in energy flashing over the ignition transformer. This causes the secondary current to move through the system too quickly. This will result in secondary energy leaking through the transformer insulation. This can result in energy leaking to ground, (Misfire), or to the “A” terminal and traveling back into the CDI. It will also result in secondary system ringing that will eventually damage the CDI and ignition transformer.
Warning: When replacing the ignition rotors used in the 1979-83 911SC and the 1978-94 3.3L 930 models the original Porsche part number 930.602.902.00, (Bosch number 1.234.332.173), Bosch has discontinued as of 02/2016. Porsche has superseded their part number to the 3.2 L Ignition rotor 930.602.902.01, (Bosch Numbers 1.234.332.200 or 1.234.332.346). These rotors will physically fit. However, they are not to be used with a CDI system as the rotor contains only a 1000-ohm Resistor. Using this rotor can result in an ignition Transformer and or CDI failures. This would be due to voltage flashing over the ignition transformer insulation. Currently, Bremi is offering an ignition rotor that has a 5000 Ohm resistor installed into the rotor.
Spark Plugs:
Spark plugs should be non-resistor plugs. However, using resistor plugs will not harm the system it will only reduce the secondary energy at the plug. When installing spark plugs on twin plug cars make sure the upper and lower plugs match in resistance. One will ideally use non-resistor plugs. Using spark plugs that are a different resistance will result in a timing shift. Due to the change, different amounts of spark energy deliver to the plugs.
Tachometer Issues:
The tachometer signal on vehicles using a 6 pin CDI is provided by the CDI box. Using tachometers that were not originally designed for use with a 6 pin CDI system (like only 3 Pin CDI tachometers). This can result in damage to the tach drive transistor in the CDI box. Also, the factory-installed fuel system cut-out relay installed in both the 911SC and 911 Turbos can induce a back-fed voltage. This, over time, can result in damage to the CDI. Using tach adaptors or other aftermarket items that tap into the tach circuit. This is not recommended unless they can use an optical isolator so as not to back feed a voltage back to the CDI.
A failed tach drive transistor can cause the CDI not to fire due to the trigger circuit being clamped. Tach circuit issues can be very hard to locate. They are hard to locate since the issue is often not present always and will require an oscilloscope connected. You will also need the tech watching the monitor at the time it occurs.
Partsklassik CDI boxes contain several protection networks designed to protect the CDI box from back-fed current spikes being delivered back. However, it may still be possible to induce a failure. Bosch CDI systems do not contain any protections for this fault. They can fail at any time due to a voltage spike.
CDI Wiring Harness:
When building a custom wiring harness or repairing a wiring harness the trigger signal wire must be shielded. Failure to shield the trigger circuit may induce false signals. This places stress on the CDI charging system and/or result in secondary system ringing due to rotor alignment.
Trigger Signal Wiring:
It is extremely important that the trigger system wiring be configured correctly. Flipping the wiring when reversing the rotation on a Bosch distributor is not required. Doing so will result in an approximately 25°-30° timing offset. This means that the CDI box will be firing when the Rotor is between distributor posts. This in turn will result in a large amount of electrical ringing. The ringing will damage the CDI and or the ignition transformer. Trigger signals should always be verified by using an oscilloscope and probing terminal 7 of the CDI box. Most definitely, they should be checked at regular service intervals especially on cars that are not regular in your shop. We have found a lot of cars that have had plugs replaced in regular service and the wires have been flipped. Also, be careful when using aftermarket twin plug distributors that use MSD pickups that the signal is indexed correctly.
The trigger signal should look like the waveform shown below
This screen shows the capacitor discharge in relation to the trigger signal. The Purple Line is the capacitors going from 450 volts to 0 volts as the energy is directed into the ignition transformer. The sudden drop occurs as the trigger signal, Yellow Line), Passes 0 volts.
The screenshot below shows what the trigger signal looks like when it is flipped.
And the subsequent firing position on the curve
Disclaimer:
All the information included in this bulletin is designed for the automotive professional and 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 and or damage to the system.
From time to time we will update this information as required.
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