Sorry If I lead anyone astray. I was thinking that the early CIS 928 originally had the air plate switch that provided the ground for the fuel injection pump relay like the 911. A few years ago I had helped a friend work on his 1978 928 that I could have sworn didn't use the tach pulse signal to an electronic fuel pump relay. Slept a few times since then....
Jrenfrew -If you have the later electronic relay, it should also provide power to the fuel pump for a few seconds when the key is initially turned ON - to prime your injection system.
Good Luck - Michael