Porsche 911 3.2l 1984, ECU

[Peter Bat]

3.2 ECU function. Does anyone know what pin 4 on the ECU does. It is connected to the starter motor, yellow, switch.

[wazzz]

The Bentley says it is "battery voltage in start position".

Checking the ECU internal schematic, it is indeed a battery + that controls an internal transistor that in turn provides a temporary ground to the DME relay (so the fuel pump can run right from start) before the ECU "decides" it is good to go and provides its own internal + to the same transistor, so that the DME relay stays grounded once the ignition key is released (and therefore the fuel pump keeps running).

That's a complicated explanation, but that's what it appears to be.

[ischmitz]

It's used to tell the DME to turn on the fuel pump. Internally there is an OR function that controls the fuel pump logic: If this pin is powered OR if flywheel rotation > 200 RPM is detected by both FW sensors the 2nd stage of the DME relay is turned on and delivers power to the fuel pump and the O2 heater.

[GH85Carrera]

Originally Posted by ischmitz

It's used to tell the DME to turn on the fuel pump. Internally there is an OR function that controls the fuel pump logic: If this pin is powered OR if flywheel rotation > 200 RPM is detected by both FW sensors the 2nd stage of the DME relay is turned on and delivers power to the fuel pump and the O2 heater.

Yea, what he said!