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Driving Concern

Explorer’s heating and cooling problem is complex Add to ...

I bought a 2001 Ford Explorer SportTrac last May. In October, I was driving on the highway on a cool early morning with the climate control knobs set to let in outside air. The air conditioner was off but the I had the temperature set to max cool (all the way to the left in the blue zone). Warm air was pouring out of the vents in front of the console – in fact, it was melting my M&M’s sitting there! During the winter the heat worked properly, but in the spring I had the same problem with warm air again. Putting the control on Max AC did give cool air. I would not say that it was really cold, but definitely cool. I've replaced the rheostat behind the knob but this did not change anything.– Don

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On a brisk autumn day with the heat on low, your M&M's should melt in your mouth – not on your dash, experts say.

“When you select vent, you should get the ambient temperature,” says Calvin Feist, instructor at NAIT in Edmonton. “Normally, nothing should be melting in the dash, unless it's 30 degrees outside.”

I asked Feist whether a bit of extra heat coming out of the dash might be normal – say, if you've just started the truck – and he said it isn't.

If it's 15C outside your truck and you have the vent on and the knob is down to blue, cold, or low, the air coming out of the dash should be 15C.

“Think of traveling with your hand out the window,” Feist says. “As you speed up it gets cooler, and that's what should be happening.”

Here's a quick HVAC refresher: If you want it cooler than it is outside, you'll need to turn on the AC. Set it to cool and the fresh air from outside gets cooled by passing over the evaporator coil.

When the AC's off, the coolest air you can get is the air from outside. If you turn up the temperature, the system takes air that’s been heated up by passing over the heater coil.

If you've set the control to medium (or, if your system lets you, at a specific temperature), it mixes that hot air with the air from outside to get the right temperature.

All this mixing of air from different places involves doors -- and that's where your trouble could be, says Centennial College professor Stephen Leroux.

“I would suspect there is an issue with the temperature blend door actuator or the temperature blend door itself,” Leroux says.

If a blend door can't open and close properly because it's sticking, broken or it's being blocked by something (Feist has seen a door blocked by a pen that fell though an air vent), then the air won't be mixed like it should be.

You'll be getting one temperature only, no matter what the control on the dash is set to.

The blend door actuator is the electric motor that moves the door. If it's not working, it's the same result – the blend door gets stuck in one position and the system can't change the temperature.

I wrote to Ford asking whether this is a common problem and hadn't heard back by my deadline.

Modern HVAC systems are complex – the blend doors might not be the problem here. But, even if it is the problem, it's not something you can easily fix yourself, Leroux says.

“If a blend door is damaged, the heater box will have to be replaced,” Leroux says. “This may require the complete dash panel assembly to be removed from the vehicle along with a number of associated components and wiring harnesses.”

Even if it's just an actuator and you're able to get to it and replace it, it would be pretty easy to break or bend a plastic blend door – and then you'd have to replace the heater box.

Feist and Leroux both recommend taking your truck in for a diagnosis and repairs.

“If you had a crystal ball and could instantly know what was wrong, you could make a lot more money because you could repair a lot more cars more quickly,” Feist says. “I wish it worked that way.”

If you have any repair or maintenance queries for Jason, send him a message at globedrive@globeandmail.com or contact him through Twitter: @JasonTchir

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