MACS Service Reports - 2015 - APR4

Wild OATs
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Outside Air Temperature (OAT) sensors, sometimes
also called ambient air temperature sensors, have one
or two jobs depending on the vehicle. If the vehicle has a
temperature display somewhere on the cluster / HVAC
/ radio, it may have an OAT to obtain that reading.
It may also be used as a very important input in ATC
(Automatic Temperature Control) systems. Unlike residential HVAC systems which have a switch to choose
between heating and cooling, in the automotive HVAC
world the ATC controller will need to make determinations based on outside conditions. For example, if a
driver selected temperature set at 70°F, does that mean
warm up to 70° (heat) or cool down to 70° (A/C)? You
can literally drive a vehicle within an hour or so from
one extreme to another, requiring use of both heating
and cooling. Similar to in-car temp sensors that need a
sample of fresh air moving over them to get an accurate
reading, the OAT sensor is mounted right in the front
of the vehicle, usually behind the grill. No problem getting air samples if you are moving down the road. It's
when you stop for an extended period of time or are
idling when you may see problems. Most OEMs have
corrected the issue of engine induced OAT heat soak.
Without a software work around (called filtering),
these sensors would end up measuring the ambient air
around the condenser / radiator, which is much hotter than the actual ambient temperature. Temperature
displays and HVAC control modules on most models
today ignore the outside air temperature reading unless
a signal from the VSS (Vehicle Speed Sensor) is sensed.
Many are done in a progressive manner, meaning that
the updated temperature within the control module
(often the same number seen on the scan tool and / or
dashboard display) will increment up in temperature at
a rate based on vehicle speed (Figure 10).

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Figure 8: P0118 would be the likely DTC with temperature this low / resistance this high.

Figure 9: Normal ECT reading with no DTCs, although the ECT could
still be slightly skewed and cause a no code drivability / starting problem.
On some models, a very believable ECT DPID could still be displaying a
"fail-soft" like the one in this scan tool screen capture. Always look at the
same DPID in OBD-II Global to compare. OBD-II Global parameters are
supposed to be the real thing, not a "limp-in" default value.

General Motors / Dave Hobbs

errors, fall outside of the operational voltage range and are
identified as such in the code description. See figures 7, 8 and
9.
In the case of the ECT, the control module has a stored
map for the operating characteristics of an engine coolant
temperature sensor. This type of code will result from three
possible sources:
1. Wiring: high resistance in the circuit.
2. Sensor: problems that affect the actual voltage
reading (calibration).
3. Cooling system: performance of the cooling
system (thermostat).

Figure 10: OAT (Outside Ambient Temperature) has been using a sophisticated algorithm (computer instructions) for many
years to improve accuracy under various road conditions.

April 2015

MACS Service Reports

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Table of Contents for the Digital Edition of MACS Service Reports - 2015