MACS Service Reports - 2015 - JUN1

Total Vehicle Climate and Thermal ManagementTM
By Paul Weissler, MACS Senior Technical Correspondent

June 2015

NEW HVAC PROBLEM IS SURFACING
circumstances takes place.
Surely you know about the algorithm that prevents A/C operation on full throttle or when coolant temperature is very high
(or not so very high as we've seen where it cuts out at just over
220°F). You may even remember that a misfire code will kick out
the A/C, although that also will turn on the Check Engine light
and have you looking to correct that issue first. But one of the
most important sensors is the one for outside air temperature,
and because of its location at the front bumper, it's often subject
to physical abuse that leads to damage of the sensor and/or its
wiring connector (Figure 1).
There's another issue however that's also rising to the top of
the list: the update rate. Say for example a car is parked overnight when it's normally cooler, and the sensor doesn't update
automatically or even as soon as the engine is started. As a result, the sensor is reading cool ambient, and that input causes
the HVAC system controls to destroke the variable displacement compressor, reducing A/C output for better fuel economy.
The motorist notices this on his way to work in the morning,
when the weather has warmed up, and he brings the car in for
a check. Let's say he parks the car in the driveway outside your
shop at night. Then, sometime after you re-open in the morning,
and eventually bring the car into a warm shop, sure enough, the
A/C isn't performing so well. You can go through all the routine
checks, even doing a complete evacuation and recharge with
your SAE J2788-compliant machine (a great diagnostic tool), but
the refrigerant charge isn't the issue.
In cases like this, you need to read that outside air temperature
sensor on your scan tool, if you happen to have a scan tool that
can read the sensor. Unfortunately, the basic "generic" testers do

motorcraftservice.com

As cars have been switched (for the most part) to electronic
controls, the so-called manual A/C systems are really electronic, even if it seems as if the motorist can make the decisions. In
many cases, there's an electronic control module in the background, deciding whether or not to allow air conditioning. And
of course with Automatic Temperature Control in Auto mode,
the motorist has limited range for command, which often leads
to confusion and dissatisfaction. When you read the special
report on this subject (which involved a lemon law suit) in the
September, 2015 issue, you'll get the idea. Lately we're seeing
these poor performance complaints that may appear to be intermittent, but really would occur every time a particular set of

Figure 1: Outside air temperature sensor is mounted on bumper beam on
this car, but that's no guarantee of freedom from damage. In fact, no front
end location seems to be immune.

ALSO INSIDE THIS ISSUE:
WHEN A CONNECTOR IS MORE THAN A CONNECTOR .................................................................................................................. 2
IS THE WIRING HARD TO REACH OR JUST HARD TO FIND? ......................................................................................................... 3
ANOTHER CASE IN THE SAME VEIN ................................................................................................................................................ 5
FAN OPERATION: WHERE'S THE STRATEGY? ............................................................................................................................... 6
A/C GROAN ......................................................................................................................................................................................... 7
CLUTCH DOESN'T ENGAGE ............................................................................................................................................................ 8
MACS Service Reports is the official technical publication of the
Mobile Air Conditioning Society Worldwide, Inc., P.O. Box 88,
Lansdale, PA 19446. The material published in MACS Service
Reports expresses the views of the contributors and not necessarily that of MACS. Every attempt has been made to ensure

June 2015

the accuracy of the content of MACS Service Reports. MACS,
however, will not be responsible for the accuracy of the information published nor will MACS Worldwide be liable in any way for
injury, labor, parts or other expenses resulting from the use of
information appearing in MACS Service Reports.

MACS Service Reports

http://www.motorcraftservice.com

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