EDS controls idle rpm and emissions devices. It sends a signal to the red plastic thing behind the IP to stabilize idle. If you pull the connector at the red plastic thing with the engine idling, idle should drop to about 500 rpm. Might even stall.

To check EDS, change settings on the idle trim device by the master cylinder. Note the number on the knob next to the indicator. Pull up on the knob turn it so a different number is next to the indicator. Push back into place. The idle should change by about 50 rpm (or maybe 25 rpm) per numeric difference; e.g., changing from setting 5 to setting 2 should drop the rpm by 150 rpm (or maybe 75 rpm).

Sixto
87 300D