The 928 has two interference suppressors. One is on the CEB and looks like a Relay, the other is located behind the clock in the center console.

This is the Relay in the CEB that is there to reduce spikes when the AC is switching of and on. (quite different from what you'd expect)

This is a section from the 1993 schematic. You see the "Relay" linked on the clutch solenoid circuit. The diode is there to suppress reverse voltage spikes that happen when you turn off the clutch power, the capacitor helps reducing spikes. I'm not sure what capacity the capacitor has.


This is the interference suppressor for the radio. It is there to eliminate whining noise, specifically coming from the alternator.