Audi Workshop Service and Repair Manuals

The signal from the altitude sender 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.
• Due to the reduction in air density (reduction in air pressure), the turbocharger needs to work harder (higher rotational speed) to deliver the required boost pressure.
• This results in a higher charge air temperature and an increased risk of engine knocking. To avoid this, the boost pressure is limited.

• ◆ Mixture composition during engine start:

• The richness of the starting mixture is preset by map data in the Motronic control unit and corrected with increasing altitude (decreasing air density due to decreasing air pressure) by a "lean" adjustment determined by the altitude sender.

Note:

The altitude sender is located in the electronics box next to the Motronic control unit.

Testing altitude sender signal

• ‒ Read individual measured value (function 09) and select channel 06.

Specifications:

Pressure variations due to weather changes are not taken into account.

• ‒ Measure the current air pressure using turbocharger tester V.A.G 1597/A (absolute pressure display) and compare it to the measured value from channel 06.
• ‒ If the measured value from channel 06 deviates so much that it seems implausible for the altitude / air pressure, check the wiring for contact resistance and if necessary, replace the altitude sender.

Checking wiring

Note:

The altitude sender is monitored by the self-diagnosis system =>Fault table, fault code 00528 - Page

• ‒ Disconnect the plug from the altitude sender.
• ‒ → Measure the voltage with a hand-held multimeter V.A.G 1526 in turn between the contacts 1 + 3 and 2 + 3.

Specified value: 4.5...5.5 V in each case

• ‒ If one of the specified values is not met turn off the ignition.

• ‒ Connect test box V.A.G 1598 with adapter cable V.A.G 1598/19 to the wiring harness for the Motronic control unit.

• ‒ → Check the following wiring connections for continuity and short circuits according to the current flow diagram:

• ‒ → If necessary rectify open circuit or short circuit according to the current flow diagram
• ‒ If the wiring is OK replace the altitude sensor.
• ‒ If after replacing the altitude sensor an implausible value is obtained again, replace the Motronic control unit.