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The Ignition Circuit Monitoring Relay

The 928 S4 and newer engines have two separate ignition circuits. Ignition circuit ONE is comprised of cylinders 1, 4, 6, 7, the right side coil and distributor. Ignition circuit TWO is comprised of cylinders 2, 3, 5, 8, the left side coil and distributor. 

If there is a failure in one of the ignition circuits, raw fuel is pumped out of the cylinders and into the exhaust system.

When the raw fuel hits the hot catalytic converters a fire may occur – OUCH!

From model year 1989 onwards, Porsche added the Ignition Circuit Monitoring Relay as a safety feature. This circuit is completely independent of the LH injection system. It is identified in the workshop manual wiring diagram as the Ignition Control Circuit. The function of the system is to turn off the fuel injection to the failing ignition circuit.

The inputs to the relay are two exhaust temperature sensors that provide input to the relay. If one of the ignition circuits fails, the “Ignition Circuit Monitoring Relay” shuts off the pulse signal to all of the fuel injectors of the affected circuit.

As you might imagine, the symptoms of a shut down ignition circuit are a significant reduction in power and a rough running engine. For trouble shooting, you need to determine if the shut-down is due to a failed ignition circuit or a failure of the Ignition Circuit Monitoring Relay itself.

The first step is to quickly identify which circuit is affected. Look at the clear relay module that is fitted next to the EZK spark control unit in the passenger compartment.

When ignition circuit ONE (cylinders 1, 4, 6 & 7) is shut-down, a RED LED is illuminated.

When ignition circuit TWO (cylinders 2, 3, 5 & 8) is shut-down, a GREEN LED is illuminated.

Check the ignition circuits and repair as necessary. If no problem is found with the ignition circuit, there may be a failure of the Ignition Circuit Monitoring Relay circuit, the signals to test are:

1. Terminal 31: ground

2. Ground must be present at terminal AL when the ignition switch is in the off position.

3. Battery voltage must be present at terminals A1, A2, 15 and 87 when the ignition is in the on position.

4. Battery voltage must be present at terminals AL and 61 when the engine is running.

5. A Voltage value of approximately 2.7v must be present at both terminals E1 and E2 when the ignition is in the on position.

6. The resistance between E1 and E1 is approximately 5 – 10 Ohms (measured at the disconnected relay socket).

The temperature sensors that fit in the exhaust system should also be inspected.

A more detailed explanation is provided in the factory workshop manuals.

If you desire further information please contact me.

Rich Andrade
If you have some specific questions diagnostic questions, please feel free to send me a note at Email: info AT electronikrepair DOT com.

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