I'm having the same problem on my '87 924S daily driver too man. So far I've replaced my Coil (stupidly, without doing proper diagnosing) DME Relay (needed it), and Speed/Reference Sensors.
SoCal has a good method for testing your ignition system:
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Go through the five minute quick test.
Check for power at the coil and at the injectors with the key on.
No power at the coil it's the ignition switch or related wiring.
No power at the injectors it's the DME relay.
Power at the coil and injectors points to a bad reference sensor. The connector may be corroded and/or not plugged in all the way or cracked and may have broken the wires inside.
Check if the sensor is working by pulling a plug wire, inserting a spare plug, grounding the plug to the intake and cranking the engine. If there is spark then the reference sensor is working.
Could be the second set of contacts in the DME relay. These are for the fuel pump. The fuel pump should run for a half second after you stop cranking. Have someone listen for it behind the passengerside wheel well. Check the fuse for the pump too.
On the early 944's you can use an alligator clip jumper between Aux fuse #2 and #3 to run the fuel pump directly. Count from the drivers door. Don't leave the clip on.
Come back with your findings.
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Here's some info on your Speed/Reference sensors that I've slowly memorized over the last few weeks... :-/
Clarks-Garage Speed/Reference Sensor info