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Thread: No power to AC

  1. #1
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    No power to AC

    Howdy all.

    So here's my problem:

    My AC randomly stopped working 2 days ago. The air temp did not degrade slowly like would happen with slowly leaking freon. The problem happened suddenly.

    now here is the rundown:

    When the AC button is pressed, the Aux Cooling fan powers up and spins like it should.

    As the Bently Manual suggests,
    I disconnected the compressor wire connection (the one a foot off of the compressor) and used a voltmeter checked for power there. There was no power to this wire with the engine on and AC switch selected.

    I used a ohmmeter to check for a failed (open) diode. It showed almost no resistance from the wire leading to the compressor and ground.

    I checked the rest of the wire till it goes in front of the radiator and the wire looked good. no frays.

    What the hell do I do now?!

    The blower fan has power and blows on all speeds. The fuses are all good.

    Is there a relay for the AC system somewhere?

    I've done a good amount of research on this and haven't found much information.

    PLEASE HELP!

    thank-you.

    .
    edit: I tried adding a bit more refrigerant to see if it possibly was low. But nothing changed.

    If a hose burst for the AC system how would I be able to tell?
    Last edited by MechE30; 08-15-2009 at 07:22 PM. Reason: Automerged Doublepost

    Gophers FTW

  2. #2
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    Electrical Troubleshooting Manual (ETM) links

    Extensive AC troubleshooting section in the ETM

    http://www.wedophones.com/BMWManualsLead.htm
    On Christ the solid Rock I stand, all other ground is sinking sand. (Edward Mote 1797-1874)

  3. #3
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    Bump! Need some help here!

    Gophers FTW

  4. #4
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    I'm not an AC expert, but I've done a little bit of digging in order to see if it's worth trying to get mine to work. There's a low pressure switch just behind the passenger side headlight - when your coolant gets low, the switch won't turn the compressor on. Even if you add coolant, it doesn't guarantee that it's throwing the switch. you have to have some sort of vacuum apparatus and a pressure gauge, etc... stuff I don't have. Some people jump the switch to make sure their compressor is working, but that can be dangerous and blow things up, I've read. Just check the forums.

  5. #5
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    Yeah I've definitely researched/read the forums and know about the high/low pressure switches. To just add refrigerant you dont need a vacuum, but you need pressure gauges and the compressor has to be working to measure that pressure, correct?

    How does one take correct pressure readings without power to the compressor? Jump it? Seems dangerous/incorrect.

    Gophers FTW

  6. #6
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    You can temporarily jump the terminals to test the compressor and measure ur pressure. As long as you don't rev the motor excessively you are unlikely to blow a line or damage something due to a high press situation.
    Shut the $%&*nig hood and drive it!

    1. 85' EtaCoupe - Sold
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    6. 88' sedan - M50 swapped DD

  7. #7
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    You can jumper the low pressure switch (or the combination switch) briefly to check the compressor clutch operation. If this works to turn the compressor on, you likely have no refrigerant.

    Not all leaks are slow. Some, like a hole in the condenser, can release the refrigerant in less than a minute.

    You don't really need the gauges to fill the system with refrigerant. You need the valves attached to the gauges. Why? Because you must first pull a vacuum to empty the system of atmospheric gases and moisture, then you fill the system without letting in air. The fill is done by weight, not pressure, so the gauges are really just to let you do diagnostics if there is a problem.

    On a related note, you never need to bypass the pressure switch on a BMW to fill the system with refrigerant, and it's dangerous to do so. Once you start filling from the low pressure side, it takes just a seconds for enough pressure to reach the drier and activate the low pressure switch.

  8. #8
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    Well since I tried filling the system without the compressor running and it did not start up, shouldn't that mean I either have a large leak capable of leaking all my pressure out quickly, or that I have a faulty low/high pressure switch? And if I have a switch failure, do I need to completely empty the system and such when I replace it, or can I replace it without emptying/filling the system again? Thanks!

    Gophers FTW

  9. #9
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    What did you use to fill with? Did you draw a vacuum? What was the pressure before you started?

    The new style dual switch screws onto a Schrader valve, and can be replaced without releasing the pressure from the system. The old style with two separate switches will open the system up when removed, and will require replacing the drier.

    But it's unlikely that the pressure switches went bad. It's much more likely that something else failed, or that you have a leak.

  10. #10
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    My AC has recently failed in a similar manner. My conclusion is that it is the AC clutch that has failed.

    My questions:

    1. Is this a common failure?
    2. Can the clutch be removed without removing lines (depressurizing the system)?


    It is an original R 12 system. If I open the lines I most likely will convert to 134.

  11. #11
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    Well I filled the proper way two years ago. Everything has been great since then. A day before the incident the system blew as cold as ever.

