MACS Service Reports - 2016 - APR1

Total Vehicle Climate and Thermal ManagementTM

April 2016

By Dave Hobbs, MACS Technical Correspondent
HVAC DATA ANALYSIS - DIAGNOSING LIKE A DOCTOR, THINKING LIKE A MODULE
Diagnosing HVAC problems has required far
more than a set of pressure gauges, a thermometer
and a multimeter for several decades now. With
electronic systems leaning heavily on cross systems
integration (one vehicle system working with and
sharing responsibilities and data with another system) the need for better understanding of our main
diagnostic tool has become more important than
ever. What main diagnostic tool is that? The automotive scan tool! Going beyond the obvious statement of every repair shop should have a scan tool
even if your specialty is HVAC, we must all admit
that most of the scan tools we use in the bay have
more information and features than we are really
that up to date on. In this MACS Service Report,
let's take up from where Paul Weissler left off with
the February 2016 report looking at electronic diagnostics in general and focus on understanding scan
tool data PIDs and bi-directional controls. First off
- just in case you've not been bitten in the nose by
that acronym "PID" - it stands for Parameter Identification. Sometimes called DPID (the word Data
on the front) it simply means a number or word of
information coming from an electronic module to
our scan tool via a serial data bus. As stated in the
July 2014 MSR, scan tools come in different levels of
accuracy and coverage depending on the manufac-

turer of the tool. "OE Level" tools are the aftermarket tools that try to do a good job of reading PIDs
and commanding functions and re-learns with every module on the vehicle that communicates with
a diagnostic tool. Factory or OEM Dealer scan tools
are tools like the GM Tech2 and Ford IDS. All they
do is work on their respective brand of vehicle but
they do it very well.
It's important to remember that everything in the
world of automotive HVAC electronic controls revolves around numerous computers each receiving
inputs from sensors and other computers, making
decisions based on the software programming from
engineers and carrying out those decisions via low
current (typically) outputs to solenoids, relays and
light bulbs. Keeping in mind that HVAC is not a
lone wolf system standing on its own, always diagnose like a (good) doctor who gives every patient
a complete examination from head to toe. This includes of course the automotive version of a complete body scan - the total vehicle, all module DTC
check. DTCs that you never would have dreamed
as recent as 10 years ago as being responsible for
fouling up HVAC system operations can and do become show stoppers for heating and A/C (Figure
1). HVAC PID analysis can be broken down into
underhood systems (cooling fan, compressor clutch

ALSO INSIDE THIS ISSUE:
Scan Tool Swiss Cheese .................................................................................................................................................................. 4
OEM Info / Subscription Security Updates ................................................................................................................................ 6
Volkswagen Mapped Cooling System ....................................................................................................................................... 8

MACS Service Reports is the official technical publication of the
Mobile Air Conditioning Society Worldwide, Inc., P.O. Box 88,
Lansdale, PA 19446. The material published in MACS Service
Reports expresses the views of the contributors and not necessarily that of MACS. Every attempt has been made to ensure

April 2016

the accuracy of the content of MACS Service Reports. MACS,
however, will not be responsible for the accuracy of the information published nor will MACS Worldwide be liable in any way for
injury, labor, parts or other expenses resulting from the use of
information appearing in MACS Service Reports.

MACS Service Reports

Table of Contents for the Digital Edition of MACS Service Reports - 2016