ACtion Magazine - March 2014 - (Page 32)

To blend, or not to blend By Steve Schaeber M photo: S. Schaeber ost of the time I can figure out why things are the way they are on a car. Even if I can't, I can usually figure out where to look for answers; something all good mechanics need to know how to do to stay on top of their game. Back in December we had a 2001 Ford Taurus in the MACS shop with a poor heating issue. Replacing the clogged heater core solved this problem, but when I tested the controls, something caught my attention. When I turned the manual HVAC selector knob to 'blend,' which allows air to flow through both the dash vents and floor vents, the compressor turned on. Why does the compressor need to run when 'blend' is selected? After all, don't we only need A/C for passenger comfort and for the defroster to remove humidity? There must be more to it, because Ford turns the A/C on during 'blend' mode as well. Looking into this further, the first thing I did was verify the compressor was supposed to activate when blend is selected. A quick check of the wiring diagram confirms this, and the Description-and-Operation section in my service information says the compressor will be enabled, but it doesn't say why. I started thinking about those tiny side windows for seeing the outside mirrors, but those side vents are also active in several other modes, so that can't be it. I went to Ford's website for the owner's manual. Not the first place I The compressor will run with these settings. 32 ACTION * March 2014 would think to look, but I was hitting a brick wall with everything else. Surprisingly, while I didn't get a full explanation, it did say that air conditioning is provided in the 'blend' mode because "for added customer comfort, when the temperature control knob is anywhere in between the full-hot and full-cold positions, the air distributed through the floor ducts will be slightly warmer than the air sent to the instrument panel registers." Hmm, I thought. That's strange. Why would someone want to have air coming out at two different temperatures? Isn't the cabin going to be either too hot or too cold everywhere? Why would you want separate temperatures between the top and bottom vents? Sure, I get the whole 'dual-zone' thing, but I've never driven a vehicle and thought that my feet needed to be warmer than my face. So, what gives here? Every vehicle manufacturer has a different strategy for their HVAC systems, and some are just better at communicating that strategy than others. I found out through contacts at MACS that A/C is used in blend mode because when the sun is shining in the windows, the temperature at the top of the cabin can become warmer than the bottom. While the feet area might feel fine, the top portion feels much warmer. You might not need full A/C, but to counteract sun load at face level, you might want it turned on just the same. This strategy was first employed decades ago, and perhaps it's been in place for so long that car companies just assume people know what it is. Mobile A/C is a mature industry that has been used in vehicles for more than 70 years. Some things may be common, but it's still important to communicate them to others so they too can learn the methods of the madness. ❆ Steve Schaeber Manager of Service Training You can reach Steve at

Table of Contents for the Digital Edition of ACtion Magazine - March 2014

Coolant pump replacement
Computer controls for Air conditioning replacement
Freeze Frame
Leonard's Law
Virtual View
Service Port
Last Watch
By the Numbers
Cooling Corner
Industry News
Association News
New Products

ACtion Magazine - March 2014