    Now I tried to fill with engine running with a can of Freeze 12 (what's in the system now). I left it on for a couple minutes with no change.

    I have the old style switches. So two separate. This means opening the system if it needs new switches. I will have to test the pressures to determine if the low pressure switch is actually bad. If the pressure is good it will mean the low pressure switch is bad. If the pressure is bad it could mean a leak somewhere huh? Where could it be? What's common?

    tell me if I'm talkin jiberish here too!

    Gophers FTW

  12. #12
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    does your blower run just the compressor won't lock on, correct?

    the power flows from cabin switch (blue button) through evap temp switch, low press switch and hi press switch. You just need to find out which one is stopping the power.

    Disco lo press switch connector, engine off but ign sw on, A/C panel A/C on, check for 12v at bk/wt wire to ground. If no voltage, either bad switch or no pressure. I recommend unless you have the right tools to check A/C, you take it to the shop.
    Last edited by aze30325softtop; 08-17-2009 at 11:32 PM.

  13. #13
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    If you filled with Freeze-12 previously and have a Bosch compressor, the leak may be a failure of the Viton seals in the compressor. I believe that Freeze-12 contains R-134a, and "some" is as bad as "100%" when it comes to damaging the seals.

    A quick check is checking the continuity of the low pressure switch.

    BMW safety switches on old-style 051 drier
    Wired lead white nylon "T" connector
    61-31-1-365-510 Low pressure A/C pressure switch (replaced by 64 53 1 386 971)
    Pipe threads, contacts close above 24-32psi, or 28-34psi
    Wired lead white nylon "T" connector is the low-pressure switch, to prevent the
    compressor from running with a low system pressure (empty or low temperatures). Some replacements sold with a green case and leads with no connector body.

    If you don't have continuity, check the system pressure. You used to be able to buy a tire-style gauge that connected to the R12 A/C fitting for about $5.


    For the other question: yes, it's possible to change the magnetic clutch with the compressor still on the car, but it's not trivial. You should first practice on a compressor on a workbench -- buy a complete donor compressor at the boneyard.

  14. #14
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    Hmmm, hopefully it's not the compressor. but if it is, I guess a good winter project would be a seal retrofit. bah.

    I will check the system pressure.

    Does the compressor need to be cycling (i.e. clutch engaged) to properly check pressures? What would taking a low pressure properly read with the compressor not running, or doesn't that matter?

    thanks guys! Much appreciated.

    Gophers FTW

  15. #15
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    bump: questions needed answered before checking pressures.

    Gophers FTW

  16. #16
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    More extensive than the Bentley

    87 ETM page (highlighting 1 of about 10 pages on the subject).

    On Christ the solid Rock I stand, all other ground is sinking sand. (Edward Mote 1797-1874)

  17. #17
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    If you have no power at the compressor connector, work up the same branch of the wiring to find the high and low pressure switches on the receiver-drier. There is also a "freeze-up" sensor that is a long probe that is inside the evaporator. See the ETM, page 6452-0. Notice that the freeze probe, low pressure switch and high pressure switch are all in series before the C106 connector at the compressor.

    With the Key in run position, depress the blue A/C button, then check for power starting at C106, then hi press, etc. Be sure you have a good ground for your volt meter. Work your way up the wiring chain, all the way to the blue A/C button.

    Also, note that the bottom 2 sliders for air direction have to be placed off of the closed position, as they actuate a "compressor enable" switch, which could also be bad.



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  18. #18
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    I measure voltages and the low pressure switch is stopping voltage (open).

    Therefore, either the low pressure switch is bad: Or I have low pressure.

    I will have to test the pressures to determine if the low pressure switch is actually bad. If the pressure is good it will mean the low pressure switch is bad. If the pressure is bad it could mean a leak somewhere huh? Where could it be? What's common?

    Does the compressor need to be cycling (i.e. clutch engaged) to properly check pressures? What would taking a low pressure properly read with the compressor not running, or doesn't that matter?

    Gophers FTW

  19. #19
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    Quote Originally Posted by MechE30 View Post
    I measure voltages and the low pressure switch is stopping voltage (open).

    Therefore, either the low pressure switch is bad: Or I have low pressure.

    I will have to test the pressures to determine if the low pressure switch is actually bad. If the pressure is good it will mean the low pressure switch is bad. If the pressure is bad it could mean a leak somewhere huh? Where could it be? What's common?

    Does the compressor need to be cycling (i.e. clutch engaged) to properly check pressures? What would taking a low pressure properly read with the compressor not running, or doesn't that matter?
    I'll go out on a limb a bit and say yes. AC operation requires among other things creating pressure differentials, and the compressor is the main player in making that happen. (If what I just said survives djb2's scrutiny, great).

    In my case with an inoperative system and the compressor unable to cycle, the low side pressure was 0 psi.

