Electrical

Ignition Circuit Monitoring Relay

The 928 S4 engine has two separate ignition circuits. Ignition circuit 1 is comprised of cylinders 1,4,6,7, the right side coil and distributor. Ignition circuit 2 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 convirters, a fire often occurs.

From model year 1989 onwards Porsche added a safety feature. The Ignition Circuit Monitoring Relay (identified in the workshop wiring diagram as the "ignition control circuit"). In reality, the function of the system is to turn off the fuel injection to the failing igition circuit.

The inputs to the system 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 fuel injection pulse to the affected circuit.

When ignition circuit 1 (cylinders 1,4,6,7) is shut down., the red LED is illuminated.
When ignition circuit 2 (cylinders 2,3,5,8) are shut down, the green LED is illuminated.

The symptoms of greatly reduced power and rougher running could be due to a failed ignition circuit or a failure of the Ignition Circuit Monitoring Relay.

The symptoms of greatly reduced power and rougher running could be due to a
failed ignition circuit. One way to quickly identify that is to look at the
module that is fitted next to the EZK spark control unit in the passenger
compartment and look for a red or green LED.

If a problem with the relay is suspected, 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 2.7 V 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)

Rich Andrade
www.electronikrepair.com