356 Auto Heating & Air Conditioning
Copyright by Goodheart-Willcox Co., Inc.
Data Link Connectors
Most temperature control computers can also be
accessed through the vehicle’s data link connector. The
data link connector (DLC) is the output terminal for the
vehicle’s self-diagnostic system. Vehicles that control
the HVAC system through the ECM or BCM may require
codes be accessed through the data link connector, rather
than the HVAC panel. Plugging the scan tool into the
DLC will automatically place the system in self-diagnosis
mode. On some older vehicles, connecting two of the
connector terminals with a jumper wire will place the
system in self-diagnosis mode. This should only be done
if specifi cally recommended by the manufacturer.
The data link connector is usually under the
dashboard or center console. On vehicles built before
1996, it may be in the engine compartment, fuse box,
or elsewhere, depending on the vehicle. Most vehicles
have only one data link connector. This connector is
used to access both engine and electronic temperature
control codes. Figure 22-4 shows some typical data link
connector locations.
Scan Tool Construction
Scan tools are small and light enough to be hand-
held. Most scan tools are similar in appearance to the one
shown in Figure 22-5. All scan tools have a keypad with
a set of function keys used to select the various functions.
Typical function keys are Enter and Exit keys, arrow keys,
and number keys. Many scan tools have specialized keys to
access specifi c diagnosis features, or to connect to a remote
computer or printer.
Some scan tools have indicator lights to tell you the
scan tool is operating, the HVAC system mode, or an obvi-
ous system defect. The display screen is used to display
trouble codes and other diagnostic results, and may also
display a menu. The menu shows possible scan tool func-
tions, often called diagnostic routines. The technician can
pick one of these routines to begin using the scan tool. See
Figure 22-6.
Cables and adapters provided with the scan tool allow
it to be attached to one or more data link connectors. Some
scan tools require adapters that plug into the data link
connector to allow the scan tool to access the temperature
control module. Some scan tools also have battery clips or
a cigarette lighter adapter to allow them to use the vehicle
battery as a power source. A few scan tools have an internal
backup battery.
Types of Scan Tools
Some scan tools can be used with only one kind of
electronic temperature control system. These are called
dedicated scan tools. However, most newer scan tools
can be used on many vehicles and are called multi-
system or generic scan tools. These scan tools use spe-
cialized cartridges and adapters which can be changed
to match the vehicle being worked on. Some scan tools
can be updated by an interface with a personal computer
or over a phone line to the manufacturer. Scan tools
with this feature have an extra terminal that can be
connected to a phone line.
Most scan tools are bi-directional. A bi-directional
scan tool is able to retrieve information from the vehicle
computer, and can send data into the computer. This ability
to transfer data two ways allows the scan tool to operate
system components for diagnosis, and to reprogram the
module if updates are necessary.
After the 1996 model year, all vehicle manufacturers
began to use the OBD II diagnostic system. All OBD II
systems use the same 16-pin diagnostic link and trouble
code format. Newer generic scan tools can retrieve trouble
codes from any vehicle using the OBD II system. An auto-
motive technical standards organization called The Society
of Automotive Engineers or SAE has standardized trouble
codes. Many post 1996 trouble codes will indicate the same
problem, no matter what make of vehicle they are obtained
from. Other trouble codes will be specifi c to the vehicle
being tested. The technician should always consult the cor-
rect model and year service information to determine what
a particular code means.
Using a Scan Tool
The following sections explain how to use a scan tool
to retrieve trouble codes and perform other diagnostic rou-
tines. Always refer to the scan tool manufacturer’s instruc-
tions for exact procedures.
Figure 22-3. Modern scan tools can be used to access diagnostic
information, as well as reprogram some computers. (Mac Tools)