MACS Monthly Newsletter - 2017 - MAR2

in my career as a service tech I can recall that using that sensor information to display on the IPC or
HVAC head was for the most part limited to luxury
vehicles. In more recent years we've seen outside
/ ambient air temperature displays added to just
about every vehicle on the road. The software tricks
utilized to ensure an accurate reading of outside /
ambient air temperatures has by in large remained
the same. What I mean by software trick is the filtering of the simple analog signal the sensor is providing. These sensors, like the majority of other
temperature sensors, are almost always negative
coefficient thermistors. A thermistor is of course a
resistor that changes its resistance when the temperature surrounding it changes. The negative coefficient part of the description simply means that
the thermistor 's resistance reacts by going up in resistance when the temperature it's measuring goes
down. (Fig. 2) If the temperature is very hot, the sensor 's resistance is very low (maybe a few hundred
ohms) and when the temperature gets extremely
cold the sensor 's resistance goes up into the thousands of ohms. A module (traditionally the HVAC
and in newer models more likely the IPC or BCM)
connects to the OAT or AAT sensor and applies the
traditional 5 volt reference voltage via a current
limiting pull up resistor. The sensor is supplied a
dedicated sensor ground by the same module as
well. As the OAT / AAT sensor 's resistance changes
with temperature, the 5 volts supplied to the sensor
(Fig. 3) changes as well. The module's A/D (analog

to digital) converter input for that sensor sees the
voltage at its fixed value current limiting pull up
resistor change. This outside temperature value is
now a digital value within the logic of the module.
With almost every module on the vehicle now communicating with each other on a communications
bus, outside / ambient air temperature can now be
passed along to systems on the vehicle in need of
this valuable information.

Software Filtering
While a few OEMs have tried some alternate
mounting locations for outside / ambient temp sensors, the vast majority of vehicles on the road have
this sensor located behind the grill. Since we have
a very warm radiator and condenser in the vicinity of the sensor, software engineers for years have
programmed various modules with a filtering algorithm that causes the sensor 's temperature increases to be ignored while the engine is running and
the vehicle is stationary. The reason of course is the
heat soak inevitable due to the sensor 's mounting
location. What good does it do for the HVAC controls to see an engine baked 100 deg. F OAT / AAT
sensor on a 45 deg. F day? The filtering (delaying
the update of sensed temperature) is delayed in a
linear manner that takes into consideration vehicle
speed and time. The sensor gradually cools back
down from engine heat soak and gradually begins
to read true outside / ambient temperature as the
vehicle's speed increases over the course of time.
This has been going on for many years.

New Uses for Air Temp

Delphi

As engine management systems continue to get
smarter, OEMs have discovered a good use for the
old OAT / AAT that was in use for driver information and comfort. With the increase of forced air induction systems (turbochargers and superchargers)
and continued drive to lower emissions / increase
fuel economy, we've seen the need for increased
accuracy in measuring just about everything. The
accuracy of sensed temperatures for air that will
eventually enter a hot engine compartment's air
cleaner inlet and pass on into the intake manifold
is one such area. Diesel engines, for example, must
balance the need for power and greater emissions
controls. The power factor is increased with variable geometry turbocharging and induction cooling
(intercoolers) and the emissions via water cooled
EGR valves and exhaust after treatment systems. In
order for the ECM / PCM to be able to monitor the
proper operation of EGR and turbocharging, various air temperature sensors are in use. Some diesels

Figure 3: A voltage divider circuit is comprised of the fixed high ohmage
resistor in the module (usually several thousand ohms) and the variable
resistance of the temp sensor. If you are curious just what that resistance
is in the module the sensor connects to, disconnect it with the ignition off
and measure resistance between the sensor signal circuit and dedicated
sensor ground (harness side) and you'll most likely see it.

March 2017

MACS Service Reports

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