What’s
worse is that so many service advisers and A/C technicians who really don’t
understand how the A/C system works go one step further and even attempt
to ‘nail down’ the restriction by telling customers that it could
be in the accumulator or evaporator! It really is surprising to hear some
of the explanations, but in fact, they don’t know what the problem is
and they are just grasping at answers to confuse the customer.
We
particularly like asking this question because in fact, it is the shortest
question you can ask an A/C service technician and quickly separate the amateurs
from the professionals. The amateurs think that A/C pressures are a lot like
tire pressures. You know, the more you put in the higher the pressure. The
truth is that A/C system pressures don’t work the same way.
In
order to truly understand what’s happening, you must first understand
that when working on auto A/C systems, you are really taking the pressures
of the refrigerant (or as some still call it, the freon). Refrigerant has
a pressure/temperature relationship. This is one of the most basic principles
that you learn about A/C. That pressure/temperature relationship means that
whatever we do to one, we automatically effect the other in the same direction.
As an example, if we do nothing more than increase the temperature of
refrigerant, we will automatically increase the refrigerant’s pressure.
Yes, automatically! And there is nothing you can do to prevent it. Likewise,
if we do nothing more than reduce the temperature of the refrigerant, we
will automatically reduce the refrigerant’s pressure.
Given
that basic information about refrigerant, you should really start to understand
what really causes increased refrigerant pressures. The fact is that increased
temperatures cause increased pressures!” Added heat will always cause
the refrigerant to increase in pressure. Now understand that poor cooling
is not the only cause of high pressures, but it is typically the most common.
We will get to the others in a minute.
Understanding
how the refrigerant’s pressure/temperature relationship works, when
you see increasing pressures on manifold gauges, you have to look towards
all the possible problems that are related to the cooling of the refrigerant.
That’s because the high pressures are most likely a result of a lack
of cooling!
There
are three basic, but distinctly different problem areas that you have to
consider when looking for poor cooling problems. First of all, the most common
place to start is at the condenser. The condenser’s sole function is
to remove the heat from the refrigerant that was absorbed in the evaporator.
When inspecting the condenser, you must be assured that the air flow over
the condenser is not obstructed and that the condenser is clean and free
of debris.
Secondly,
you must assure that fan shrouds and air damns are secure and in place. If
working with electric cooling fans, it is critically important that all fans
work at all speeds. (Some vehicles utilize dual cooling fans and others have
fans that operate at two speeds.) You must be able to confirm that the fans
are all operating as they should be.
Last
but not least, don’t overlook engine cooling problems. Yes, and overheating
engine radiator can and will transfer heat to the condenser and cause poor
cooling conditions for the A/C system.
In
actual fact, what you will most likely find is a combination of these problems.
It’s important that your visual inspection is complete. In some cases,
you may even have to remove part of the fan shrouding or radiator support
brackets in order to inspect between the condenser and radiator. In so many
cases, although the front of the condenser may be clean, the area between
the radiator and condenser becomes clogged with debris and seriously restricts
air flow.
Earlier
we said that there are other possible reasons for high pressure gauge readings.
Those high pressures could be as a result of an excessive oil charge, an
overcharge of refrigerant or air in the system. However, understanding the
repair history of the vehicle will quickly eliminate those problems as possible
causes of the high pressure gauge readings.
So
what about the restriction? What really happens when there is a restriction
in the A/C system? What really happens is that the pressure gauge readings
drop! Yes, they drop and become lower than expected! When you think about
it in the way we are going to explain it, it will make perfect sense to you.
First
of all, understand that when there is a restriction in the A/C system, it
is almost always at the expansion valve (or orifice tube if so equipped)
or at the liquid line receiver drier. By virtue of their design and construction,
evaporators and accumulators are almost impossible to ‘plug up’.
We only mention those components because those seem to be the ones that are
typically ‘pointed to’ by service techs making the wrong diagnosis.
When
looking at the expansion valve or liquid line drier, understand that they
are both part of the high side of the system (between the condenser and the
evaporator). For the purposes of this discussion, let’s assume a restriction
at the expansion valve. When that happens, where would you expect the refrigerant
in the condenser to go? Now remember, it’s a closed loop system. Typically,
on a properly operating A/C system, the refrigerant would leave the condenser
and travel down the liquid line through the expansion valve. With a restriction
in the expansion valve, the refrigerant in the condenser has no place to
go. It simply sits there (or is moved out of the condenser very slowly).
Now consider what else is happening at the condenser? What are the cooling
fans doing? Typically, the cooling fans continue to operate. Therefore, consider
the basic information we already know about refrigerants. We know that
refrigerants have a pressure/temperature relationship. Because of the pressure
temperature relationship, when we reduce the temperature (as in when the
refrigerant is being over cooling because the restriction causes it to remain
in the condenser), we automatically reduce the refrigerant’s pressure.
Yes.. the pressures are actually reduced!
When
you have a restriction in the A/C system (which is typically at the expansion
valve or liquid line receiver drier), the refrigerant pressures will drop.
Not increase! Don’t get confused. |
This
was probably one of the best books on recharging air conditioning that I
have seen...
