Tweeting sound coming from A/C vent in car?

[bordelond]

Strange symptoms ... the vehicle is a 2003 Ford Taurus SES, 6-cylinder.

The passenger-side A/C vent seemed to "tweet". It's not any belt under the hood -- it sounded like the tweet is coming from inside the dash (directly behind the far-right passenger vent), and the sound was drowned out when you open the hood with the engine & A/C going.

The A/C kept working this way -- if noisily -- for about three weeks (the tweet actually came, went, and came back again). Since Sunday night, the A/C won't blow at all on the first three settings (Low, Medium-Low, Medium-High), but will blow barely-cool air at full blast on the High setting. Recirculated cabin air vs. use of outside air (i.e. "Max A/C" vs. just "A/C") makes no difference.

What might cause such symptoms? Thanks in advance for any pointers or advice.

[BwanaBob]

My parents' old Corolla did this too. I hope someone has an answer.

[bordelond]

Uh oh -- the compressor's got it's own belt that can squeak (http://www.aa1car.com/library/2003/ic50350.htm):

The most common symptom of a compressor failure (besides no cooling) is a seized compressor. It will not turn when the magnetic clutch engages, and you may hear squeals of protest from the drive belt. Or, the belt may have already broken or been thrown off its pulleys.

Loss of lubrication is unquestionably the most common cause of compressor failure. This can happen when there is a refrigerant leak somewhere in the system that allows refrigerant and oil to escape. Typical leak points are hoses, hose and pipe connections (O-rings and flange gaskets), the evaporator, condenser or the compressor shaft seal. An electronic leak detector or dye should be used to find the leak so it can be repaired.

A restriction inside the A/C system can also starve the compressor for oil. Oil circulates with the refrigerant, so if the orifice tube or expansion valve is blocked it may cause the compressor to run dry and seize.

Even if a compressor is still turning, it may have to be replaced if it is leaking, making excessive noise or not working correctly. Some compressors are naturally noisier than others, but loud knocking noises can sometimes be caused by air in the system (the cure here is to vacuum purge the system to remove the unwanted air, then to recharge the system with refrigerant). Metallic noises and bearing noise are usually signals that the compressor is about to fail. "http://www.aa1car.com/library/2003/ic50350.htm

The most common symptom of a compressor failure (besides no cooling) is a seized compressor. It will not turn when the magnetic clutch engages, and you may hear squeals of protest from the drive belt. Or, the belt may have already broken or been thrown off its pulleys.

Loss of lubrication is unquestionably the most common cause of compressor failure. This can happen when there is a refrigerant leak somewhere in the system that allows refrigerant and oil to escape. Typical leak points are hoses, hose and pipe connections (O-rings and flange gaskets), the evaporator, condenser or the compressor shaft seal. An electronic leak detector or dye should be used to find the leak so it can be repaired.

A restriction inside the A/C system can also starve the compressor for oil. Oil circulates with the refrigerant, so if the orifice tube or expansion valve is blocked it may cause the compressor to run dry and seize.

Even if a compressor is still turning, it may have to be replaced if it is leaking, making excessive noise or not working correctly. Some compressors are naturally noisier than others, but loud knocking noises can sometimes be caused by air in the system (the cure here is to vacuum purge the system to remove the unwanted air, then to recharge the system with refrigerant). Metallic noises and bearing noise are usually signals that the compressor is about to fail.

Hope this doesn't necessitate a brand-new compre$$or :(

[bordelond]

My parents' old Corolla did this too. I hope someone has an answer.
Did the A/C keep running for a while? Did the A/C ever quit?

[Gary T]

If the tweet is a more or less steady sound, it's almost certainly a vacuum leak. Engine vacuum is used to operate most of the controls and devices in the HVAC system, and the symptoms you describe are consistent with loss of vacuum to the function selector switch. It may be a hose come loose, or it may be a component that's cracked. It will take some exploration behind the dash to find it.

[Rick]

The A/C kept working this way -- if noisily -- for about three weeks (the tweet actually came, went, and came back again). Since Sunday night, the A/C won't blow at all on the first three settings (Low, Medium-Low, Medium-High), but will blow barely-cool air at full blast on the High setting. Recirculated cabin air vs. use of outside air (i.e. "Max A/C" vs. just "A/C") makes no difference.

What might cause such symptoms? Thanks in advance for any pointers or advice.
Does the tweet sound like "I taught I taw a puddy tat? ;)
When you say it won't blow at all on the first three setting, do you mean the fan is not running, or do you mean you can't feel any air out of the face level vents? If it is the latter, try putting your hand over the defrost vents, and down by the floor vents to see if the air is coming out there.
If the answer to this is yes, I think Gary T nailed it, you have a vacuum leak. If however the fan motor is not running, and there is no air coming out of any vent anywhere on the first three speeds, then there is some type of electrical failure. Coupled with the tweet, it could be a fan motor that failed.
I have also seen cases where a leaf gets into the system, and acts like a reed on a woodwind instrument.

[bordelond]

Does the tweet sound like "I taught I taw a puddy tat? ;)
When you say it won't blow at all on the first three setting, do you mean the fan is not running, or do you mean you can't feel any air out of the face level vents?
... mmm ... I believe the fan is not running on the first three settings. It sure doesn't sound like it. And yes, along with that, no air comes out of the face level vents.

But then you kick it up to High, and it all comes alive, roaring at full blast, just less cool than it ought to be (but not hot air).

ETA: oh, and when you kick it up to High, the tweet is gone. Going full blast sharply reduced the tweet when the A/C was still running, too.

[BwanaBob]

Did the A/C keep running for a while? Did the A/C ever quit?


No it never quit - it worked fine except for the "tweet".

[Gary T]

On this system, a vaccum-operated switch provides power to the blower resistor, which is used on all speeds except high. I believe it also provides power to the compressor. Do not pursue any electrical aspect of the problem until the vacuum aspect has been rectified.

[bordelond]

On this system, a vaccum-operated switch provides power to the blower resistor, which is used on all speeds except high. I believe it also provides power to the compressor. Do not pursue any electrical aspect of the problem until the vacuum aspect has been rectified.
Thanks to everyone for your responses.

Gary T/Rick/anyone knowledgeable: the fix Gary describes in the quoted text above -- is it considered "no big deal", or is it a huge dashboard-ripping quest for a needle in a haystack that racks up huge labor charges?

[Gary T]

Thanks to everyone for your responses.

Gary T/Rick/anyone knowledgeable: the fix Gary describes in the quoted text above -- is it considered "no big deal", or is it a huge dashboard-ripping quest for a needle in a haystack that racks up huge labor charges?
It depends on where the leak is. It's not likely to be very quick and easy, and it's not likely to require removal of the entire dash. But as to which of these extremes it's closer to, that won't be known until the leak has been found.