this technical article I will be focusing on the steps involved in
diagnosing, repairing and recharging your air conditioning system on the
1984-92 BMW E30 3 Series. Keep in mind that this technical article is
written with my own BMW 325is in mind, however the procedures and
information herein can be applied to nearly every modern car with air
conditioning out there on the road today.
First off, lets begin by first
explaining in laymans terms how air conditioning works.
Here is a list of the main system components and what they do.
main function of the compressor is to compress and pressurize the
refrigerant in the system, and also to keep the refrigerant moving through
the system when the A/C is turned on. The compressor is belt driven off the
engine. The compressor takes in cold, low-pressure gaseous refrigerant and
compresses it. As it compresses, it builds up heat. This now-pressurized hot
gas is sent to the condenser.
condenser then receives the hot gaseous refrigerant from the compressor.
Usually, condensers are placed in the front of a car, where it receives
oncoming airflow from the radiator fan. The condenser then turns the hot gas
into a liquid. This liquid
(still under high pressure) is then sent to the receiver drier.
receiver drier next receives the hot pressurized liquid refrigerant from the
condenser. It is essentially nothing more than a desiccant tank. It removes
moisture from inside the hot liquid refrigerant. Moisture in the lines is a
potential problem, as refrigerant can eventually react with moisture and
corrode the insides of the system. The process is completed when the hot
liquid refrigerant then exits the receiver drier and passes through the
expansion valve receives the hot liquid refrigerant. It allows the pressure
of the liquid refrigerant to drop. The drop in pressure causes the liquid
refrigerant to cool down. The result is cold liquid refrigerant, which is
then sent to the evaporator.
The evaporator receives the cold
liquid from the expansion valve. The
evaporator is usually located inside the car under the dashboard. When you turn on the A/C fan, it channels air over the
evaporator, cooling the air; this air is then blown through the vents of the
car. As the cold liquid passes through, it evaporates from a liquid back
into a gas, and is then routed back to the compressor to start the entire
there are two more components to the system that control the operation when
you turn the compressor on and off.
The A/C Clutch is used to engage
or disengage the compressor when you press the A/C button on the dashboard.
When the A/C is turned off, the belt that drives the compressor spins freely
on the A/C Clutch, which is mounted on the front of the compressor. Now when
you push the A/C button, it sends current to the clutch, which locks it and
allows the compressor to begin turning.
Pressure/Low Pressure Switch
two switches are a built-in safety feature. When the system pressure is too
low it interrupts the voltage going to the clutch on the compressor, which
then disengages the drive belt from the compressor, and turns off the A/C.
It works the same way when it senses there is too much pressure in the
now that I have explained a little about how the system works, we can now
begin to go over the steps involved with recharging your A/C system. The
first thing to know is that there are two types of refrigerant used in most
modern A/C systems, R-12 and R-134a.
until about 1991, all major automobile manufacturers used R-12 as a
refrigerant in A/C systems. It was discovered that R-12 contains CFC, or
chlorofluorocarbons, which destroy the ozone layer. Hence it was necessary
that they come up with a replacement. This came in the form of R-134a.
Nowadays every car uses R-134a. There has been much debate over the years as
to how well R-134a works as an automobile refrigerant. When R-134a was first
used in cars, it was not uncommon to notice that the new refrigerant simply
did not cool as well as the older R-12. It was also pretty common for the
A/C system to emit unpleasant odors after running for long periods of time.
These days engineers have sorted out nearly all of the problems first
two types of refrigerants cannot be used together. For instance, you cannot
put R-134a directly into a system that originally used R-12. Thus we run
into one of the major problems with recharging A/C systems. R-12 is no
longer available to the general public. I have heard of certain vendors that
do still make R-12, however it is unlikely that one could purchase it. These
days, A technician has to have a special license to purchase and charge R-12
systems. What this means is that you can still have your old R-12 system
charged, however the increasing costs of stockpiled R-12 is making this less
and less of an option. If you choose to have youre A/C recharged at a
shop that still services R-12 systems, expect to pay $50 a pound or more. As
most cars use two pounds or more of refrigerant, you can quickly see why
R-134a becomes a more attractive option.
I also have heard of products on the market that are a direct
replacement for R-12, such as Freeze 12 or Duracool, however I have no
direct experience with using them. I have heard from various sources that
Freeze 12 seems to work very well as a direct replacement.
may be asking now what is involved in converting to 134a. Well, there are
many kits on the market you can buy at your local auto parts store
that say all you have to do is replace the fittings on the fill ports, and
recharge using R-134a. I wish it were this easy. The simple fact is that a
lot of preparation work needs to be done in order for the A/C to work
correctly with the new refrigerant. To correctly convert to 134a, you need
to replace all the seals in the A/C system. The original seals in the system
were designed to work with R-12. The chemical composition of the R-134a will
eventually degrade the seals, causing the refrigerant to leak out. You will
also need to purchase a receiver drier specifically for R-134a. The last
step is to also convert to a R-134a
compatible compressor. I have heard that it is possible to use an R-12
compressor to do this, however it will require a total flush out of the
compressor, as well as replacing all internal seals with R-134a compatible
seals. When you get into this realm, it is easier to simply replace the
compressor. I have also heard that depending on the climate, you may also
need to replace the condenser to a unit with a larger surface area. If you
were to take this to the shop to have it done, the cost and labor involved
in doing this can easily exceed $1000 or more.
lets begin going over your system. The first step is to verify that the
compressor kicks on when you hit the switch. Start the engine, then turn the
A/C fan on to maximum and press the A/C button on. Now open the hood and
look down at the compressor on the passenger side down near the radiator.
