MACS Service Reports - 2015 - DEC6

Volkswagen
Figure 8: To more accurately control the variable displacement A/C compressor's output, a smart sensor is used on the high side of the A/C system.

Instead of the pressure transducer varying an
analog output, these unique sensors use a microprocessor to put out a variable duty cycle. In the
example, a low pressure output of 13% duty cycle
represents 1.4 bar (20 psi) and a high pressure input of 90% duty cycle represents 37 bar (536 psi).
Naturally the PCM will take the various duty cycled square waves from these sensors and translate
them into the proper bar or psi data PID for a scan
tool. A problem comes into play however when you
don't have a scan tool that can communicate with
Volkswagen modules beyond generic OBD-II data
PIDs. In the event of any sensor going rogue, the
module that particular sensor reports to may resort
to a default value referred to as fail-soft. This is a
calculated value based on other known values. In
the case of the A/C high pressure sensor, that might
be the value of the output to the A/C compressor 's
variable displacement control valve, coupled with
engine RPM and coolant temperature data. If the
HVAC module was looking at a slightly high low
pressure transducer and decided to run a full duty
cycle of near 95% to the compressor control valve
while the engine was running at 2500 RPMs with an
ECT at 180°F, the HVAC control logic would know
that the high pressure might be around 280 PSI. If
the sensor was in default or fail-soft mode, a made

December 2015

up value might be 280 PSI as viewed on the scan
tool, as opposed to the real value of something less
believable from a broken sensor, such as 0 PSI. If
this is the case, then a DTC should also set.
If the discussed scenario were an engine management sensor, such as a TPS or coolant sensor, the
true non-default fail-soft value is always displayed
in the generic OBD-II section of your scan tool.
However, in the case of the A/C pressure sensor,
it is highly unlikely you will find that data PID in
OBD-II generic, meaning you won't be able to verify the problem or the fix with your scan tool. That
means you'll need to reverse engineer the diagnostics.

In the case of the VW pressure sensor, you would
see a scope pattern or frequency counter on your
DMM, indicating a rock solid 50 Hz as the percentage of duty cycle ramps up and down based on pressure. All of that information came out of a couple of
small but valuable slivers of information afforded
to us from VW service documentation. Armed with
the knowledge of how the sensor works and how
to test it, you could then watch a pressure gauge as
you command the duty cycle output from the PCM
to the variable displacement compressor control so-

6

MACS Service Reports



Table of Contents for the Digital Edition of MACS Service Reports - 2015

MACS Service Reports - 2015 - JAN1
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