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Audi A4 Brake Servo Pressure Sensor Replacement
 
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Pelican Technical Article:

Audi A4 Brake Servo Pressure Sensor Replacement

Steve Vernon

Time:

1 hour1 hr

Tab:

$40

Talent:

*

Tools:

Small flathead screwdriver

Applicable Models:

Audi A4 (2002-05)
Audi A4 Cabriolet (2003-06)

Parts Required:

New brake servo pressure sensor

Hot Tip:

Use care working on the plastic stubs

Performance Gain:

Proper brake booster function

Complementary Modification:

Replace brake booster

Without a doubt, your braking components comprise the most important systems on the car. Having a proper functioning brake system is the highest priority you should have on a running vehicle.

The brakes on the Audi A4 are vacuum assisted. This means that the brake booster uses vacuum pressure from the engine and a secondary vacuum pump to assist in applying force to the brakes or master cylinder. If you are starting to notice a different "feel" to your brake pedal by either needing more, less or a modulation in the pressure needed to be applied to the pedal, you should check things right away.

One part of the system that you will want to check is the brake servo pressure sensor.

The pressure sensor on the brake servo piping informs the ECU of the amount of vacuum or pressure in the system, in turn telling the brake servo when to activate or deactivate based on that reading. If this sensor fails, you can get inconsistent different pedal feel than what is normal.

To get access to the sensor you will need to first remove the battery cover and cowl (red arrow).
Figure 1

To get access to the sensor you will need to first remove the battery cover and cowl (red arrow). They both just pull off.

The brake servo pressure sensor is located just to the right of the master cylinder attached to a vacuum hose going into the brake booster (red arrow).
Figure 2

The brake servo pressure sensor is located just to the right of the master cylinder attached to a vacuum hose going into the brake booster (red arrow).

I have removed the master cylinder so that you can get a better view of the sensor (red arrow).
Figure 3

I have removed the master cylinder so that you can get a better view of the sensor (red arrow). You do NOT need to remove the master cylinder to change out the sensor.

The sensor attaches to a connection in the vacuum hose and can be replaced without removing the hose from the booster.
Figure 4

The sensor attaches to a connection in the vacuum hose and can be replaced without removing the hose from the booster. Begin by using a small flathead screwdriver and unclipping and removing the sensor connection (red arrow).

The sensor is held in place by two plastic stubs (red arrow, one shown).
Figure 5

The sensor is held in place by two plastic stubs (red arrow, one shown).

Use a flathead screwdriver and gently pry the sensor over the stubs (red arrow).
Figure 6

Use a flathead screwdriver and gently pry the sensor over the stubs (red arrow). If it is really stubborn, try a little lubricant around the stubs. Pry the sensor off and remove it.

The new sensors probe will sit in the air flow of the hose.
Figure 7

The new sensors probe will sit in the air flow of the hose.

Since the small O-ring on the sensor helps create a vacuum seal, make sure the stubs are in good condition.
Figure 8

Since the small O-ring on the sensor helps create a vacuum seal, make sure the stubs are in good condition. If you have damaged or broken them when removing the sensor you will need to replace the mount. The sensor needs to sit flush and make a vacuum seal with the mount. Installation is the reverse of removal.


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