    To check for leaks, a typical method discussed on the forums is to add a dye (UV if I remember correctly), let that circulate, then use the appropriate light and conditions to view it. If it's been done previously, there usually is a sticker in the engine compartment stating they've added dye.

    Prayerfully the one place you don't want to find a leak is in the evaporator core or in the hoses on the passenger compartment side of the firewall, given how well they are hidden.

    With the inoperative system (90 325i vert, Denso 10PA15C compressor, single high-low pressure switch), used that as an opportunity to convert to R-134a. They pressure tested the system (something you can do with R-12 but not R134a -- that held up OK). Then they changed fittings, pulled vacuum, then refilled with R-134a. I've been doing fine with that setup in Vegas temperatures since April of this year.
    Last edited by RomeoMike; 08-19-2009 at 02:13 PM.
    On Christ the solid Rock I stand, all other ground is sinking sand. (Edward Mote 1797-1874)

  20. #20
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    If you have refrigerant in the system the pressure will be much higher than atmospheric pressure, even with the compressor off. It depends on the temperature, but expect 40-100 psi (70-85 psi at room temperature).

    The pressure doesn't depend on the quantity of refrigerant in the system. Even a tiny amount of liquid remaining will create full pressure -- enough to close the low pressure cut-out switch.

    Pressure switches are pretty reliable. I would guess that it's 100X more likely that a non-functioning system is empty than the pressure switch has failed.

  21. #21
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    So I tried putting some more refrigerant into the system. I found that the hose running from the compressor to the heat exchanger by the radiator has a leak in it (from rubbing against the body of the car).

    How hard is this hose to remove? Anything special about changing this hose?


    thanks everyone for your help.

    Gophers FTW

  22. #22
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    That's the hose with the highest heat, pressure and flexing.

    Luckily it's also the least expensive hose in the system -- it used to be about $25, but I see that's its now about $35 from autohausAZ. Be certain to buy a new drier and o-rings all around at the same time.

    Replacing the hose is easy, although the threaded fittings tend to seize in place a bit.

    To get to the condenser side remove the complete headlight assembly -- it's only three screws and the electrical connection. Always use two wrenches working against each other. Otherwise you'll twist the tubing which can lead to cracks.

    When reassembling, remember that the o-ring makes the seal, not the pressure from the threaded fitting. Slightly more than snug is all that is needed.

    64-53-8-391-052 Compressor to condenser
    Rubber hose section 30mm, 11.75" labeled
    "A/C HOSE 1/2 IN (13 MM) 4860", "NBR PET CIIR"
    the Aluminum tube is 0.500", formed end is 0.460", 11.6mm, about ~15/32"
    22mm, 7/8", wrench for both ends,
    Uses two 64-50-1-468-464 11.1mm o-rings

  23. #23
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    Also, note that the bottom 2 sliders for air direction have to be placed off of the closed position, as they actuate a "compressor enable" switch, which could also be bad.[/QUOTE]

    Just in case you overlooked this, this little known switch had a faulty connection at it's plug, causing me your same troubles a week ago.

  24. #24
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    Where would the world be without you DJB2. ahhh. thanks AGAIN (and again) for your help.

    maybe someday my very limited knowledge can help you...that's a big maybe, haha.

    Quote Originally Posted by djb2 View Post
    That's the hose with the highest heat, pressure and flexing.

    Luckily it's also the least expensive hose in the system -- it used to be about $25, but I see that's its now about $35 from autohausAZ. Be certain to buy a new drier and o-rings all around at the same time.

    Replacing the hose is easy, although the threaded fittings tend to seize in place a bit.

    To get to the condenser side remove the complete headlight assembly -- it's only three screws and the electrical connection. Always use two wrenches working against each other. Otherwise you'll twist the tubing which can lead to cracks.

    When reassembling, remember that the o-ring makes the seal, not the pressure from the threaded fitting. Slightly more than snug is all that is needed.

    64-53-8-391-052 Compressor to condenser
    Rubber hose section 30mm, 11.75" labeled
    "A/C HOSE 1/2 IN (13 MM) 4860", "NBR PET CIIR"
    the Aluminum tube is 0.500", formed end is 0.460", 11.6mm, about ~15/32"
    22mm, 7/8", wrench for both ends,
    Uses two 64-50-1-468-464 11.1mm o-rings

    Gophers FTW

  25. #25
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    DAMN!
    So while muscling off the Compressor - Condenser hose, part of the Condenser decided to SNAP OFF. Ugh. Guess I'll be finding a new condensor, and waiting 10k till I have to do a timing belt sway to get in there.

    I'm correct in assuming I can't remove the condenser without pulling the radiator, right?

    Man it'd be nice to not have to...

    Gophers FTW

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