Your car’s HVAC system doesn’t get much thought — until it stops working

If any single system on a vehicle is taken for granted more than any other, it has to be the humble HVAC unit. When it works as designed, it’s completely unobtrusive and when it fails (whether in the heat of summer or the iciness of winter) it can drive us right round the bend.

No matter how advanced your ride might be, the basics of its HVAC system are pretty much the same as a ride from the 1940s.
In behind your auto’s dash lives a miniature furnace-slash-A/C unit complete with ventilation fan and exit ports for the ductwork. The heat and cool-air sources are small finned cores. The heater’s core looks and operates like a tiny radiator and the A/C’s evaporator core resembles a shrunken A/C condenser.

Hot engine coolant flows through the heater core, and cooled refrigerant gas moves through the A/C evaporator. When a fan blows over these, you get warm or cool air depending on your dash control settings.

Directional controls (whether the air comes out the defrost or other vents) are pretty much handled by small electric servos operating plastic duct doors. Formerly this job was handled by cables or vacuum controls, and there are still plenty of older vehicles on the road that use these methods.

HVAC issues and how to fix them

The most common concern with HVAC systems is lack of heat. It can almost always be traced to a low engine coolant level. As the heater core sits higher than the radiator and engine coolant passages, when the coolant level drops due to a leak, the heater is the first to run dry and lose heat. If your engine coolant level seems OK, your ride may have a circulation problem (due to blockage) or a control problem. If a top-up on the engine coolant solves the problem, you might want to find out how and where it lost some volume before you risk engine overheating.

When the fan stops blowing on one or more settings but still seems to work OK when set to its highest speed, the blower motor resistor is the likely cause. This small electric piece reduces the battery voltage to the blower motor depending on the dash control setting. When you select the highest fan speed, the resistor is completely bypassed and full voltage goes to the blower motor.

Many times these resistors are accessible by simply dropping the glove box down on its hinges, and their replacement can usually be an easy DIY job. Check the electrical connector for any heat damage when replacing a resistor, for often when they fail, they will overheat first and melt the plastic connector shell.

Pre-emptive maintenance

When good air flow is a safety necessity, such as in winter for glass defrosting, before winter hits, check to see if your ride has a cabin air filter and inspect it if it does. A dirty cabin filter can drastically reduce the volume of air flowing though the ducts, meaning longer defrost times.

Most carmakers make the filter easily accessible in behind the glove-box. This compartment usually hinges completely out of the way by pressing in slightly on both sides to release its catches. A normal good filter should appear off-white in colour and have no debris. If you’re not sure and the filter is over a year old, change it anyway. They’re relatively cheap (often less than $15) and are available from just about any auto-parts store.

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