Spent some quality time testing this today... It lives behind the passenger side A-pillar.
I'll update s2central.net with detailed info and a graph etc but here's a summary of findings that confirms what I got from the OFRM - and my tests went further.
This sensor is essentially a variable potentiometer (pot) that provides a voltage reference into one pin on the ECU which it uses to derive altitude and therefore ambient pressure information.
As ambient air pressure decreases with increased altitude this is very important... The turbo boost needs to be reduced at high altitude in thin air to prevent it overspinning.
WIth a voltmeter it is worth doing a quick test on your altitude sensor - applies to 3B, AAN, ABY & ADU engines here - though note location is slightly different on AAN.
Measuring voltage across pin 1 and pin 3 you should observe the following:
You need the ignition turned on, and if you have VAGCOM you can observe the ambient pressure in Group 2 with the engine running...
4.16V at 1020.8mbar (I live close to sea level so thats about right)
If you disconnect the sensor, the ECU throws up a fault code and assumes a pressure of 968mbar (somewhat high altitude).
By manipulating the sensor internals I was able to plot an approximate graph of the devices dynamic range.
Its essentially linear from a minimum value of 0.35V at ~600mbar to a max of 4.3V at 1034bar.
I took a test drive with the value fudged at the minimum for max altitude - expecting to see a reduction in boost pressure at WOT - that wasn't always the case and I got several boost cutouts along the way. Very odd - clearly the ABY doesn't like this much.
I then read in the OFRM for ABY that the alt sensor voltage should be no lower than 0.5V and actually no higher 4.0V - I dunno how accurate that needs to be but it tells me something.
On the RS2, with its support for trailer coupling via the Motronic coding plug - it pulls this pin to 0.3V on the ECU when the trailer is attached to emulate 600mbar and a max boost reduced performance of 162kW while towing (versus 232kW on factory chips).
The altitude and temperature compensation maps in the boost chips are still puzzling me - I nearly had it, but not quite...
I think I need to dig deep and get an EPROM emulator to do this more thoroughly...
Anyway - I thought this info is worth sharing - especially the horrible driving at WOT - looks like a faulty alt sensor can throw off the ECU without raising an alarm... One to check if chasing boost cutouts !
Paul
I'll update s2central.net with detailed info and a graph etc but here's a summary of findings that confirms what I got from the OFRM - and my tests went further.
This sensor is essentially a variable potentiometer (pot) that provides a voltage reference into one pin on the ECU which it uses to derive altitude and therefore ambient pressure information.
As ambient air pressure decreases with increased altitude this is very important... The turbo boost needs to be reduced at high altitude in thin air to prevent it overspinning.
WIth a voltmeter it is worth doing a quick test on your altitude sensor - applies to 3B, AAN, ABY & ADU engines here - though note location is slightly different on AAN.
Measuring voltage across pin 1 and pin 3 you should observe the following:
You need the ignition turned on, and if you have VAGCOM you can observe the ambient pressure in Group 2 with the engine running...
4.16V at 1020.8mbar (I live close to sea level so thats about right)
If you disconnect the sensor, the ECU throws up a fault code and assumes a pressure of 968mbar (somewhat high altitude).
By manipulating the sensor internals I was able to plot an approximate graph of the devices dynamic range.
Its essentially linear from a minimum value of 0.35V at ~600mbar to a max of 4.3V at 1034bar.
I took a test drive with the value fudged at the minimum for max altitude - expecting to see a reduction in boost pressure at WOT - that wasn't always the case and I got several boost cutouts along the way. Very odd - clearly the ABY doesn't like this much.
I then read in the OFRM for ABY that the alt sensor voltage should be no lower than 0.5V and actually no higher 4.0V - I dunno how accurate that needs to be but it tells me something.
On the RS2, with its support for trailer coupling via the Motronic coding plug - it pulls this pin to 0.3V on the ECU when the trailer is attached to emulate 600mbar and a max boost reduced performance of 162kW while towing (versus 232kW on factory chips).
The altitude and temperature compensation maps in the boost chips are still puzzling me - I nearly had it, but not quite...
I think I need to dig deep and get an EPROM emulator to do this more thoroughly...
Anyway - I thought this info is worth sharing - especially the horrible driving at WOT - looks like a faulty alt sensor can throw off the ECU without raising an alarm... One to check if chasing boost cutouts !
Paul
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