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The A/C on my 2004 Ford Freestar is not putting out cold air. The compressor is kicking on and off.

What could be the problem?

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    Have you checked the refrigerant levels? Is it cold outside? – JPhi1618 Sep 26 '16 at 17:58
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The compressor turning off immediately is usually a sign of a safety mechanism kicking in. A sensor is most likely causing the compressor to disengage. It's probably due to one of two conditions:

  • An over-pressurized line. It would be dangerous to compress liquid where the system is already in an overpressurized state. The sensor prevents this from happening and the compressors cuts out.

  • An under-pressurized -- or empty -- line (a far more likely scenario). In addition to refrigerant, there's also another important component present in the refrigerant line: oil. This flows with the refrigerant, lubricating the compressor. Without lubricant, the compressor could seize up and destroy itself. The A/C system will not allow the compressor to run when it is not pressurized, assuming that the lack of refrigerant could mean a lack of lubrication flow. So, it cuts the compressor off to prevent damage.

The low-pressure situation is the most common. Your system has lost refrigerant and the compressor cycles on and off, trying to detect pressure each time. It's possible that you need to simply add more refrigerant, but it's more likely that your system has a hole somewhere in the closed loop system. A/C systems are not meant to be "topped off" from time-to-time. They are closed, with the intention of none of the refrigerant escaping.

A low pressure situation likely means that some component in your loop is leaking. That could be the a/c lines, the compressor, the evaporator, the filter dryer, or the radiator (a/c radiator, different from the engine radiator). UV dye and a black light are usually used to detect leaks in the loop. You can even buy refrigerant with UV dye already added.

Repair

1) Fix the failed component. Once you identify the component, you'll need to replace it. If it's the evaporator, you need to rip the entire dash apart in order to get at the heater box, then take it apart. Just getting the dash apart can take an entire day, and there are a ton of electrical connections that you'll need to take notes for. If the issue is in the engine compartment, you have a much easier chore ahead, but you'll need the correct tool to disconnect A/C lines.

2) Pull down a vacuum. Before you can recharge the system with refrigerant, you need a vacuum pump in order to vacuum out the a/c lines. The lines must be completely empty before filling, including air. A set of gauges and 30 minutes should prove to you that the vacuum is holding and that the hole is fixed.

3) Recharge lines. Only once is the system is repaired, closed, and holding a vacuum for an extended time should you attempt to recharge it. Refill the refrigerant lines up to the correct pressure using the gauges; refer to service manuals for the correct pressure or the correct output temperature at the vent that you should be targeting. It's very important to not overfill the system.

EDIT

As a side note, my answer about how the entire a/c loop works can be found here.

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Like JPhi1618 mentioned, check if it's cold outside. Your A/C won't work below certain temperatures.

The A/C system contains refrigerant which will be 'exhausted'(correct me pls) or leaked out. When there is not enough pressure, the A/C keeps turning on/off in the beginning. After some time it won't go on then.

Bring your car to a A/C mechanic who can connect a machine to it and run some tests to determine if there is a leak. This machine also fills the system with new refrigerant fluid. It takes around 1 hour to complete this process.

Don't try to fill the system without doing a leak test. It is very harmful to your health when you inhale it.

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    What do you mean by "Your A/C won't work below certain temperatures."? You realize the A/C functions (on most vehicles) when the HVAC is set to defrost mode? This is there to clear the air of moisture from your breath so the windshield doesn't freeze up during cold winter times. – Pᴀᴜʟsᴛᴇʀ2 Oct 27 '16 at 10:22
  • Well I don't know about that but the compressor is having a hard time to compress the liquid at that temperature and could easily break down. – com2ghz Oct 27 '16 at 10:31
  • But, R134a is a gas? You can turn on the air conditioning in low temperature. Other than to dehumidify there's no point though. – Ben Oct 28 '16 at 21:51
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The first place I would check is the thermal expansion valve just before the evaporator core in the passenger compartment. If it is clogged with debris it would prevent the AC from cooling as it prevents the evaporator unit from functioning properly. This in turn prevents the compressor from receiving both refrigerant and oil which will overheat the compressor and destroy it.

If the plenum temperature is below 20 C and or if the air flow is restricted the evaporator core will freeze up. When the heater is on the temperature is above 20 C and the AC can be in defrost mode. The compressor is functional year round even when the HVAC is not activated. This is to maintain system pressure and keep the seals from drying out.

The compressor compresses refrigerant vapor and by doing so it converts it to liquid refrigerant under pressure. This is similar in a way to when you pump up a bicycle tire and the pump becomes hot. This is why the condenser coil is hot because of the absorbed heat. The AC fans then cool the coil.

The next process is leaving the condenser and to the dryer to remove any water that could freeze and block the TX valve. The TX(thermal expansion) valve gradually releases the pressure into the evaporator coil. When the refrigerant pressure is reduced it releases heat and becomes cold.

If the AC button light is flashing it indicates a system problem. If not check any codes as the problem may be elsewhere.

Refrigerant poisoning is extremely serious. Consult with a trained professional. Safety first.

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