Avoiding Expansion Valve Woes (Part 1)

By Richard Hawkins

On March 2^nd, an article was published in this blog covering a class that was held at the recent MACS Training Event and Trade Show.  The class title was TXVs, Superheat, MOP, and More w/Gene Dianetti. 

While sitting in that class and listening to the instructor, it brought back memories of two categories of tech calls I had been on over the years that involved expansion valves. The first category involved evaporator freeze-up and the calls would generally go something like this:

The Tech Call

Me:  Hello, this is Richard with A/C Tech Support.  I had a message to give Roger a call about an evaporator freeze-up issue on a (it could have been most any vehicle that used an expansion valve system).

Roger:  This is Roger.  Thanks for calling.  Man, this thing is kicking my butt.  I’ve had it in the shop for the better part of a week and put 2 different expansion valves on it and can’t get the the evaporator to stop freezing up.  It’s icing up all the way out to where the metal pipe connects to the rubber line.

Me:  What was the problem when the vehicle first came into the shop?

Roger:  The evaporator freeze-up problem.

Me:  That’s what I figured, but just wanted to be sure there hadn’t been more going on with it.  Did your customer say how long the problem had been occurring?

Roger:  He said the system had cooled fine up until about two weeks ago and that was when he contacted me about it. He brought it in, and we changed the expansion valve.

Me:  Has there been anything else done to the system other than you changing the expansion valve twice?

Roger:  Nope, that’s it.

Me:  Why did you change the expansion valve?

Roger:  Because that is the most likely problem when an evaporator is freezing up.

Me:  Why did you change the expansion valve a second time?

Roger:  Because I must have gotten a bad valve.  Now it seems like this valve is bad too.

Me:   OK. We need to discuss this. The expansion valve is not the most likely problem when there is an evaporator freeze-up issue. In fact, the expansion valve is not the problem at all. Please see figure #1.

Roger:  Sure, it is.

Me:  There is a misconception out in the field about expansion valve operation. They DO NOT control evaporator temperature.

I’ve been doing air conditioning work for a long time and I’ve never heard anyone say that before.

Roger:  Sure, they do.  I’ve been doing air conditioning work for a long time, and I’ve never heard anyone say that before.

Me:  And I’ve been doing air conditioning tech support for a long time, and I’ve had this same conversation with a lot of technicians over the years.  Improper expansion valve operation can affect evaporator temperature, but expansion valves do not control the evaporator temperature and a malfunctioning expansion valve is not going to cause an evaporator to freeze up.

Roger: Why not?

Me:  Because there is a temperature control system that controls the evaporator temperature.  The system you are working on uses a fixed displacement compressor and has a cycling system.  There is an evaporator temperature sensor that provides input to the computer which controls the cycling of the compressor.  If everything is working properly, the computer is going to cycle the compressor off before the evaporator gets cold enough for it to freeze.  If the expansion valve controlled the evaporator temperature, that system would not be needed.  Other systems that utilize variable displacement or variable output compressors control evaporator temperature by varying compressor output.

Roger:  Hmm.  That makes sense. I had never thought about it that way before.  So, you don’t think there is any chance the expansion valve is causing the evaporator to freeze-up.  Please refer to figure #2.

Me:  That’s right.  There is another problem there that needs to be diagnosed.

Check back in next week and we will discuss the most likely possible problems that would be causing the freezing up and cover the second category of expansion valve related tech calls.