Pretty old schoolThe Bosch tester plugged in between the car's wire harness & the ECUs (computers). One then turned a rotary switch to run though testing the various inputs (from the sensors). As far as I know, it did not test the ECU itself. If all the inputs checked out OK the nest step was 'replace ECU'. Its main advantage was that one could fairly quickly test all the components and it also tested that the inputs were getting to the ECU - if a component failed you were advised to check at the sensor to rule out a fault in the wiring from the sensor to the wire harness plug.
It wasn't unti lthe 1990 model with Motronic injection that you had any diagnostics. With Motronic, you had a single ECU and a check engine light that would flash a code if you hit the button behind the seat by the ECU.Noob question here, but does the S3 and earlier have a diagnostic port? Seems like the ECU does not have this. I ask because I am into electronics and curious if anyone has ever made a device to read all sensors and such.