MACS Monthly Newsletter - 2017 - MAR1

©

Total Vehicle Climate and Thermal ManagementTM

March 2017

By Dave Hobbs - MACS Field Correspondent

Outside Air Temp / Ambient Air Temp Sensors - New Roles and New Problems
Background and Operation

Dave Hobbs

Every general tech or HVAC specialty tech knows
that the Outside Air Temp Sensors (OAT), also referred to as Ambient Air Temp Sensors (AAT), plays
an important role in HVAC systems. (Fig. 1) But did
you know on some vehicles this mundane sensor
plays a role in powertrain electronics and can actually cause low engine power and / or a warning
indicator for the traction control or stability control system? In this month's MACS Service Report
we'll cover those crazy systems interactions, the en-

Figure 1: OAT (Outside Ambient Temp) and AAT (Ambient Air Temp) sensors have been around for decades but in more recent years have had their duties
expanded beyond HVAC and IPC / DIC / Radio duties. Now it's a serious PID
to look at if you have drivability problems or DTCs and even Chassis electronics
DTCs.

Dave Hobbs

hanced role of these sensors on today's vehicles and
practical ways to diagnose OAT / AAT problems.
Traditionally utilized in Automatic HVAC systems,
these simple thermistors mounted outside the vehicle (behind the front grill in front of the condenser)
provided an input to the HVAC head / programmer in order for it to determine if the driver was
requesting 70 deg. F (for example) of cooled duct in
summer conditions or 70 deg. F of warmed up floor
or defrost air in winter conditions. At one time

Figure 2: This chart shows the relationship between a sensor's resistance and temperature.

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

March 2017

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.

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MACS Service Reports



Table of Contents for the Digital Edition of MACS Monthly Newsletter - 2017

MACS Monthly Newsletter - 2017 - JAN1
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