MACS Monthly Newsletter - 2021 - APR2

on the type of compressor, swash plate slider shoes may be overloaded and
piston rods in variable displacement units can be bent. Bad news all around.
See Figure 1.
If the specific year/make/model is prone to slugging by design, there is
little a service technician can do to change the system design. Installing an
" anti-slugging " module may be the cure. This module is installed in the compressor clutch coil circuit. These modules rapidly cycle the clutch on/off upon
start-up to safely clear the compressor of liquid, before allowing the 100%
duty cycle of the clutch relay to run the compressor as designed.

When a system is in good health and functioning properly, the compressor
might run forever. When that same system develops a small leak, the system
will continue to operate until the refrigerant level reaches a point where the
control system will intervene and turn the system off. Operation and environmental conditions may allow it to run again for the short term without the
customer noticing, but eventually modern systems are shut down when the
controller determines the charge is too low.
Let us examine what happens to a compressor in this scenario. If refrigerant carries oil to and helps to cool the compressor, what happens as the
system continues to operate as the charge leaks out?
Exactly...the low refrigerant level causes reduced oil flow back to the compressor. A small refrigerant leak can have the system operating at a belowoptimal level of charge for an extended period in possibly severe conditions.
This can result in compressor damage. See Figure 2.

Mike Bailey

As service technicians we can reduce the chances of either
causing slugging or aggravating slugging in a vehicle prone to the
problem.
1. Strictly adhere to the manufacturer's refrigerant charge
specifications. Research TSBs before service to determine if slugging is a problem and possible changes to refrigerant charge
specs.
2. Take the time to carefully balance the oil charge during
service. Strictly adhere to manufacturer's instructions on oil balancing when replacing components. Be mindful of previous service and possible oil overcharging. A complete system flush and
manual draining of oil from the compressor may be in order if you
suspect oil overcharging. Start from scratch if in doubt and install
the OEM specified volume of oil after flushing.
3. Install an anti-slugging module if the vehicle is prone to slugging. Consult TSBs to determine if the vehicle in your bay is a candidate. See Figure 1.

Figure 3 shows a GM R4 Radial compressor. This compressor model functioned perfectly fine
as a light weight and
space saving unit. It
replaced the old Frigidaire A6 in the early
days of Orifice Tube
systems. Unfortunately,
back then technicians
could " top up " or even
recharge leaking systems. These systems
held a large refrigerant
charge and would run Figure 3: GM's R4 compressor would
for a decent amount of fail from reduced lubrication due
time with a small leak. to leaks causing a low refrigerant
As the system ran and charge.
the refrigerant level
dropped, the compressor ran on a reduced amount of oil. This resulted in the
unit eventually seizing solid after starving for oil and grinding its innards to
bits while discharging them into the system. The low pressure/cycling switch
was supposed to protect the compressor, however continuous cycling as the
charge dropped only added to the oil starvation problems. Accumulator/
dehydrators left in service during repair also contributed to R4 compressor
failures. Saturated desiccant would break apart and plug the oil return hole
at the bottom.
Why did that Frigidaire A6 rarely seize up? These early compressors which
also included the York/Tecumseh and Chrysler V2 designs had built-in features which allowed them to operate without refrigerant in the system. They

Staying cool
Sure, the end game is to cool the passenger compartment. What about
cooling the compressor? Manufacturers have taken compressors from being right on top of the engine in a relatively " cool " environment and tucked
it away in a tiny space, possibly between a subframe and an engine block
under an exhaust manifold, bolted to the engine. Sometimes you can barely
see the thing.
What cools a compressor? There are several ways a compressor sheds
heat. Some are intended and some are happenstance.
Air flowing over the compressor body will carry a small amount of heat
out of the unit. Mounting pad contact to the engine block will also remove
some heat. Neither of these are considered major contributors to compressor
cooling though.
Heat from the compressor is typically shed through the condenser. That
is right...refrigerant and oil returning to the compressor from the evaporator
can still absorb and carry heat out to the condenser.
Some vehicle and evaporator designs will superheat the refrigerant vapour before returning it to the compressor. Still, enough heat can be added to
the refrigerant and taken away to keep the compressor working.
Lubrication and refrigerant level
Most technicians are aware that refrigerant carries oil. During service we
need to add the appropriate amount of the correct oil to keep the compressor
working.
Consider the average vehicle that comes in for Air Condition service. Electrical and air flow problems are common, but studies show the most popular
failure to be low refrigerant charge or the system is empty.

April 2021

MACS Service Reports

Mike Bailey

Figure 2: A healthy swash plate (left) and a damaged
swash plate (right) caused by a leaking system, running
undercharged for an extended period.

MACS Monthly Newsletter - 2021

Table of Contents for the Digital Edition of MACS Monthly Newsletter - 2021