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Thread: DIY: A/C Troubleshooting

  1. #1
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    DIY: A/C Troubleshooting

    I'm writing this DIY to help folks understand the operation of their car's Air Conditioning System, so that basic troubleshooting can take place. I've included some definitions, simple testing and charging procedures using a standard R-134a gauge set, available for rental at most major auto parts chains. As always, exercise common sense and good safety practices when working on your car, and keep in mind I'm not an A/C expert. If you aren't comfortable with the amount (or lack) of detail in this DIY, then please do not attempt any of these procedures. If I run across some good pictures I'll edit this post with them at a later date.


    A basic A/C system is a pretty simple mechanism; there aren't a lot of moving parts to worry about. These are the basic pieces in order of operation:

    • Low Pressure Port (Smaller Connector)
    • Compressor - Compresses refrigerant into near-liquid state
    • High Pressure Port (Larger Connector)
    • Expansion Valve - releases/sprays compressed refrigerant into Evaporator Coil
    • Evaporator Coil - allows heat contained in cabin air to be absorbed by refrigerant
    • Condenser Coil - Acts as a radiator for cooling the refrigerant
    • Filter/Drier - Traps any H20 or debris and keeps it from entering the compressor


    The variables include (but are not limited to):
    • How well the compressor is working (a moving part)
    • How much refrigerant is in the system loop
    • How well the Expansion Valve is working (on a BMW, a moving part)
    • Outside Air temperature
    • How well the air is circulated past the Evaporator Coil (Inside the cabin),
    • How well the air is circulated past the Condensor Coil (Outside the cabin)
    • How well refrigerant is circulated through the system


    In troubleshooting, the first thing you'll want to do is to hook up a gauge set (You can 'rent' these from most local auto parts chains at no cost) to the High Pressure and Low Pressure ports of the system, to get an idea of how well the system is functioning and to perform some simple diagnostics.

    Keep in mind that the reason the ports are called Low and High "sides" is directly related to which 'side' of the compressor they are located. Remember, the compressor's job is to create high pressure, so the 'high' side is after the refrigerent goes through the compressor.

    There are two ports on the system:
    The Low Pressure Port
    Its location in the flow of the system is before the compressor. This port's connector is the smaller of the two ports. This is where you would add refrigerant (if required) and the hose color on the gauge set is generally Blue.
    The High Pressure Port
    Its location in the flow of the system is after the compressor. It has the larger of the two ports and the hose on the gauge set is generally Red.


    Connect The Gauge Set
    With the engine off, hang the gauges from the hood latch mechanism so that you'll minimize 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 the valves.

    Read The Gauges
    Even though the gauges have different scales, they should read the same pressure if the engine has been off. You should see around ~125psi on both gauges. If you are reading significantly lower than that, then the system probably needs more refrigerant. If the gauges read zero, the system has a leak and is not holding pressure at all.

    Test The Air Conditioning
    First, verify the hoses are clear of any moving parts in the engine compartment. Start the car with the A/C on full and roll down the windows. Make sure the compressor's clutch has engaged, let it run for about 5-10 minutes and take a reading. If the compressor's clutch won't engage, the system either has an electrical problem, the compressor's clutch has failed or the refrigerant pressure is too low to allow the system to switch on. A minimum pressure level is required to allow the pressure switch to activate the clutch. 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.

    Read the Gauges Again
    With the A/C on and the car running, you should see about ~20-30 psi on the low side (Blue) gauge in 70/75-degree weather and about ~200-300psi on the high side (Red). In 100-degree weather, you could see up to ~45psi on the low side. A good rule-of-thumb is that the high side should be roughly 10 times the low side, if the system is operating properly.

    If the gauges show lower overall pressures but the high side is still roughly 10 times the low side, then you are probably low on refrigerant, and may have a small leak.

    If the gauges show pressure but are still equal while the compressor is engaged, then your compressor is not functioning properly. (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 somwhere, likely the expansion valve.

    It is normal for the compressor to 'cycle' on and off and you will see this reflected in your readings. Understanding when the compressor is on and when it is off will allow you to take proper readings. You'll be able to hear the change between modes.

    Adding Refrigerant
    If your troubleshooting has determined that you are low on refrigerant, then you'll need to add some to the system. A valve should come with the rental of the gauge set that allows connection to a standard can of 134a refrigerant.

    Attach the R134a Can
    Unscrew the handle on the valve all the way, then thread the fitting tightly onto the can of refrigerant. Attach the output of the valve to the Yellow Hose which is connected to the center of the gauge manifold. Close all the valves on the manifold. With the can upright, screw in the valve handle all the way so that the top of the can is punctured by the pin in the handle, then back off the valve a few turns to retract the pin, which allows refrigerant out of the hole. You should hear the hiss of the refrigerant entering the yellow hose.

