By Jim Kerr

Although summer is not quite with us yet, the warm sun quickly heats up a vehicle interior. Many drivers turn on the air conditioning for a few minutes when starting the vehicle to bring the temperature down. The first caresses of cold air bring comfort, but sometimes they are accompanied by a “musty” smell. Usually the musty smell disappears in a few seconds, but sometimes it will persist as long as the air is blowing. Persistent musty smells are unpleasant, but there are ways of correcting it.

To understand why musty smells occur, and how to prevent them, an understanding of the refrigeration cycle of the a/c system will help. When the system is turned on, the compressor (a pump driven by the engine) pushes the refrigerant, usually R134a or R12, to the condenser in front of the radiator where it is cooled and turned into a liquid. This liquid flows through a hose to the evaporator unit (like a small radiator) located inside the heater housing where it expands into a vapour and absorbs heat from the air passing through the fins of the evaporator.

The refrigerant is then pulled back into the compressor to repeat the cycle again. This is a simplified description. Control valves, pressure switches, driers, filters, and accumulators also are needed to make the system work properly but their operation only controls the basic refrigeration cycle described previously.

Water vapour, present in the air, condenses on the cool fins of the evaporator in a manner similar to the water that forms on a cold glass jar removed from a refrigerator. This water traps dirt from the outside air as it passes through the evaporator fins on its way to the passenger compartment. Normally this water and the dirt trapped in it runs into the bottom of the heater-a/c housing and drains through hoses leading beneath the vehicle. The liquid we see leaking beneath many vehicles on a warm summer day is actually this water.

If the vehicle has been operated in a dirty or dusty climate, all the dirt may not be washed away. It can collect in the bottom of the housing or on the evaporator fins and become a perfect place to grow mould and mildew. This is the cause of a persistent musty smell.

There are two recommended methods of removing the musty smell. The most expensive method is to remove the evaporator unit and physically clean it and the heater-a/c housing. The second method is to spray the inside of the housing with a disinfectant to destroy the mould. Both methods may be required to control an odour. I would try the disinfectant method first. Cleaning the evaporator can restore weak air conditioning airflow too.

The disinfectant is often sprayed into the heater-a/c housing through the opening where the blower motor resistor block is mounted. The resistor block must be unbolted, but leave it connected to the wiring. The blower motor must be running during the cleaning process. Caution! The resistor block gets very hot during operation. Spray the disinfectant into the heater housing and onto the evaporator unit, but be sure not to spray any onto hot engine surfaces. Disinfectant solutions specifically to destroy AC mould are available through most automotive parts suppliers and dealerships.

Ventilate the interior of the vehicle with auxiliary fans while spraying the disinfectant. Then let the vehicle sit for five minutes, and rinse the housing by spraying clean water (minimum of 2 1/2 litres) through the opening. Finally, turn the fan off and reassemble the resistor block.

Preventing mould build up in the housing can be difficult. Check the drain tubes and unplug them if required. General Motors has designed an accessory afterblow module Part #25533406 to help prevent odours from developing. This module turns the heater fan on five minutes after the ignition has been off for fifty minutes and the air conditioning has been used for more than four minutes. The airflow from the heater fan will help to dry the housing and prevent mould growth. This module will adapt to other vehicles, or you can simply operate the fan for a few minutes with the a/c turned off to help dry the housing.