Checking altitude sensor: Audi 1.8 (AGU)

The altitude sensor signal influences:

The boost pressure control

The boost pressure control prevents the turbocharger from over revving when the altitude increases and consequently the air density decreases.
A decrease in air density (lower air pressure) causes the turbocharger to increase its charging rate (higher speed) to produce the required boost pressure.
This results in a higher charge air temperature and an increased risk of engine knock. To avoid this, the boost pressure is limited.

Composition of mixture when starting the engine

The richness of the starting mixture is determined by mapped characteristics stored in the engine control unit and is leaned out according to a correction factor determined by the altitude sensor as the altitude increases (lower air density as the air pressure decreases).

Switch on the ignition.
Select the function "Reading measured value block" (function 08) and Display Group 18
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Read measured value block 18
⇒
1
2
3
4
Display readout:

Note:

The altitude correction factor is determined by the air pressure (barometer reading) and by the altitude of the workshop.

Observe display zone 4.

Display in / display zone 4 · Air pressure / in mbar
0 % · 1013 mbar / (sea level)
-6 % · 950 mbar / (altitude 500 m)
-10 % · 900 mbar / (altitude 1000 m)
-20 % · 800 mbar / (altitude 2000 m)
-30 % · 700 mbar / (altitude 3000 m)

If an implausible value appears in display zone 4:

Pull off connector on the sensor.
Switch on the ignition.
Connect the V.A.G 1526 hand held multimeter for voltage measurement between the following connectors contacts:

3-pin connector on the wiring harness, contact · Specified value
1 + 3 · approx. 5 V
2 + 3 · approx. 5 V

If the specified values are not achieved:

Switch the ignition off.

Connect the V.A.G 1598/22 test box to the wiring harness for the engine control unit
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The following wiring should be checked for a short to positive or negative and an open circuit:

3-pin connector on the wiring harness contact. · Test box V.A.G 1598/22 contact.
1 · 61
2 · 62
3 · 67

If necessary, rectify open circuit or short circuit.

If no wiring fault is detected:

Replace the altitude sensor (F96).
If an implausible value is still displayed after the replacement of the altitude sensor, replace the engine control unit => Page
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