Fixing your BMW's Air Conditioning

[genphreak]

Recently I had to do a whole bunch of work on my car's AC, so learned a bit in the process. This post is to help those not familiar with Air Conditioning understand them enough to carry out basic troubleshooting steps.

Included are definitions, simple testing and the simplest of re-charging steps using a standard R-134a gauge set- a vital tool you will need to rent or purchase, e.g.

AC involves a pretty simple mechanism with only a few moving parts, and works by circulating refrigerant through the following components. The process transfers heat collected from the cabin to a special radiator mounted in front of the engine radiator. This transfers the heat to the air passing through the front of the car. This radiator is called a condenser because it causes the warm refrigerant gas to lose its heat to the passing air. The gas, with the help of the compressor, then condenses back to liquid form for another round back to the cabin where it will absorb more heat.

If the system used gas only, no liquid, barely any heat would be transferred. The state change makes the process more efficient, and easy to control using pressure. The state changes are made to happen at the right time and place, (why the heat exchanger inside the cabin is called an evaporator and the one at the front of the car is called a condenser.

These are the parts of the system it helps to know about:

• Compressor. Compresses the refrigerant to a near-liquid state, powered by a tension controlled fan-belt
• High Pressure Port (Larger Connector, sometimes coloured red)
• Expansion Valve. This releases/sprays compressed refrigerant into Evaporator Coil
• Evaporator coil. The HVAC system in the cabin passes cabin air through this radiator to allow heat contained in cabin air to be removed by the refrigerant
• Condenser coil. At the front of the car, this radiator uses air entering the front of the car to cool the refrigerant.
• Filter/Drier. Hopefully traps any H20 and debris, stopping it entering the compressor
• Low Pressure Port (Smaller Connector, sometimes coloured blue)
• Refrigerant is now returned to the compressor and the cycle repeats

What affects performance (usually):

• How well the compressor works (a wear item)
• The refrigerant in the system loop (can leak or stop circulating properly)
• How well the Expansion Valve works (a wear item)
• Outside air temperature (varies)
• The amount of air circulated through the Condenser Coil (at the front of the car)
• The amount of air circulated through the Evaporator Coil (in the cabin)

Before attempting anything- be sure to exercise common sense and apply good safety practices when working on your car. Seek assistance when you need it, and try to perceive what you are yet to understand about what you are working on before you end up breaking something. Something I have learned to do through bitter experience as I am not an Air Conditioning expert.

Also, know that the gas in your system is worth money, and is very damaging to the environment. It is easily evacuated and that your local shop will happily remove it for you (and replace it when you come back) if you want to open the system to fix anything. So whatever you do, don't let it out!

Troubleshooting
Initial checks:

1. Check the AC belt for damage and belt tensioner for smooth operation and pressure on the belt. This is vital to the system working well.
2. Replace the lower engine cover if you've lost it, it makes the air movement through the front of the car - and your AC, more effective.
3. Check the condenser coil is clear and free of debris
4. Check the fan clutch is working properly as these wear out on BMWs and people notice the AC misbehaving before they see the engine having heat troubles.
5. Make sure the compressor clutch is engaging (you can usually hear it click under the hood when you turn on the A/C). If it won't engage, the system either has an electrical problem or the compressor's clutch has failed, but it is common on old cars for the refrigerant pressure to become too low to allow the system to switch on. The pressure switch that monitors this can fail, but it is highly uncommon. (Easy to check; look for continuity as a minimum pressure level is required to allow the pressure switch to activate the clutch). Do not bypass it to check for longer than a second or two, as this is a safety feature that prevents the compressor from overheating if there is not enough refrigerant in the system to allow for adequate compressor lubrication.

Diagnosing
The first thing to do is hook up your gauge set. Once connected to the the low & high pressure ports of the system it is easy to get an idea of how well the system is working and start working out what is wrong.

Bear in mind the reason the ports are described as being on the Low or High "side". Which side they are on relates to which 'side' of the compressor they reside: Because the compressor's job is to create high pressure, the 'high' side is after the refrigerant passes through the compressor.

