1980 - 1984 US/Japan L-Jetronic Fuel Injection No-Start Diagnostics

More older Porsche 928s are coming back to life. I'm assisting an independent shop with an early 1980s Porsche 928 that has L-Jetronic fuel injection. A known good tester L-Jet module was sent and the engine will not start, so more diagnostics is needed. I created this overview to assist such diagnostic testing.

Where the newer 1985 - 1995 928s (as well as Euro/ROW 1984) use digital based control where the engine can start with no connection to the coolant temperature sensor and mass airflow sensors, the earlier L-Jetronic must have valid voltages at the module before injection will start. I outlined these connections and how to verify the circuits.

Also worth checking the airflow meter on these - if the measuring plate isn't returning to its rest position properly the ECU won't see correct voltage and the injectors won't fire.