A common problem on these ecu's (L1, L6 and L8) is the ground path on pin 10 failing. Usually when one starts the engine without proper grounding.
The track on the board from pin 10 to a diode will melt if to much power goes through it. Also the diode needs to be replaced.
The diode is very near pin 10 and is mounted in the direction from connector to front of the ecu.
The track can be repaired by soldering a wire. (From memory - it's been more than 20 years since I did this - on the back of the board.)