For 944s through 88, there are two hall-effect sensors. These are both situated to get crank position information from the ring gear and flywheel. These sensors are referred to as the speed sensor and the reference sensor. They are fairly reliable but a first place to look if problems crop up.
If your engine is hard to start, you may want to check your speed sensor. It senses movement of the flywheel and signals the DME to turn on the fuel pump. When set correctly it is positioned very close to the ring gear teeth (0.8 mm). If set too far away from the gear, it may be slow to sense movement, too close and it gets chewed up by the ring gear.
I do not recommend people casually set out to adjust the sensor holder (bracket). The sensors are difficult to reach and often ruin an otherwise fine afternoon. The speed sensor can usually be more easily verified by testing for fuel pressure during engine cranking or via the tach bounce (see notes). Besides, the sensors themselves are generally quite reliable as they have no moving parts.
They do have a weakness associated with the connectors (which are much more accessible) and many emergency no-start troubleshooting sessions begin with wiggling the harness/sensor connectors. The female part of the connectors can be replaced and cost about $10 each. The male portion of the connector is very hard to find/buy but comes with a replacement sensor. The Porsche/Bosch sensor itself can be replaced by a BMW/Bosch sensor that fits and works perfectly and it is somewhat cheaper. Its only downside is that the cable for it is about 6 inches longer than needed. Hint: when you replace your sensors, keep one of the old ones and use it (together with a glued, thin washer) as an adjusting tool.
In the past we have offered a simple sensor tool for measuring and adjusting the speed sensor but this small tool (see machinists depth gauge, below) does a fine job and can be bought at most hardware stores for cheap.
To use the tool, you must first remove the speed sensor. Just adjust the tool's sliding t-bar to be 0.8 mm longer than your speed sensor and "figure 8 washer" and then insert the tool into the speed sensor's holder, orient it so the scale faces the front of the engine - this way the tool will touch the top edge of the ring gear teeth. In this position and the sensor holder properly adjusted, the tool's sliding T-Bar should just barely touch the top of sensor holder. Adjust the sensor bracket as needed. Remove the tool and install your sensors into the bracket.
Another way is to simply glue a small washer that is about .08 mm thick to the bottom of your old sensor, install it so the washer touches the ring gear. Tighten the bracket and then replace the old washer/sensor with new sensor.
Modifying your bellhousing may seem a little extreme, but it sure makes accessing the clutch easier - just pull the transaxle/torque tube and the bellhousing slides away without having to mess with the sensors (which on turbos means pulling the intake).