MACS Service Reports - 2015 - DEC7

Volkswagen
Figure 9: VW's A/C pressure transducers are 3-wire smart sensors. Unlike other 3-wire sensors that have a variable analog output between 0-5 volts, these
sensors put out a digital pulse that increases it's duty cycle (on time) as pressure goes up.

lenoid to ramp up. If your scan tool struggles doing
that, several tools to control variable displacement
compressor solenoids are available to do the job.
See Figure 10. As duty cycle increases, the pressure
sensor duty cycle should increase as well in the case
of the VW.

SBD Step 7: Isolate the Root Cause
When you want to avoid the dreaded comeback,
simply replacing a faulty component that was not
the cause but rather the effect is asking for failure.
For example, it's common in the business when a
catalytic converter goes bad to say "cats don't just
die, they're murdered." That phrase became popular as techs discovered that over-fueling and ignition misfires that dumped raw gasoline into the
converter, overheated it which caused its early demise.
Thinking HVAC, it can also be said that compressors don't just die; they're often murdered, too!
They will kick the bucket if they lack oil or have

December 2015

7

the wrong oil, and go to an early grave if they have
a low charge which robs them of a good flow of
refrigerant to move that oil. So when a compressor
grenades shrapnel into the system as it fails, be sure
to determine if at all possible what caused the failure before installing the new unit.
Even when diagnosing something as simple as a
faulty blend door motor, isolate the root causes described in SBD Step 7. In that example, you could
take the cover off the motor (if possible) and look
for a cracked plastic gear. Yes it could have just got
old and brittle and broke, but it's a very good possibility the gear met too much physical resistance
in the form of a rusty, bent or binding blend door
shaft. Replacing a motor may only fix the vehicle
temporarily.

SBD Step 8: Repair and Verify Fix
Finally, perform dynamic diagnostics by first
verifying that the vehicle has a problem. Using the
power of observation, applying critical think

MACS Service Reports



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

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