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Speed Reference Sensor Replacement
 
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Pelican Technical Article:

Speed Reference Sensor Replacement

Steve Vernon

Time:

1-3 hours

Tab:

$80 to $300

Talent:

****

Tools:

10mm socket or 5mm Allen, extensions

Applicable Models:

Porsche 944 Turbo (1986-89)

Parts Required:

Sensor(s)

Hot Tip:

Remove the intake manifold

Performance Gain:

Proper running engine

Complementary Modification:

New spark plugs

When trying to track down hard starting problems or a vehicle that won't keep idle, one of the areas you will want to check is the speed reference sensors. There are two sensors on the Porsche 951 and both sensors are located beside each other on the top left hand side of the bell housing. The sensor closest to the engine is the sensor used to locate TDC or Top Dead Center. The sensor measures the engine's timing via one single peg that is on a parallel row next to the row of flywheel teeth. Each complete revolution the engine makes, the peg swings past the sensor and sends a signal to the DME indicating TDC. The DME uses this to readjust the timing retarding or advancing, to prevent knocking and pinging in conjunction with the knock sensor. The second sensor is a speed reference sensor. This sensor uses the actual flywheel teeth as the reference to indicate engine speed to the DME by magnetic induction. The magnet in the sensor notes the number of flywheel teeth that pass by per second and the signal is sent to the DME. If this sensor registers less than 300 RPM it will cut off the fuel pump.

Like most things on the 951, the sensors are located in a very difficult location to service, and the sensors have a tendency to get corroded in place.

Begin by disconnecting the ground cable from the battery. While people have been successful removing and adjusting the sensors from the top of the engine without removing any components, the work is a lot easier if you remove the intake manifold.

The sensors are located behind the motor and air oil separator on the rear of the left side of the bell housing (red arrow).
Figure 1

The sensors are located behind the motor and air oil separator on the rear of the left side of the bell housing (red arrow). Both electrical connections should be accessible behind the intake manifold (yellow arrow)

When changing the sensors make sure to change one at a time and do not mix up the sensors and connections (red arrows).
Figure 2

When changing the sensors make sure to change one at a time and do not mix up the sensors and connections (red arrows). Follow each sensor's wiring up to the connection and change one at a time.

Here is a photo of the sensors mounted in the bracket on the bell housing.
Figure 3

Here is a photo of the sensors mounted in the bracket on the bell housing. The red arrow indicates the TDC sensor and the yellow arrow points to the speed reference sensor. I have removed the intake manifold, turbo and air oil separator to get this picture; you will be not be able to have access to them like this unless you remove the components.

Before removing the 10mm bolt (red arrow) for the sensors, make sure to tape off or stuff a rag into the viewing port on the bell housing.
Figure 4

Before removing the 10mm bolt (red arrow) for the sensors, make sure to tape off or stuff a rag into the viewing port on the bell housing. If you drop something down in the hole, there is a good chance you will need to separate the bell housing to get it out, and you do not want to do that! Once you remove a sensor be sure to cover the hole right away.

It is a good idea to spray the sensor with some penetrating oil before trying to remove it and let it soak overnight.
Figure 5

It is a good idea to spray the sensor with some penetrating oil before trying to remove it and let it soak overnight. Wiggle and pull the sensor until you can remove it from the mount. Sometimes pulling it up twisting and pushing it down then doing it over and over will help get a stuck sensor out. DO NOT pull the sensor by the wire cable. The speed reference sensor has a locking plate between the sensor and the mount (red arrow); make sure to transfer this over to the new sensor. The height adjustment on the sensors is critical. Failing to reinstall this plate can cause the sensor to make contact with the flywheel teeth and ruin the sensor.

If you need to adjust the height of the mount, use an old sensor and glue a 0.
Figure 6

If you need to adjust the height of the mount, use an old sensor and glue a 0.8mm thick washer to the bottom of the old sensor (red arrow).

There are two bolts on the mount that you will use to adjust the height of the sensor from the flywheel.
Figure 7

There are two bolts on the mount that you will use to adjust the height of the sensor from the flywheel. The locking bolt (yellow arrow) locks the mount in place while the pivot bolt (red arrow) pivots the height of the mount. Loosen the locking bolt and insert the sensor with the washer glued to it. Adjust the pivot bolt until the sensor with the washer is touching the teeth on the flywheel; make sure to have the locking plate between the sensor and mount while doing this. Lock the mount in place and then remove the old sensor with the washer on it. Install the new sensor, locking plate and tighten everything down. Now the sensor height is set. Installation is the reverse of removal.

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