MACS Monthly Newsletter - 2021 - JUN3

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So, when a person sweats, the sweat doesn't evaporate
-which would produce a cooling effect-the way it would
with low humidity.
What about Arizona, where the relative humidity is
typically low (the famous " dry heat " that people often
talk about)? The relationship between humidity and the
temperature people feel (called " sensed temperature " )
changes as the ambient (and cabin) temperature goes
up. So, if it's very hot, we still feel the heat.
If the relative humidity is low (say 20% in a very dry
area) and the ambient temperature is just 70°F, it feels
like 66°F, and someone in a short sleeve shirt and shorts
might actually want it warmer. However, if the ambient
temperature is 100°F, with 60% relative humidity, it will
feel like 132°F, oppressively hot. And when the temperature goes up to 110°-115°F, the A/C can't cool anyone
fast enough. And even if the humidity is very low, say
20%, but the ambient temperature is 100°F, it will feel like
100°F, and even though there may not be much condensate draining from the HVAC case, motorists will expect
the A/C to provide comfort by lowering it.
So, it's not surprising that a combination humidity/temperature sensor has come into wide use on Automatic
Temperature Control systems. The early sensors -we're
talking ten years ago (yes, they're not exactly brand-new
additions to the A/C scene) could be placed anywhere,
including into a seat. Shortly thereafter, we began to see
them placed in a better location-on the windshield, with
Stellantis (Fiat Chrysler) and General Motors the prominent examples that we're covering in this report.
The windshield-mounted humidity sensor measures
the relative humidity of the passenger compartment side
of the windshield. It also detects the temperature of the
windshield surface itself on the passenger compartment
side. Both values are used as control inputs for the HVAC
control module (or body control module if that is what
the car uses) to calculate the fog risk on the windshield
compartment side, and if indicated, turn on the defroster
circuit and A/C compressor.
This information even may reduce fuel consumption by
decreasing A/C compressor displacement or usage to

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Figure 6: With cover off, release retaining clip (3) from humidity
sensor (4) can be removed from bracket (2). Unplug connector
(1).

Figure 7: Jeep Cherokee windshield humidity sensor wiring
diagram. Note LIN connection to body control module.

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the needed minimum without causing any fog. The sensor also may enable partial recirculation mode in order
to improve heating-up performance of the passenger
compartment under cold ambient temperature conditions and without the risk of mist build-up on the windshield. The humidity sensor element's temperature sensor also supplies the temperature of the humidity sensor
element itself. But, this information is really only needed
if the thermal contact between the humidity sensing element and the inside windshield surface is not sufficient.

Figure 5: Chrysler windshield humidity sensor shown with
cover (3) on. Press release tabs (2) and (4) to release
cover from bracket (1).

June 2021

MACS Service Reports

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MACS Monthly Newsletter - 2021

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