MACS Monthly Newsletter - 2019 - SEP1

Total Vehicle Climate and Thermal ManagementTM

September 2019

By Dave Hobbs, MACS Technical Correspondent

In-Car and OAT (Ambient) Temperature Sensors
An Auto Electronics Quick Review

In-car temperature sensors and ambient / outside
air temperature sensors (sometimes referred to as
OAT sensors) have been around for decades. Simple
NTC (negative temperature coefficient) thermistors
are all they are, right? Yes, and that's always been
true for most all temperature sensors. A resistor
that changes resistance as the temperature changes.
Most are of the NTC design that see their resistances go up as temperature goes down (and vice
versa). It's what the engineers do with in-car and
OAT sensor circuitry and software coding details
that can lead to misunderstandings with diagnostics. Details like how they are supposed to be
mounted, supported with airflow (in the case of
in-car temperature sensors) and how these sensors'
readings are updated and processed by the various
HVAC related ECUs that makes it worth spending
some time discussing in this month's MACS Service
Report.
Sure, we can just say, "Read the DTCs and follow
the trouble trees," but we've got the service manual
for 90% of the occurrences where things are that cut
and dried on the vehicles coming into our bays. It's
the other 10% where we wish we knew a little bit
more about what it is we're working on. That tricky
10% is what we'll cover in this report.

ECUs (Electronic Control Units) of all names / acronyms are powered by 12-volts, even those used in
high voltage hybrid / EV models. As we all know,
the 12-volt system isn't always at 12-volts. A fully
charged 12-volt dc battery will most likely be in
the 12.6v to 12.8v range (surface charge removed)
at 70°F but may dip down to around 10-volts or so
when the engine is cranking and then back up to
anywhere between 12.9 and 14.9 volts depending
on ambient / battery temperature and the type of
charging system the vehicle has.
Having stated that, we know that thermistors are
simply temperature variable resistors and of course
resistors restrict (drop) voltage. In a typical thermistor circuit, the voltage drops are actually "divided" up between a fixed value resistor (called a
pull up resistor - Figure 1) inside the ECU / control
module and the thermistor in the dash or in front of
the condenser. As we stated before, anywhere from
10-volts to 15-volts is system voltage. So "dividing"
up a thermistor 's circuit voltage in a "system" voltage powered circuit is not practical when that system voltage is all over the place.
Hence the 5-volt circuit we've had for close to
40 years when OBD-I first appeared. Since the
variables of system voltage are too much to trust,

ALSO INSIDE THIS ISSUE:
Condenser Replacement: Now I Need to Calibrate a Radar? Maybe! ........................................................................ 6

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

September 2019

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 Monthly Newsletter - 2019

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