MACS Monthly Newsletter - 2021 - JUN4

and the most accurate way to test is with the scan tool
that has the Stellantis software.
Stellantis' scan tool software looks for data items for
relative humidity, windshield glass, cabin air and dew
point temperatures on most vehicles with the sensor,
and it is possible to confirm the trouble code as for a
bad sensor. With the doors closed and the system set
for 70°F on Automatic Temperature Control, select the

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DIAGNOSING THE SENSOR CIRCUITS
STELLANTIS HUMIDITY SENSOR DIAGNOSIS
There are trouble codes of course, and if your scan tool
has the needed software, you can pull them. But Stellantis provides only two codes, P1009-00 for humidity module failure and U1190-00 for lost communication between
the humidity sensor module and the BCM (body control
module). But as you'll see later in this report, the codes
are not always available.
The sensor is attached to a plastic bracket with a metal
clip, and that plastic bracket is mated to the windshield
with a special adhesive. So, when you replace the sensor, be super-careful detaching it, for if the bracket is
damaged, the windshield has to be replaced with an OE
replacement that includes a new bracket. The Stellantis
sensor has four tabs, two on each side (Nos. 2, 4), which
must be released from the bracket (No. 1) to free the cover (No. 3). See Figures 5, 6.
Incidentally, that windshield can't just be removed, and
the bracket repaired on a workbench. The windshield is
part of the structural support for the roof and its urethane adhesive is hard to clean, making windshield reinstallation after removal a questionable proposition. If
the job really has to be done, make sure the automotive
glass shop you deal with has a replacement windshield,
with the new sensor bracket in place and with new seals
and adhesive, ready to start the work.
Ready to continue diagnosis? See Figure 7. Check the

Figure 9: GM humidity sensor unplugs from windshield
bracket.
scan tool HVAC data items, starting with the humidity.
Hold a cup of steaming hot water just under the sensor
and the humidity reading should go up slightly. Wait until
the humidity reading drops to normal, then with a fine
spray bottle, spray warm water on the windshield at the
sensor and the reading should rise significantly. Turn on
the defroster and the humidity reading should drop as
defrost action begins (the A/C compressor should turn
on, of course).

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GENERAL MOTORS HUMIDITY SENSOR DIAGNOSIS
The General Motors windshield module system has a
harness (several wires to the HVAC control module) and
individual trouble codes for each possible failure (Figure
8). As a look into the new models (near future), there is
no LIN connection, because GM is going to a new electrical/electronic architecture that uses no LINs at all.
Rather, it remains primarily a CAN (Controller Area Network) design with some Ethernet, the most widely used
networking protocol in non-automotive commercial use.
And unlike Stellantis' 12-volt ignition connection, the
GM sensor has a five-volt reference wire (labeled No. 1
in the drawing, No. 3 as it goes to ground in the HVAC
control module). The windshield module has three sensors, each with a wire to the HVAC control module: No.
2 is from the humidity sensor, No. 4 from the windshield
temperature sensor, and No. 5 from the humidity sensor's temperature sensor.
Each sensor failure could trigger at least one code, or
even two (open or shorted circuit, or high or low voltage). A failure of the five-volt reference voltage signal
would produce a separate code for voltage over 5.5 or
under 4.5.
The humidity sensor failure codes are: B1395-07 for

Figure 8: General Motors (Buick Enclave shown) wiring diagram for windshield humidity sensor. Note wiring goes
to HVAC module.
wiring: 12V at the ignition terminal (No. 3), a good ground
from the ground terminal (No. 1). No. 2 is the terminal for
the LIN (local interconnect network) wiring to the BCM,

June 2021

MACS Service Reports

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MACS Monthly Newsletter - 2021

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