AC in a car: a luxury that became a necessity
Remember when AC in a car was a luxury? Me neither, because today it is almost impossible to buy a car without it. And with good reason. Driving in the heat without AC is not just uncomfortable, it is dangerous because it reduces concentration and increases fatigue.
But when the AC compressor gives up, the repair is not cheap. A new compressor costs EUR 300 to 800, plus charging the system, plus fitting. You easily get past EUR 1,000. For an older car, that is often too much.
A used AC compressor costs EUR 80 to 250, which is a significant saving. But the AC compressor is a specific part and there are some traps you need to be aware of. The compressor does not live alone, but in a system with the expansion valve, dryer and lines that can kill it within weeks if you ignore them.
How a car's AC system works
To understand why the compressor is so important (and expensive), you need to understand how AC works. The AC system works on the principle of compression and expansion of a gas (refrigerant).
Here is the simplified process:
- The compressor compresses gaseous refrigerant, which heats it up
- The hot compressed refrigerant flows through the condenser (in front of the radiator) where it cools and turns into a liquid
- The liquid refrigerant flows through the expansion valve where it expands suddenly and gets very cold in the process
- The cold refrigerant flows through the evaporator (inside the ventilation system) and cools the air entering the cabin
- The refrigerant returns to the compressor and the cycle repeats
So the compressor is the heart of the system. Without it, nothing works. It is also the only mechanically driven part of the system (driven by a belt from the engine), so it is the most prone to wear.
Types of AC compressors
The market has several types of compressors, and each has its own characteristics:
Piston compressor
Uses pistons to compress the refrigerant. It can have fixed or variable displacement. Variable piston compressors (like the Denso 6SEU or Sanden PXE) adjust their displacement to demand, which reduces engine load when full cooling is not needed.
Scroll compressor
Uses spiral elements for compression. It is quieter and has less vibration than a piston compressor. Used in some Japanese and Korean cars.
Electric compressor
Driven by an electric motor instead of a belt from the engine. Used in hybrid and electric vehicles. As a used part it is specific, because it requires checking the electronics alongside the mechanics.
Why AC compressors fail
Understanding the causes of failure helps you avoid the same problems with a used compressor:
- Lack of refrigerant - the refrigerant also serves as a lubricant for the compressor. When there is not enough of it (due to a leak), the compressor runs "dry" and destroys itself.
- Lack of oil - besides refrigerant, the AC system also circulates a special oil (PAG or POE) that lubricates the compressor. If there is not enough of it, the compressor overheats.
- Moisture in the system - moisture reacts with refrigerant and creates acids that attack the internal parts of the compressor
- Contamination - metal dust from a previous compressor failure, or contamination from the system
- Electrical problems - a faulty electromagnetic clutch that engages and disengages the compressor
- Natural wear - after 150,000 to 200,000 km, the internal parts wear out
Checking a used AC compressor
Checking an AC compressor is tricky because you cannot test it without a complete system (refrigerant, condenser, evaporator). But there are some things you can check:
Visual check
- Electromagnetic clutch - turn the clutch by hand. It should rotate smoothly. If it catches, the bearing is worn.
- Shaft - check whether there is play on the compressor shaft. Side-to-side movement points to a worn shaft bearing.
- Connections - refrigerant line connections must be undamaged and clean
- Housing - no cracks, dents or oil leak traces
- Pulley - the pulley must be straight, with no signs of belt wear
Rotation test
Turn the compressor shaft by hand (using a connected tool or improvising). It should turn with moderate resistance, but smoothly. If it cannot be turned or turns with heavy resistance, the internal parts are jammed or damaged.
If it spins completely freely with no resistance, that is also a bad sign because it means there is no compression (worn pistons or valves).
Oil check
Drain a little oil from the compressor. It should be clear or slightly yellowish. If it is black, grey or has metal particles, the compressor is internally damaged and you should not buy it.
Used AC compressor prices
- VW/Skoda/Seat (Denso/Sanden): EUR 60 to 150
- BMW (Denso 6SEU): EUR 80 to 200
- Audi (Denso/Sanden): EUR 70 to 180
- Mercedes (Denso 7SEU): EUR 100 to 250
- Renault/Dacia (Sanden): EUR 50 to 120
- Ford (FS10/FS18): EUR 50 to 130
- Opel (Delphi/CVC): EUR 50 to 120
A new compressor for the same models costs EUR 200 to 600, and a remanufactured one EUR 150 to 400.
On top of the compressor price, add EUR 100 to 250 for fitting and EUR 60 to 120 for charging the system with refrigerant.
What to replace alongside the compressor
When you replace the AC compressor, there are parts that should be replaced or checked:
- Receiver-dryer (filter dryer) - mandatory replacement every time the AC system is opened. Costs EUR 20 to 50. Contains a desiccant that absorbs moisture from the system.
- Expansion valve - if it is old or suspect, replace it (EUR 20 to 60)
- O-rings - all seals at the joints should be new (a set costs EUR 5 to 15)
- Compressor oil - put in fresh PAG oil of the right viscosity
- Belt - if the belt driving the compressor is worn, replace it
If the previous compressor failed catastrophically (jammed and broke apart internally), it is essential to flush the entire system of metal particles before fitting the new compressor. Otherwise the metal dust will destroy the new compressor too.
Refrigerant type: R134a vs. R1234yf
An important detail many people forget. Older cars (up to roughly 2015) use R134a refrigerant, and newer ones use R1234yf. These two refrigerants are not compatible and require different compressors and oils.
R134a is cheaper to charge (EUR 60 to 80), while R1234yf can cost EUR 100 to 180 to charge because the refrigerant itself is significantly more expensive.
When you buy a used compressor, always check which refrigerant your car uses and buy a compatible compressor.
Common myths about car AC
Let's bust a few myths that get repeated:
- "AC uses a lot of fuel" - yes, but open windows on the motorway create aerodynamic drag that uses more fuel than the AC. At speeds above 80 km/h, AC is the more economical option.
- "AC should be used all year" - this is actually true, not a myth. Using the AC in winter helps dry the air and prevents windows fogging up. Plus, it keeps the seals and compressor in shape.
- "When AC cools less, just top up the refrigerant" - refrigerant does not disappear on its own. If it is low, something is leaking somewhere. Topping up without fixing the leak is throwing money away.
When to buy used and when new
A used compressor makes sense when:
- The car is older and its value is low
- The compressor comes from a low-mileage car
- You have a mechanic who can check the condition before fitting
A new or remanufactured compressor is the better choice when:
- The previous compressor failed catastrophically (metal particles in the system)
- The price difference is small
- You need a warranty because you use the car daily
When a used compressor really pays off
A used AC compressor can be an excellent saving, especially for older cars where the price gap to new is significant. The key is checking the bearings, oil and overall condition of the compressor.
Do not forget to replace the receiver-dryer and o-rings every time you swap the compressor, and always use the right type of refrigerant and oil. If you need help finding an AC compressor, send a request via PoDi and you will get offers from vetted suppliers.