The Low Pressure Port
Resides on the low side before the compressor. Its connector is the smaller of the two ports, and is where you can add refrigerant (if necessary) and the hose colour on the gauge set is sometimes Blue.

The High Pressure Port
Resides on the high side after the compressor. It is the larger of the two ports and the hose on the gauge set is sometimes Red.

1. Connect the Gauge set (Engine off)
Hang the gauges from the hood latch so that you minimise tangling of the hanging hoses.
Attach the hoses to the high and low pressure ports (they are sized differently, so that you won't confuse them) and open their valves.

2. Read the Gauges
The gauges have different scales, but when you read them you will see they show the same pressure if the engine has not been running. You should see something like 125psi on both gauges. If they read lower than that, the system probably needs more refrigerant. If the gauges read zero, the system has a leak and is not holding pressure at all.

3. Test
Start the car with the A/C on full and lower the windows to ensure the cabin does not cool down and allow the thermostat to kick in and turn off the compressor. Make sure the compressor's clutch has engaged (you can usually hear it click under the hood when you turn on the A/C) before leaving it to run for 5-10 minutes. If the compressor's clutch won't engage see Troubleshooting clutch problems above).

4. Read the Gauges Again
With the A/C on and the car running, you should see 20-30 psi on the low side (Blue) gauge in 20-25C (68-77F) weather. On the high side (Red) you should see 200-300psi. In 36C (100F) weather, you will see up to ~45psi on the low side. Some say you should see about 10 times the pressure on the high side to that you see on the low side, if the system is operating properly.

- If you see lower overall pressures but the high side is still roughly 10 times the low side, then you may be low on refrigerant (or have a small leak that will soon make it that way soon).

- If the pressure is still equal while the compressor is engaged, your compressor is not functioning appropriately. Check to make sure both gauge valves are open when you are taking the readings.

- If the low side is very low and the high side is very high, then there is a blockage/clog in the system somewhere, likely the expansion valve.

Note that it is normal for the compressor to 'cycle' on and off and you will see this reflected in your readings. Be sure to notice when the compressor is on and when it is off, you must listen to hear the change between modes (the sound of more effort/labouring means compressor is on)

How to add refrigerant
Think first. It is always better to find the leak first as having it leak out again on the first hot day with all your friends in the car won't leave you feeling very intelligent:

• Refrigerant does not leak out over time, it leaks out due to pressure and gaps- usually worn seals. Leaks are often cheaply repaired as there may only be one and it is easy to change an O-ring or two. One good way if you don't have the time is to simply take it to a local shop to check for a leak. They too can do this quite cheaply- probably by adding refrigerant to the depleted system and at the same time add UV dye. They'll load it up and test it for leaks, and if nothing is obvious give it back to you to use until it fails again. Then they'll be able to find the leak as the dye shows up (under UV light) at each leak point. You can do this yourself too, but some really decent gear is advisable. You just have to wait for it to pressurise enough to leak before going back to find the root cause.
• Obviously, just adding refrigerant when a system low may only work temporarily- as soon as the first hot day comes and the system has to do some serious work, it will pressurise far more and force gas to leak. Beware also: If the system has been messed with before, there may be some water in the system; doing the same thing again will prolong the effects and cause more serious problems (such as compressor failure). ***Never refill if the system has no residual pressure.***
• Whenever the system is opened, you must change the dryer and evacuate the air before filling- you will need a vacuum pump (see the links below) or a local shop to do this.
• If you accidentally vented the system to the atmosphere, note: You should change the dryer before refilling (for if any air entered the system water will form/reform into droplets and steam whenever the system is used, destroying the compressor.
• After performing any repairs that 'open' the system to the atmosphere, you must evacuate air from the system with a vacuum pump. Then verify that the system can 'hold' a vacuum for 30 minutes before recharging the system.
• If your system has zero pressure, resist this temptation to add refrigerant as it is going to be a total waste. You will need to fix the cause of the leak first- and to do this adding dye as well as refrigerant is quite advisable. You must first evacuate the air from the system with a vacuum pump, verify that the system can 'hold' a vacuum for 20 minutes or so, and then re-charge the system.
• If you don't know how your system came to be depleted but just want to know if the compressor is bad or not, you might be able to get it going for a few runs by adding in a can of refrigerant, but this is silly as you would need to check for leaks after replacing a compressor anyhow, so should also add dye. It is worth doing this with the help of your local shop as you may need to remove the gas a couple of times before it is all running reliably. Try not to worry about the compressor too much, they do not dye out very often and are very cheap from the boneyard or reconditioned. There are many replacement new ones also which are nowhere near BMW parts prices.
• Whatever you do, never mix refrigerants, as this is illegal for many good reasons and will make the system unserviceable in future.

