My car has AC but it is not really cold - Diagnosis

 

 
 

This is usually where I come in and at this point I start asking questions. Many of the answers I would really like to know as they may require specialist knowledge but there are some questions you could ask yourself and make some judgements upon. More recent cars present more problems for owners and are less likely to be detectable without specialised equipment so the following really applies to older models.


1. Bearing in mind the rule of thumb in the previous page, how long since it was last charged? If you have owned the car for less than about two years and have no other evidence then look further.


2. Is there enough air coming through the face-level-vents? This is extremely important. If you can feel some air but it seems weak, put the air control to face-level, turn the blower speed to maximum and then feel how much air is coming out. Insufficient is most likely to be the result of blocked pollen/cabin filters as most modern cars have them or possibly a dislodged piece of air trunking under the dash or a vent control that has stuck or even a blower motor not pulling it's weight. Cabin filters are usually replaced at annual services - they last only about 9 to 12 months. Try pushing the recirculation button (your handbook will show you the logo for this) - if more air comes out of the face-level vents then suspect the pollen filters. If you cannot feel any air, can you hear the blower motor turning? If necessary turn the engine off, turn the ignition on again without starting the engine and listen hard for the sound of the blower motor. Try all speeds in turn. If the blower motor is not going at all then the AC will usually not work at all either. Often if the blower refuses to work at one speed the AC may be turned off as well.


3. The heater may be turned off but is it still working a little and spoiling the AC? Turn off both heater and AC and with the car running on tickover for say ten minutes, the bonnet closed and the blower operating, check that the air from the vents is only at ambient temperature and is not heated.


4. Is the AC compressor actually running at all? Look under the bonnet at where the drive belt is running and get a friend to switch the AC on and off a few times, not too quickly as some systems do not start for several seconds after the switch has been pushed. You should hear a distinct click as the clutch operates and you may then be able to identify which is the compressor, you should then be able to observe that when the AC is turned off the belt turns the pulley only but when it is switched on, the outer plate (which is connected to the compressor internals) turns also. If it does not turn, look first at the fuses - it could be something really simple. Should an AC fuse have blown, before you replace it, check that any electro fan on the radiator is able to turn freely - if it is stiff it may draw excessive current and blow the fuse again. If no fuse has blown probably the most likely scenario is that the gas pressure is so low that the safety switch has operated to prevent the compressor turning and risking damage. Double-check that the interior fan is actually running - if this fails the AC will not run.

If the car is post 2000 you may have a system which uses one of the newer clutchless compressors. These appear to be working all the time whether the AC is switched on or off and thus require more equipment to diagnose. If you seem unable to turn it off but it still underperforms then you need a specialist - its not really suitable for self diagnosis.


5. Once you have ascertained that the compressor is running you could try following the path of the thin pipe coming from the bottom of the condenser (the front radiator). This will often go into a type of metal bottle (the receiver/dryer) that will occasionally have a small glass eye set into the top of it or possibly in a separate small block (this is the sight-glass). Clean the glass and observe it whilst the compressor is running. Is it virtually completely clear? If you can see a continuous stream of bubbles or even what looks like foam then almost certainly the system needs recharging. It is perfectly normal to see a few bubbles each time the compressor starts and stops and if you have the later R134a refrigerant a few bubbles may be seen continuously on a very cold day but otherwise the sight-glass should show only a continuous flow of liquid. These sightglasses are much less frequently encountered on more modern vehicles, only the Japanese seem to still have faith in them.

6. Could it be that cold air is coming out of one vent and hot air out of another. We meet this occasionally and the distribution of the cooled air is sometimes quite complex. Occasionally it is the result of a small plastic cog wheel wearing out on the most commonly used teeth. Sometimes it is the result of a stepper motor failing.

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