Knock Sensor Control Unit
WARNING: This page does not describe the selected car, but rather 6 other vehicles, including the 1987 Volkswagen Scirocco, 1987 Volkswagen Quantum, 1987 Volkswagen Jetta, 1987 Volkswagen Golf, and 1987 Volkswagen GTI. However, it is still accessible from the selected car via links, so may be relevant.
The knock sensor control unit receives signals from the knock sensor and also a vacuum signal from the inlet manifold. It transmits signals to the TCI control unit to adjust ignition timing.