A valve should come with the rental of the gauge set that allows connection to a standard can of R134a refrigerant. Read the instructions on the can, which generally describe a process like:

1. Attach the can to the connection valve
2. Unscrew the handle on the valve as far as it will go
3. Thread the fitting tightly onto the can of refrigerant.
4. Attach the output of the valve to the yellow hose (connected to the center of the gauge manifold).
5. Close all the valves on the manifold.
   I put the can upright. Some put it in a bucket of warm water up on a horizontal surface to make it work faster and get more out.
   Beware: this is dangerous, if the water is hot you could blow your A/C, your arms, your hands and face to pieces just like hand grenades do.
6. With the can upright, screw in the valve handle all the way so that the top of the can is punctured by the spike in the handle, then back off the valve a few turns to retract the spike, which allows refrigerant out of the hole. You should hear the hiss of the refrigerant flowing through the yellow hose.
7. With it all connected up, check the hoses are clear of any moving parts in the engine bay.
8. Start the engine and turn on the A/C, set it to high.
9. Open the Low Side (Blue) Gauge Valve to allow the refrigerant to flow from the can into the system while monitoring the gauge, and keeping the can upright.
10. Do not attempt to turn the can upside down.
    It is normal for the can to become chilled as the refrigerant expands out of the can.
    If the refrigerant does not flow, check for an intermediate valve which has not been opened (some gauge manifold sets have a separate, additional shut off valve for the yellow hose).
11. Watch the gauge and when the pressure is within range, close the Low Side (Blue) Gauge Valve, then close the can valve.

Repairing your system
If the system has been opened to the atmosphere, you will need to replace the drier/filter to ensure that any moisture in the system is absorbed by the dessicant in the drier. Suppliers do not warrant compressors unless the drier/filter is replaced at the same time. After performing any repairs that 'open' the system to air, you will need to evacuate the air from the system with a vacuum pump, verify that the system can 'hold' a vacuum for 20 minutes or so, and then recharge the system with refrigerant.

If you replace your compressor, remember to add the correct oil (there are different types for different models, countries and production runs). Otherwise, your compressor can overheat and fail. The oil is designed to co-exist with the refrigerant, so you should ask the parts guy to look up what kind of oil to use, and how much to put in. If your old compressor blew up, you may need to flush the entire system, and you may need to completely replace the evaporator and condenser coils, because they are nearly impossible to 'flush' adequately. As soon as they spit even a small piece back into the compressor after its been replaced, you'll have to replace it again!

Some cool DIY Links:
Reciever/Dryer
Evaporator/Expansion valve
Compressor

Updating your car to use r134a refrigerant (for earlier BMWs such as e21, e23, e24, e26, e28, e30, (and early e32, e34, e36)

Hope this helps 'yall!

[genphreak]

I am replacing the compressor on my 1990 at present as it is pre-R12 and its better to just replace the one I have as I can't get it apart and its probably cactus now anyhow. Has anyone ever used a Unicla (Nagoya, Japan) compressor? I have a new UP170 here with the 12V 5-rib clutch- looks like it will work well fine.