About the Sensors

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.

Some report that the gap is not really that critical but too close is definitely bad as the ring gear will eat it.

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.

  

Adjusting the Sensors

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. 

Special Note: The speed sensor is mounted in the bracket with what some call a "figure eight washer" (see red image above). Forgetting to include the washer when measuring for the sensor's location can result in a sensor that gets chewed up by the ring gear.

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.

Notes:

  1. A regular scare occurs if you drop things into the small holes around the sensors and some report there are unusual vortex forces that draw small objects into these holes. Getting objects out is not always easy, the best approach seems to be removing the starter. Just a simple rag stuffed into the holes can prevent the problem.
  2. Adjusting the sensor holder (aka bracket) involves loosening the socket-head bolts - they is hard to see, hard to reach and often very tight. Like many things Porsche, you can see it or you can feel it but you can't do both at the same time. It may seem unbelievable but many times breaking the bracket is the only way to get it off. A new bracket costs about $35 but be sure to get the correct one for your model year. 
  3. A mis-adjusted setscrew (on the flywheel) can cause a broken sensor bracket.
  4. The speed sensor senses crankshaft movement and 1) feeds the tachometer, and 2) enables the ignition and fuel pump via the DME relay. A basic check of the speed sensor is to see the tach's needle bounce during engine cranking. Side note: This is not a great test because, when cranking, sometimes the tach "bounce" is only the slightest of flutters.
  5. What about the reference sensor? The reference sensor is used to tell the DME about the crank position.  It is adjusted by setting the height of the the small set screws which are threaded into the flywheel. Those two setscrews tell the DME exactly when to fire the ignition and injectors.
  6. Because the starter is located close to the sensors, a somewhat common problem can occur with electromagnetic interference. Basically, the magnetic field generated by a rotating starter motor can induce a false trigger of the reference sensor which causes the ignition to fire at the wrong time. This problem is more common on the small-body starters of 87 and later. The sensor cables are shielded to protect them from interference but the shield wire also connects through the connector and sometimes that connection corrodes. This results in loss of the shield and what some call the "starter-thunk" problem.
  7. Also, with the small body starters, the sensor bracket was redesigned in 87 to add a tube or collar for the reference sensor (see image below). This was aimed at helping with electromagnetic shielding. Special note, the sleeve below costs $60.

    

 

The Sensor Mod

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).