MACS Service Reports - 2015 - SEP5

September 2015

Figure 4: Chart shows temperature changes in airflow between dash registers
and floor outlets under damp morning conditions, simulating a drive to work.
System was set for 74°F and bi-level. Operation was source of customer complaint, but the computer strategy had a logical approach. This took place in midsummer.

MACS Service Reports

Paul Weissler

we were given on this case, because it was being recorded to present as evidence. One drive cycle was
45 minutes, replicating a drive to work, and it took
place during a humid morning. The ambient temperature at the start was 81°F and was rising. The
system was in bi-level and the A/C set point for the
cabin was 74°F.
The airflow from the floor registers went up from
a starting point of 78.6°F (actual cabin temperature)
to the mid-90's and then started to drop. By the end
of the trip the floor air was at 86.3°F. While the airflow temperature from the upper registers steadily
dropped from the outset, by the end of the trip they
were at 52°F, about what one could expect on a humid day. A possible strategy: switch from bi-level to
dehumidified cold air on the driver's face and chest
and the system delivers that, while the cabin also is
dehumidified. The floor duct air is not cooled and
its temperature rises somewhat. See Chart, Figure 4.
The next test on this car was a middle of the day
drive on a hot day with low humidity (43%). The
ambient temperature was 95°F and the car had been
in a hot soak (parked in the sun) for some six hours.
The air in the upper registers was 120°F; in the floor
outlets the air was 109°F at engine startup. The fan
was set for a high speed and the A/C for 73°F. Once
the air in the ducts had cleared out, the dashboard
registers were blowing 87°F air, the floor outlets
96.9°F air. However, in a matter of a few minutes
the air from both upper and lower ducts was cool
and within a few degrees, with the upper registers a
bit cooler (Figure 5). That's pretty much what most
people would consider normal from a manuallycontrolled system. If the system always performed
this way, the motorist likely would not have complained.
But here's the third set of operating conditions,
which were considered truly abnormal. It's the end
of the workday drive back home. See Figure 6. The
ambient temperature was 89°F and the A/C was set
to 74°F. For the first 12 minutes the system operation is like what you'd expect on a hot day. Both upper registers and floor outlets are pumping out cold
air. However, the humidity is rising and after 12
minutes a light rain starts. The rain becomes heavier and heavier as ambient temperature drops, first
to 79°F, then to 77°F. When this happens, the airflow
from the A/C registers continues to cool. Bi-level
cooling ends and the temperature of the air from the
floor registers rises gradually into the upper 90's.
Basically, although this change seems odd, it's really a combination of starting with the hot and dry
day. When the rains come, it switches very quickly
to upper registers only, with cooling and dehumidification of the high humidity air.
The owner may not like this strategy for one rea

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