When you select ‘defog’ on the HVAC controls, certain decisions have to be made to get the desired result.
Recently we had to remove the radio from a Toyota Matrix, which required removing a panel that holds some of the HVAC controls. It’s a typical manual HVAC system, with three knobs and two buttons. One knob operates a cable connected to the temperature blend door, one is the fan speed switch, and one knob is connected to the cable that operates the vent selection door. One button on the panel is labeled “A/C,” and it allows the driver to turn off power to the compressor clutch. The other button has that little “recirculation” icon, and it operates an air intake door motor to choose between outside air or recirculated cabin air.
The A/C button communicates with a control unit, but the ‘recirc’ button is a mechanical switch that latches in when pushed and unlatches (pops out) when pushed again. When we removed the switch panel, we saw a cable connected to this switch that leads to the vent selector knob. When the switch is latched to select recirculated air, turning the vent knob to the defog position moves this cable and mechanically releases the latch, automatically forcing the system to draw in outside air.
While this cable/switch arrangement is unusual, the operating strategy is almost universal. That’s because when humans breathe inside a closed cabin, the air gradually becomes more humid, and you need dry air to defog a windshield. If the A/C is operating, the air is dried as it flows past the evaporator before being blown at the windshield. When the heater is operating, air is dried by the heater core before being blown at the windshield. But what happens if the A/C is off and no heat is being requested?
We were on the Merritt Parkway in Connecticut, a scenic four-lane divided road built in the 1930s. It’s an old design, with no shoulders and short exit lanes, but traffic typically moves at 50 mph with barely two seconds between cars. On this December day, the temperature had warmed to the low 50s in moderate fog. Inside the Audi, the seat heaters were on and the climate control system was set to minimum temperature. With ‘recirc’ selected to keep out exhaust fumes, the cabin stayed warm without assistance from the heater. Sure enough, the windshield began to fog inside, but when I pressed the ‘defog’ button, the windshield instantly became completely opaque. In near panic mode, I opened the window and stuck my head out while yelling at my wife to start pushing buttons to “make it go away.”
The climate control system wasn’t letting air pass through the heater core because the inside temperature was above the requested temperature, and the A/C compressor wouldn’t run because the ambient temperature was too low. When ‘defog’ was selected, the air intake door opened and the fan went to maximum speed, blowing humid outside air at the inside of the cold windshield. We had to request an uncomfortably high cabin temperature to clear the windshield, forcing us to continue our trip with the windows open.
This will never happen on my Mini Cooper. The electric recirc door operates as decided by the driver, not as an engineer decided long ago and far away.
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