MACS Monthly Newsletter - 2021 - JUN5

high out of range (greater than 5.5 volts for more than 50
milliseconds), and B1395-03 for under 4.5 volts for 50ms.
It also could produce the out of range voltage code
(B1395-07) or one of the specific sensor codes for over
4.9 volts. Other codes -for short to ground are B018A-02
for the windshield temperature sensor; or B048C-02 for
the humidity sensor, B048F-02 for the humidity temperature sensor. A code suffix of 05 is for open circuit/high
resistance or short to voltage.
If the humidity sensor fails, the HVAC module inserts
a substitute value of 60% relative humidity to keep the
system operating. If one of the temperature sensor failures occurs, the module uses the last temperature value.
When there's a sharp temperature change since the last
operation, this can be a tipoff to a performance problem.
The windshield sensor circuit is typically operative
only when an ATC system is in an Auto mode, so if system performance is normal when switched to the manual mode, that bit of information also can help with diagnosis. The humidity sensor unplugs from the bracket for
the rearview mirror (Figure 9).

is requested, and the compressor clutch is engaged, so
apparently the PCM is okay, but something is producing
a request signal to the PCM, but what? Because the A/C
button seems to be working normally when the technician presses it, the likely source for an A/C request is
from the defroster circuit. As we now know, the windshield sensor circuit can activate the defroster when the
humidity and temperature conditions indicate that windshield fogging is about to occur.
The technician's scan tool did include data items for
the humidity sensor, but it didn't produce the humidity sensor failure code. However, he did get a dewpoint
reading of 328°F when the normal limit was 185°F. So, it
obviously had gone into default. He also got what appeared to be normal readings for windshield glass and
sensor interior air temperatures, but the data item listing
for the relative humidity simply displayed that the " signal not available/not programmed, " and we couldn't find
the numerical signal on later Cherokee models either.
The software for other Stellantis products is apparently
where the trouble codes appear, but the signal unavailable data item should be sufficient information for diagnosis.
He tried unplugging and then reinstalling the humidity
sensor, and pressing hard on the sensing block on the
side of the sensor, and it produced a humidity reading
of 1%, obviously nothing realistic. But it did enable him
to turn the compressor on and off with the A/C button,
which seemed to confirm the humidity sensing issue.
When he stopped pressing and the system went into
default, the compressor remained engaged-the button
wouldn't turn it off.
Yes, installing a new sensor solved the problem. The
scan tool showed a normal dewpoint temperature and
a 45% relative humidity, so apparently when the signals
are good, they are displayed on the Cherokee. v

IS THE WINDSHIELD SENSOR EVER A REAL-WORLD PROBLEM?
You bet it can be, and not always in an obvious way.
Here's an example, on a 2017 Jeep Cherokee Trailhawk
(the trail-rated version) with the 3.2-liter V6 engine. The
symptom: A/C compressor engages, and the system
blows cold air even when the control panel has the A/C
off but the fan on. The A/C button was off, and the button does function physically, registering on and off when
the technician pushes it. He connects a scan tool and
checks the PCM data items and even with the button
in the off position, the data item list shows A/C is requested.
Lots of possible causes. Is a PCM driver faulty? Is there
a short in the control panel? The PCM shows that A/C

techinfo.toyota.com

That Flow Sensor in Toyota and Lexus Vehicles

Figure 10: DENSO compressor has refrigerant flow sensor, which incorporates a spring-loaded spool valve.

June 2021

MACS Service Reports

http://techinfo.toyota.com

MACS Monthly Newsletter - 2021

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