    Let it Flow!
    First, verify the hoses are clear of any moving parts in the engine compartment. With the hoses on the gauge manifold connected to the ports, start the engine and turn the A/C on high. Open the Low Side (Blue) Gauge Valve to allow the refrigerant to flow from the can into the system while monitoring the gauge. Keep the can upright, and do not attempt to turn the can upside down during the filling process! It is normal for the can to become very chilled as the refrigerant expands out of the can. If the refrigerant does not flow, verify there is not an intermediate valve which has not been opened, as some gauge manifold sets have a separate, additional shut off valve for the yellow hose. Watch the gauge and when the pressure is within range, close the Low Side (Blue) Gauge Valve, then close the can valve.


    Repair Considerations
    If the system has been opened to outside air, you'll need to replace the drier/filter to ensure that any moisture inthe system is absorbed by the dessicant in the drier. Most auto parts chains won't warranty the compressor unless the drier/filter is replaced with the compressor. 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 re-charge the system with refrigerant. If you replace your compressor, remember to buy oil (different types for different cars and refigerant systems) to put into the system, so that your compressor doesn't overheat and fail. The oil is designed to co-exist with the refrigerant. You'll need to ask the parts guy to look up what kind of oil and how much to put in. If your old compressor 'cored out', you may need to flush the entire system, and you may need to completely replace the evaporator and condenser coils, becuase they are nearly impossible to 'flush' adequately.

    Thanks to SecretAznMan4u for getting me started on this, and for his assistance in diagnosing my own A/C problem.
    Attached Images Attached Images
    '98 328 Alpinweiss Cabriolet, 150K, CD43, CHA-S634 MP3 Changer, Chromeline Interior Trim, 'Redlined' Gears & Diff, Z3 Chromed 16" Rims, Billet Front Stress Bar, aFe CAI, 'Golf Tee' Exhaust Mod, Powerdiscs, SS Brake Lines, CDV delete, JTD Underpanel, BAV Lowering Springs, Bilstien Sports, Bimmerworld Rear Lower Control Arms and Front Camber Plates, E46 RSM's with reinforcing plates, Electric Rad Fan, Alum. T-Stat Housing, Mishimoto Hi-Capacity Aluminum Radiator, Hi-Performance Water pump, 3.23 LSD, Sharked.
    Sometime to come: New Clutch with LWFW, Clutch Stop, M50 Manifold, M3 Cams, Supercharger (I can dream).

  2. #2
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    Nice writeup!

    -HoB

  3. #3
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    finally. thanks a lot

  4. #4
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    Where is the low refrigerant switch on an e36??

  5. #5
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    The "refrigerant switch" is actually a Safety Pressure Switch, Part # 64538362055, priced at $152.89 according to RealOEM.com. According to page 610-10 of the Bentley, the A/C Pressure Switch on 1992-1998 E36 vehicles is located in the "Right front of engine compartment".
    I'd look on the compressor first, since that fits the description.
    Let us know!
    '98 328 Alpinweiss Cabriolet, 150K, CD43, CHA-S634 MP3 Changer, Chromeline Interior Trim, 'Redlined' Gears & Diff, Z3 Chromed 16" Rims, Billet Front Stress Bar, aFe CAI, 'Golf Tee' Exhaust Mod, Powerdiscs, SS Brake Lines, CDV delete, JTD Underpanel, BAV Lowering Springs, Bilstien Sports, Bimmerworld Rear Lower Control Arms and Front Camber Plates, E46 RSM's with reinforcing plates, Electric Rad Fan, Alum. T-Stat Housing, Mishimoto Hi-Capacity Aluminum Radiator, Hi-Performance Water pump, 3.23 LSD, Sharked.
    Sometime to come: New Clutch with LWFW, Clutch Stop, M50 Manifold, M3 Cams, Supercharger (I can dream).

  6. #6
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    Thanks for this! Excellent post. I think my pressure is a little low so this will help me diagnose.
    128i

  7. #7
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    quick question i noticed on my switch in my car that it did not have the two lead wires running off of it that i have seen in diagrams and from the parts list at bmw is there a different version of this switch. Mine seems to just have a clip on the top and lacks the two lead wires that run to the two switch hookups. Im only asking because it broke and my brother was retarded enough to throw out the broken peice so icant even get a # of it.

  8. #8
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    Quote Originally Posted by Glepp View Post
    quick question i noticed on my switch in my car that it did not have the two lead wires running off of it that i have seen in diagrams and from the parts list at bmw is there a different version of this switch. Mine seems to just have a clip on the top and lacks the two lead wires that run to the two switch hookups. Im only asking because it broke and my brother was retarded enough to throw out the broken peice so icant even get a # of it.

    The pressure switch you have is part number 64538362055. E36's 1995 and up came with pressure switches that do not have any leads coming out of it. Cars older than 1995 have the switch with 4 leads and 2 connectors which is a different part number than the one i gave you.

