MACS Service Reports - 2015 - AUG5

INCREASING MPG BY KEEPING A/C FROM GETTING TOO COLD!
Keeping with traditional design, the air temperature coming out of the evaporator to be connected to
the vehicle's ductwork are kept just above the freeze
point of water thanks to the control valve. Since the
driver and passengers don't always want duct temperatures as low as they can go, the HEBA (Heater
Evaporator Blower Assembly) reheats a portion of
the cooled air to a higher temperature courtesy of
the heater core. The key word for fuel economy is
REHEAT. Think about that. You take some gasoline
to create heat in the form of expanding (pressurized) gasses to move the piston down to get the vehicle moving. A bunch of that heat energy from the
gasoline is wasted to the air via the vehicle's cooling system. Then you make the engine's drive belt
turn a (hard to turn) A/C compressor to make the
passenger compartment colder than it needs to be.
To keep the passengers at temperatures they really
want, we take some of that wasted heat energy created in the combustion process and use it to reheat
the air. Sounds like a lot of waste, right? Getting
away from this is what engineers have called Series
Reheat Reduction. It's enough of an increase on fuel
economy (around 0.4 MPG increase) and change in
technology that has caused serious changes to the
HVAC systems you are starting to see in your service bay.

evaporator freeze up protection duties. This sensor
is not a duct sensor either like those you might see
on a scan tool for each duct in an automatic HVAC
system. Those sensors are for fine tuning the temp
doors so the sometimes fooled aspirated in car temp
sensor doesn't have the total responsibility for helping an automatic HVAC system maintain the perfect
temperatures. This EOAT is usually mounted near
the evaporator core and is a very important input to
the HVAC control module so it can cycle the clutch
for temperature control that does NOT require the
blend door to heat up A/C air that is too cold.
Now since we know that a flakey evaporator
temp sensor (mounted on the evaporator core) on
some older vehicles can keep a compressor running
too much or not allow it to run at all, it makes sense
that this newer sensor mounted near the evaporator on clutch cycled pneumatic controlled variable
displacement compressors can do the same thing.
See Figure 8.

Does Econ Still Shut Off The Compressor?
If you've studied hybrid A/C, you know that EOATs have been used to allow the Ford and GM hybrids with mechanical compressors as well as the
hybrids with electric compressors to allow compressor shut off during idle to reduce fuel usage
(idle stop)when A/C temps are cold enough to keep
the customer happy. Remember, on mechanical
compressors this is the time when that load on the
compressor drive belt dampens fuel economy the
worst. On some stop start hybrids such as the GM
E-Assist, if you press the A/C compressor request
(snowflake button) and there's a green LED on, that
means the EOAT is going to let the HVAC controls
know when the gas engine has to be restarted during a prolonged idle stop in order to get some cold
refrigerant moving through the evaporator again.
Press the button to get an amber LED and now you
are telling the HVAC "I don't want a dip in A/C

Are We Still Cycling Clutches? EOAT and Series
Reheat Reduction

August 2015

Dave Hobbs

One method to accomplish Series Reheat Reduction with a variable displacement compressor is to
use a sensor (thermistor) in the evaporator case. On
these vehicles, the system utilizes a pneumatic DCV
in the compressor to maintain the lowest non-freeze
point evaporator temperature and pressure. It simply cycles the clutch when the control head temperature request and EOAT (Evaporator Out Air Temp)
sensor tell the HVAC controller that compressor
operation is not required to achieve / maintain the
desired temperature at that moment in time.
Don't confuse EOAT with OAT. OAT is Outside
Ambient Air Temperature, which is the sensor in
front of the condenser telling the HVAC controls to
either warm up to 60°F or cool down to 60°F, depending on outside air temperatures. This newer EOAT
might not even be the same as the sensor stuck to
the evaporator coils to help with clutch cycling /

Figure 7: The electronic DCV is controlled by the PCM.

MACS Service Reports

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