Bosch Motronic - Component Datasheet

Altitude / Ambient Pressure Sensor (F96)

The altitude correction sensor (also known as ACS) is located near the ECU. The location is behind the passenger side A-pillar where a 3-pin connector attaches it into the Motronic loom.

Referred to as F96 on the Motronic schematic, this device provides an input to the ECU that indicates the ambient atmospheric pressure. A barometric cell within F96 moves a potentiometer which provides a resistance value for the ECU that changes with atmospheric pressure and therefore altitude.

Altitude is very important to any turbocharged engine. If the engine electronics do not take atmospheric pressure into account when running at high altitude then severe damage to the turbocharger can result as it over-spins in a desperate attempt to generate boost in such a thin air environment. I know this seems counter-intuitive as one could think of a turbo as a means to increase air input and counteract the low ambient air pressure. But, as any visitor old enough to remember the crazy days of turbocharged F1 technology will tell you - the performance losses that they used to face in all the high altitude races were hugely significant. Normally aspirated cars are much less sensitive to altitude.

The ambient pressure sensor is used for boost pressure regulation when it detects altitudes in excess of 3300ft. Turbo boost is reduced as altitude increases to prevent the turbo from an over-spooling frenzy. In the event of failure (or disconnection) of F96 then the ECU interprets an altitude reading of 13124ft which limits boost to an absolute minimum.

This is all but irrelevant for S2 owners in the UK, except when they pop across to continental Europe where the ECU will permit much more boost pressure in the lowlands of the Netherlands than the alpine passes of Switzerland !

Follow this link to related ACS info on SJM's site, but bear in mind that some info is specific to the 200TQ and UrS4/S6 applications with 3B and AAN engines respectively.