[winker]

The 5.1v zeners across the injector and vss signals are doing nothing, assume they are to limit the input voltage to 5.1V. To work I believe they require at least 5mA before they will dump excess voltage across them.

The current circuit shows 100k resitors in series, so to get 5ma we would need a voltage of v=0.005x100000=500v (worst case in reality of a car is about 100v, but more likely 60-80v)!

Assume that the supply is 12v and the zener is a 5.1v.

We need to drop 6.9V (12-5.1) across the resistor @5mA (+a few uA for the load)

R=6.9/0.0051=1352, but recommend 1K or 1.2K (to be on safe side)