AUTO TECH: How to get rid of those summer A/C odours

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This article was published 24/05/2013 (4878 days ago), so information in it may no longer be current.

Although summer is not quite with us yet, the warm sun quickly heats up a vehicle’s 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’re accompanied by a “musty” smell. Usually this odour 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 they occur, and how to prevent them, a primer on 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 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 vapor 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 they only control the this basic refrigeration cycle which is similar to how the refrigerator in your house works.

But here’s the rub: 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 mold and mildew. This is the cause of a persistent musty smell.

There are two recommended methods of removing the odour. 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 mold.

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 mold are available through most automotive parts suppliers and dealerships.

Ventilate the interior of the vehicle with auxiliary fans while spraying the disinfectant. Let the vehicle sit for five minutes and then 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 mold buildup in the housing can be difficult. Check the drain tubes and unplug them if required. Some manufacturers use an after-blow sequence to help prevent odours from developing. A module turns the heater fan on for five minutes when the air conditioning has been used and after the ignition has been off for several minutes. The airflow from the heater fan will help to dry the housing and prevent mold growth.

You could do the same manually if your vehicle tends to develop A/C system odours. Simply operate the fan for a few minutes with the ac turned off to help dry the housing.

Jim Kerr is an experienced mechanic, instructor of automotive technology, freelance journalist and member of the Automobile Journalists’ Association of Canada.

kerr.jim@sasktel.net