Was troubleshooting a highly intermittent problem on a '75 FI engine recently and thought I'd share the solution.
The PCM has a single, large interface plug with which the wiring harness black plastic plug mates. The end opposite the wiring harness has a hook on it that must be inserted under a steel pin by pivoting the wiring harness plug to about 60°, inserting it under that pin and then scissoring the harness plug until it snaps in place and is held by a spring-steel catch at the other end of the plug. Enough background info . . .
If the black plastic housing of the wiring harness plug has ever been removed (to feed plug through chassis hole, for instance), make sure that both the tie-wrap at the wiring harness end AND the small screw at the "hook" end of the plastic housing are installed.
If that little screw is left off the "hook" end, it will force the plug up into the black plastic housing when you "scissor" it in place on the PCM and it will not make proper contact with the electrical connectors at that end . . . result: highly intermittent, difficult to trace runtime problems!!
Hope this keeps someone from chasing their a** for days . . . weeks . . . months . . . longer? Just check that it's there!!!