MACS Monthly Newsletter - 2021 - APR8

The following procedure will exercise a mechanical control valve as well as
test computer control of an electronic control valve on most vehicles. Think
" heat load " .
If we place the system under severe stress by creating a large heat load,
it should respond by commanding the compressor to maximum stroke. If I
continue testing by removing the previously applied heat load, the system
should respond by forcing the compressor to minimum stroke. This can be
accomplished in the service bay without special tools.

that temperature down 30°F at the interior ducts is working very well.
Keep the system running and proceed with Step 2.
Minimum heat load...Step 2
Quickly change HVAC settings as follows.
1. RECIRCULATED air
2. Temp to FULL COLD
3. LOW blower
4. Mode set to DASH and FLOOR
5. CLOSED windows and doors
6. Stabilize and record readings
7. LOW-Side reading _____psi
8. HIGH-Side reading _____psi
9. Ambient Temperature _____°F
10. Duct Temperature Left _____°F
11. Duct Temperature Right _____°F
12. Clutch Duty Cycle _____%

Maximum Heat Load...Step 1
Configure the vehicle and record.
1. Gauges connected
2. Hot Engine
3. Hood open
4. Fresh air (cabin filter OK)
5. Temp to full " COLD " (not MAX)
6. High blower
7. Mode set to DASH and FLOOR
8. Open windows, closed doors
9. Stabilize and record readings
10. LOW-Side reading _____psi
11. HIGH-Side reading _____psi
12. Ambient Temperature _____°F
13. Duct Temperature Left _____°F
14. Duct Temperature Right _____°F
15. Clutch Duty Cycle _____%

Theory: In Step 1 an extremely high heat load was applied to the system.
Step 1 also cooled the vehicle interior and the air handling system. We are
now using this to our advantage in Step 2.
Step 2 will largely reduce total heat load on the system. Switching to RECIRCULATED air will now pull that cool interior air the system produced over
the evaporator. LOW blower will move less of that cool interior air (still containing heat) over the evaporator. Closing the windows stops warm outside
air from entering the vehicle.
The evaporator, now cooling " already cooled air " will approach freezing,
Low-Side pressure will temporarily drop a few psi and the system should respond by driving the compressor to Minimum stroke. Watch the High-Side
gauge. As the compressor is forced to minimum stroke the High-Side gauge
will drop steadily and heavily, possibly ending up at around 100 psi. When
readings stabilize the compressor should be in MINIMUM stroke and discharge air temperature will have dropped lower than it was after Step 1.
Reverse the HVAC settings and the system should then respond by moving
the compressor back to Maximum stroke.
Remember this testing is cause and effect produced by evaporator
temperature.
Poor airflow over the evaporator can cause it to operate too cold if the air
containing the intended heat can not flow over the core as it should. This will
" latch " the compressor into minimum or less than required stroke. Confirm
cabin filter is clean, ducts are free of rodent nests and the evaporator does
not have a thick film of refrigerant oil and dust blocking airflow.
Videos of the described Variable Displacement tests and other goodies are
available on the author's website www.theautoprof.com under the Technician Resource tab. v

Theory: The HOT ENGINE and HOOD OPEN will provide a large amount of
heat which we can use to load the refrigeration system. The FRESH AIR and
HIGH BLOWER settings will draw hot air off the engine and across the evaporator core. This will warm the evaporator core and is detected by an evaporator temperature sensor on an electronic system, or the mechanical control
valve will detect rising suction pressure. Mode set to DASH and FLOOR and
WINDOWS OPEN allows less restriction and more air flow across the evaporator.
The system should respond by forcing the compressor to maximum stroke.
IN MAXIMUM STROKE gauge readings should be the same as a fixed displacement compressor under the same severe conditions. High-side
gauge reading will vary depending on humidity but should be at least ambient plus 100 psi. The Low-Side gauge reading will rise above 30 psi. Why? We
are loading the evaporator core with so much heat the liquid refrigerant in the
core is completely vaporized mid-way through. This will superheat the vapor
on its way out of the core, increasing pressure. The engine cooling fan should
also run continuously on high speed.
Measure the temperature of the air entering at the cowl. A system pulling
MACS Service Reports is published monthly by the Mobile Air Climate Systems Association. It is distributed to members of MACS and
is intended for the educational use of members of the automotive air
conditioning service and repair industry. Suggestions for articles will
be considered for publication, however, MACS reserves the right to
choose and edit all submissions.

Editors:
Elvis Hoffpauir, Steve Schaeber
Production Designer:
Laina Forcey
Manager of Service Training: Steve Schaeber
Mobile Air Climate Systems Association
P.O. Box 88, Lansdale, PA 19446
Phone: (215) 631-7020 * Fax: (215) 631-7017
Email: membership@macsmobileairclimate.org
Website: www.macsmobileairclimate.org

Unless otherwise noted, all photos/art by author.

April 2021

MACS Service Reports

http://www.macsmobileairclimate.org

MACS Monthly Newsletter - 2021

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