Air conditioning is a lifesaver in summer heat, but it is useful in winter too: dry air clears a misted windscreen quickly. The system consists of a compressor driven by a belt from the engine, two heat exchangers (the condenser at the front of the car and the evaporator behind the dashboard) and a sealed refrigerant circuit.
Recharging, leak detection and compressor repairs are jobs for a garage with the right equipment. However, the part of the maintenance that most affects air freshness and system performance is easy to do yourself.
What you will need
Important: Do not open the refrigerant circuit or unscrew the service valves: pressurised refrigerant can cause frostbite. Only work under the bonnet with the engine switched off and cold.
Step 1
Replace the cabin filter
Air entering the cabin passes through the cabin filter, which traps dust, pollen and other pollutants. Paper filters are cheaper and reduce dust and pollen, while activated carbon filters cost more but also absorb some of the harmful substances from other vehicles' exhaust.
The filter sits in a housing behind the glovebox, under the dashboard or in the engine bay; the owner's manual shows exactly where. Replace it at least once a year, and more often in dusty cities or on unpaved roads. While it is out, vacuum the housing and note the airflow arrow on the new filter.
Step 2
Get rid of bad smells
If the cabin still smells musty after a filter change, bacteria and mould have built up on the evaporator and in the ducts. Use WD-40 Specialist Air-Conditioner Cleaner to clean them. It is usually applied to the evaporator through the air intake or drain with the filter removed, strictly following the instructions on the can, after which the system is blown through with the fan.
Do not sit in the car during treatment and ventilate it well afterwards. Fit the new filter once the cleaning is done.
Step 3
Clean the condenser
The air conditioning condenser sits in front of the engine radiator and is the first to collect insects, seed fluff and road dirt. A clogged core sheds heat less effectively, so the system cools less well. Clean it carefully with a soft brush and a gentle stream of water without bending the thin fins. Keep a pressure washer well back.
Step 4
Check the belt, connectors and fixings
The compressor is often driven by the same belt as the alternator and water pump. Inspect it for cracks and slipping: if it squeals, have it checked or replaced.
With the engine off, clean the connectors for the compressor clutch, cabin blower and radiator fan with WD-40 Specialist Contact Cleaner: a corroded contact can stop the compressor from engaging. Treat mounting bolts and brackets that rust from salt and moisture with WD-40 Multi-Use on the outside, keeping it away from the belt and pulleys.
Step 5
Check the drain and system performance
In summer, clean water should drip under the car after a drive with the air conditioning on: this is condensate from the evaporator. If there is none and the passenger footwell is damp, the drain tube is blocked and needs clearing.
Turn the air conditioning to maximum. If the vents blow air at room temperature, the problem lies with the compressor or the refrigerant circuit, such as a leak or low gas. In that case the system needs a garage diagnosis with pressure and leak tests.
Tip: Run the air conditioning for at least a few minutes once a week, including in winter: this keeps the circuit seals lubricated and stops the compressor seizing. A few minutes before the end of a journey, switch off the cooling and leave just the fan running to dry the evaporator and discourage odours.
What next
Replace the cabin filter yearly or more often, clean the condenser in spring and have the system fully checked at a garage according to the manufacturer's schedule. Care of rubber seals and locks before winter is covered in our guide to preparing a car for the cold season.
These uses come from the experience of WD-40 users in many countries. Always follow the instructions and warnings on the product label.
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