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Pelican Technical Article:

Knock Sensor Replacement

Steve Vernon

Time:

1 hour1 hr

Tab:

$70 to $400

Talent:

**

Tools:

13mm socket, torque wrench

Applicable Models:

Porsche 993 Carrera (1995-98)
Porsche 993 Carrera 4 (1995-98)
Porsche 993 Carrera 4S (1996-98)
Porsche 993 Carrera S (1998)
Porsche 993 Targa (1996-98)
Porsche 993 Turbo (1996-97)

Parts Required:

Knock Sensors

Hot Tip:

Torque the bolt correctly

Performance Gain:

Proper running engine

Complementary Modification:

Check top of the motor

There are two knock sensors on the Porsche 993; one for each bank, and they play an important part of the engine management system. Porsche utilizes two bridges that each span three cylinders, this allows one knock sensor per-bank of cylinder. The intake sensor helps the DME determine the base timing and if knock is determined by the sensor the DME will retard the timing. The sensors detect knock or ping in the motor which is pre detonation and can cause serious and expensive damage to the motor.

The sensors are basically speakers that "listen" for the knock and they will fail over time. Fortunately, the sensors are easy to replace once you get access to them. Unfortunately you will need to remove the shroud air diverter and alternator. Note: a special torque wrench is required to properly torque the sensors.

Note: The knock sensors are torque sensitive and over or under torque them will result in incorrect readings from the sensors and poor performance from the motor.

There are two sensors for each bank (1-3 and 4-6).
Figure 1

There are two sensors for each bank (1-3 and 4-6). On the Varioram motors the electrical connection for the sensors for bank 1-3 are located behind the heater blower motor (red arrow) and you will need to remove the blower to access the connection. Please see the article on blower motor removal for additional instructions. The bank 4-6 sensor connection is just behind the A/C compressor near the number 4 intake runner (yellow arrow).

On the non-Varioram motor the electrical connection for bank 1-3 is located behind the blower motor (red arrow, which we have already removed) and the electrical connector for bank 4-6 is attached to the right side intake manifold (yellow arrow).
Figure 2

On the non-Varioram motor the electrical connection for bank 1-3 is located behind the blower motor (red arrow, which we have already removed) and the electrical connector for bank 4-6 is attached to the right side intake manifold (yellow arrow).

To access the sensors you will need to remove the shroud air diverter (red arrow).
Figure 3

To access the sensors you will need to remove the shroud air diverter (red arrow).

You will also need to remove the alternator, fan and housing.
Figure 4

You will also need to remove the alternator, fan and housing. Please see our article on alternator replacement for further instructions.

With the fan housing removed you can see the bridge (yellow arrow) that is attached to all three cylinders and the sensor (red arrow) sits in the middle.
Figure 5

With the fan housing removed you can see the bridge (yellow arrow) that is attached to all three cylinders and the sensor (red arrow) sits in the middle.

In this photo you can see the bridge (yellow arrow), the sensor (red arrow) and how the wiring passes through a grommet in the shroud (blue arrow) for the 3-6 bank.
Figure 6

In this photo you can see the bridge (yellow arrow), the sensor (red arrow) and how the wiring passes through a grommet in the shroud (blue arrow) for the 3-6 bank.

We are showing you how to replace the sensor on the 4-6 bank but the procedures are the same for bank 1-3.
Figure 7

We are showing you how to replace the sensor on the 4-6 bank but the procedures are the same for bank 1-3. Begin by disconnecting the wiring connection by squeezing the metal clip (red arrow) and pulling them apart.

Swing out the retaining clip (red arrow) from the bottom and slide the connection out from the mount.
Figure 8

Swing out the retaining clip (red arrow) from the bottom and slide the connection out from the mount.

Pass the connection through the grommet in the shroud (yellow arrow).
Figure 9

Pass the connection through the grommet in the shroud (yellow arrow).

Use a 13mm socket and remove the sensor.
Figure 10

Use a 13mm socket and remove the sensor. Installation is the reverse or removal. The sensors are torque sensitive and should be torqued to 14 lbs. Do not over torque the sensor.

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