I am guessing a combination of the speed and ref signals from the flywheel, but I wanted to make sure and get more detail about the process.

 

Also, at what point, relative to crank or cam angle, does this event happen in the 951?

 

The DME needs the ref sensor to indicate the relative position of TDC of one
of the two pairs of cylinders (2 up, 2 down). Once that's known, the DME counts
the ring teeth (speed sensor) to determine the "next" TDC and calculate the
appropriate timing. Since the DME counts the teeth, it no longer needs the ref
sensor input once the engine starts.

Note: A good test for bad or missing ring teeth is to disconnect the ref sensor.
If the engine starts to run poorly, it has a bad tooth/teeth as the DME's counter
mis-calculates successive TDCs.

 

If memory serves the DME fires the injectors when engine RPM hits 200 or so. Once the engine starts the DME polls the AFM for an air flow signal. If the air flow signal is invalid the DME turns the injectors off and the engine shuts down.

 

"Once the engine starts the DME polls the AFM for an air flow signal. If the air flow signal is invalid the DME turns the injectors off and the engine shuts down."

It doesn't wait for the engine to start. The DME samples all the sensors once it
"sees" the ref & speed signals. This causes the 8051 to branch to its' run mode
versus startup mode. The injectors will "fire" without any AFM signal, although the fuel
quantity will be inadequate without the AFM signal causing the
engine to not run.

Actually, a 944 can be made to just run (no accel) without the AFM by increasing the
fuel quantity injected by raising the fuel pressure enough (not easy) or disconnecting the temp sensor.

 

There is also a signal wire from the ignition switch to DME terminal #4 during cranking.

 

"There is also a signal wire from the ignition switch to DME terminal #4 during cranking."

That only causes to fuel pump run before the speed & ref signals.
It does not cause the injectors to fire.

 

So does anyone know the exact time in the crank cycle that the injectors fire? (in degrees)