    2011 328i X-Drive~Mercedes ML55 AMG~2000 E320~1998 Chevrolet Corvette~2001 BMW 740iL

  9. #9
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    /ac systems are so easy. all my friends bring their cars to me when they mess the a/c up. personally i never use on my car because it feels like it puts to much darg on my engine and i dont like it.

  10. #10
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    great set of info there.

    I have a question, I have a 2000 E39, and my A/C will only blow cold air if its less than around 85F outside, otherwise it blows hot air. The Aux fan comes on fine, and the blower in the car works. I bought a set of gauges today, and with the compresser running I have around 80 low / 150-160 high. I am thinking I have a bad compressor. Any thoughts?

  11. #11
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    ^I think you are right, when I fixed & filled my system I showed, 45 low, 165 at idle, so it seems like your high side pressure should be greater, since the low side is. I get about 45 degree air at the dash vents no matter what the weather.

    Robert


    but this one goes to eleven

  12. #12
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    what i find strange is how it works when its not too hot outside. is that because the freon hasn't expanded much due to the lower temperature, and the 'barely hanging on' compressor is able to do its job with the denser freon, but once its hot the compressor cant handle it?

    what did you do to fix yours? is there a certain oil i need? Also, what is the procedure for recharging from a system at vacuum?

  13. #13
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    Mine suffered from a bad condensor, so I replaced it and the receiver/dryer assembly, and then pulled a vaccuum on it for about 1/2 an hour, then let it sit with the system still at a vaccuum for about 1/2 an hour more, then 3 cans of R134 later I was in business. Yours seems more like a compressor problem, but the leak check/vaccuum process is the same.


    but this one goes to eleven

  14. #14
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    sweet, thanks!

  15. #15
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    This DIY thread was very helpful, thanks for posting Mr. Jazz.
    Last edited by mt_goat; 05-20-2009 at 11:28 AM.

  16. #16
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    Early E36 have a sight glass on the receiver-dryer. You should add Freon until the bubbles go away.

    I usually put a wedge in throttle to bump the RPMs to 1500. Makes adding Freon faster.

  17. #17
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    Just a tip, when refilling with R134a the can will get cool/cold and seem to stay at that pressure for a while. Fill a small pot with hot water and submerge the can in the water. It will help the coolant to change to a gas and move into your system.

    I have a 98 328i and the pressure switch is on the dryer/reciever, which is the silver canister in the "right front corner of the engine compartment".

  18. #18
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    I took my car to a local shop advertising $39 a/c check up.

    So they checked all the pressures... everything is good!

    The only problem is... on the work order...

    Dual Function Switch on the Evaporator Dryer...

    Is this the same at the safety pressure switch on the Evaporator Dryer?

    Let me know... if it is... I will order from Pelican Parts... or somewhere else if someone has any other suggestions.

  19. #19
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    Thanks for all the help guys! AC blowing cold!!! Took it to Midas for AC inspection. They gave me a list of what needed to be done.. and quoted me approx $400 to complete. I bought the part online for $75 landed to my door. Friend of mine took it in and fixed it... and I will send him a Timmy's Gift Card! THANK YOU!!!

  20. #20
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    My compressor won't turn on when you first turn the A/C on. You have to drive it about 10 minutes before it cuts on.. Its a 97 318i convertible. Full of freon, climate control lights are on, and the aux fan is working.. Any thoughts??

  21. #21
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    Shoudn't the windows be up? I have never seen anyone check a system with the windows down.
    Not everything that counts can be counted, and not everything that can be counted counts.

  22. #22
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    They do it with windows down, doors open etc to make sure the internal t-stat in an auto mode doesn't start cutting the compressor or fan down. Everything is supposed to be at max cool/ max fan when doing a a/c check.
    David M.
    '96 328 Sedan

  23. #23
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    a/c

    I have a 1986 bmw 325es. I got the a/c checked out by a friend and he said that he couldn't find a leak and it is probably electrical. The air blows but it blows hot. So what could it be? Could it be the compressor? Any ideas?

  24. #24
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    Quote Originally Posted by baseball3 View Post
    I have a 1986 bmw 325es. I got the a/c checked out by a friend and he said that he couldn't find a leak and it is probably electrical. The air blows but it blows hot. So what could it be? Could it be the compressor? Any ideas?
    Don't have people look at your car that don't know what they're doing. You can't see leaks in the system. You have to use dye and a UV light. Your friend should have started with the compressor. Check that first. If it starts spinning when you switch on the ac it's fine. If the clutch stays still, it's probably bad.

    If the clutch does not engange, verify there is power going to it. If so, replace the compressor. If not, you have a wiring problem.

    If the compressor is good, check the system pressure. If it's low, fill it up. If it's where it should be, check your low and high pressure swithces, and your evap temp switch. All of those could cause the system to blow hot if they're not working properly.
    -Chris
    My e30 build thread/color change here

  25. #25
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    do u know were i can find a parts diagram and were is the flapper door i think i have to replace mine but im not sure what i need to pull to check and i dont want to pull something i dont have too

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