Look at the front of the compressor.. Is the clutch engaged? What you are
looking for is the plate on the very front of the A/C compressor. When the
clutch is engaged the plate spins at the same rate as the belt. When it is
disengaged, the plate remains stationary. Have a friend toggle the button in
the dash on and off to check for operation. If the clutch does not engage
when you press the button, you will need to determine why. Unplug the
electrical connector going to the compressor and check for 12volts. If no
voltage is present out of the connector, the next step is to check the
low-pressure switch on the top of the receiver drier.
receiver drier is located on the passenger side of the car in front of the
wheel well and directly behind the headlights. You will see two switches on
the valve for the receiver drier as well as the sight glass. What you need
to do is unplug the low-pressure switch electrical connection, and check the
leads for voltage with the A/C button turned on. If you read 12 volts from
the leads, then the low pressure switch has tripped and interrupted the
circuit proving voltage to the compressor. If you jumper the low-pressure
leads together, the A/C compressor clutch should engage. (CAUTION: when
attempting this, be sure to only run the compressor for a few seconds to
prevent possible damage to the unit.)
look at the top of the sight glass on the receiver drier. A steady stream of
bubbles usually indicates that the system charge is low and needs to be
topped up. Streaks on the inside of the glass usually indicate that there is
no refrigerant left in the system. Once you have finished checking the sight
glass, shut off the engine.
next step is to determine if the system will hold a charge. On all R-12
systems it is normal for the system to leak refrigerant through the lines.
This is to bleed off any excess pressure that builds up in the lines. If the
system has not been used in quite some time, it is possible for all the
refrigerant to bleed out of the lines. This is the reason that R-12 is no
longer used; it leaks out of the lines, and the resulting CFCs can damage
the ozone layer. The newer R-134a lines are called barrier lines, and do not
determine if the system will hold a charge, you first need to have the
system evacuated. This is where a vacuum pump is hooked up to the system and
using vacuum pressure, all the air and refrigerant left in the system is
drawn out. Typically when a system is evacuated, it is left on the vacuum
pump for a period of about 30-45 minutes. This will insure that you have
pulled all moisture and air out of the system. Most people do not have
access to a vacuum pump, however any decent garage will be able to do this
for you at a minor fee. Evacuating the system will also determine if there
are any leaks in the lines. If your system does not hold pressure, you will
need to trace down the leak, and fix it accordingly. In cases where you
remove A/C lines, it is important that you always replace the receiver
drier. When you open up the system to outside air, you are essentially
saturating the receiver drier with moisture and can render it useless.
Evacuating the system will also tell you the condition of other system
components as well, such as the expansion valve and condenser. If either of
these parts are fault, you must replace them and have the system evacuated
the system has been tested for leaks and has been evacuated, its a good
idea to fill the system with about 4 ounces of oil charge. You can get this
any local Pep Boys, Kragen, etc. Be sure to check the label on the can if it
is for R-12 or R-134a systems. Luckily, you can still purchase R-12 oil
charge over the counter.
add the oil charge, you will need to locate the low-pressure return line.
This is the line that returns the cold gas back to the compressor. In the
case of the 325is, it is the line that runs directly in front of the
passenger side wheel well. Locate the schrader valve on the top of the line.
Remove the black protective cap on the top, and you will see the valve
will need to have a fill hose such as the one shown here. These too are
available in any Pep Boys. This special hose allows you to transfer the
contents of the can into the line. With the cars engine off, Connect the
hose to the low pressure valve on the car. You will want to do this first
with the engine OFF. Next, on the other side of the hose, turn the screw on
top counter clockwise until it stops. Load the can of oil charge into the
clamp on the hose and lock it in place.
When the can is secure in the clamp, turn the screw on top clockwise
all the way until it stops. This will pierce the top of the can and allow
the oil charge to flow in. Turn the car back on, turn on the A/C and put the
fan to maximum.
a good idea to put the can in a pot of warm water while you charge the
system. As you open the can, the pressure drops, causing the can to get
cold, and it slows down how fast the charge can get into the car. Put the
can in the warm water and rotate the screw on top counter clockwise all the
way. This will open the valve and allow the oil charge to flow in. I must
also stress at this time that the can of charge must be upright at all times
during charging. Failure to do so could result in not all of the oil or
refrigerant entering the system.
the valve open on the fill hose until the cans of oil charge are empty and
then turn off the engine. We are now ready to add the refrigerant to the
system. I was fortunate to have a previous supply of R-12 lying around from
back when it was still commercially available.
BMW e30 specifies 2 ½ lbs. of charge to completely fill the system. I
grabbed three one-pound cans and repeated the process I used for the oil
charge. Im specifying using can type refrigerant here as it is the method
most commonly available to the do it yourselfer. There is another process
for charging the system using a tank, however this type of charging requires
use of special gauges and hoses, so its best left to the professionals.
Cans of R-134a are available at your local auto parts store for about $13 a
pound. The procedure for adding these cans is the same, although they use a
different kind of fill hose and ports. Cans are available in different ounce
measurements, so calculate how much you need and then buy accordingly.
you charge the system, look down at the sight glass on the receiver drier.
Keep an eye out for when the bubbles in the system stop flowing and
you see a steady stream of liquid. When you see this, close the valve on the
fill hose as the system is now fully charged.
that the system is fully charged, you should be able to feel youre A/C
blowing cold once again.
Well, there you have it - it's
really not too difficult at all. If you would like
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