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An expansion valve myth explored

By Richard Hawkins, MACS contributor

Let’s explore the expansion valve myth.

I have encountered many tech calls where a technician had replaced an expansion valve because of a problem with an evaporator freezing up and was frustrated because that did not fix the problem.  Yes, numerous times I have heard someone on the other end of the phone in a conversation that went something like this: 

Me:  Hello, this is Richard with A/C tech support. I had a message to give Gary a call about an expansion valve issue.

Technician:  This is Gary. Thanks for calling. The expansion valve is buried inside of the dash, and it took me about 6 hours to replace it and I think this new one is bad. I’d like to take a stick of dynamite and blow this car up.

Me:  Yes, changing out an expansion valve can be very time consuming on some vehicles. It would make our lives a lot easier if the design engineers would just go ahead and locate them all on the firewall like they do with a lot of vehicles.  However, we know their primary concern is not making a vehicle easy for us to work on.

Technician: For sure.  

Me:  What type of issue are you having with the new expansion valve?

Technician:  The evaporator is freezing up.

Me:  Why was the original expansion valve replaced?

Technician:  Because the evaporator was freezing up.

Me:  You don’t have an expansion valve problem.

Technician: How do you know that.?  You haven’t even asked for any pressure readings.

Me:  Because expansion valves do not control the pressure and temperature of an evaporator.

Technician: Sure they do and if there is a problem with one, it can cause the evaporator to freeze up.

Me:  Problems with expansion valves can affect the pressure and temperatures in an evaporator but they do not control the pressure and temperature.  If they did, there would be no need for cycling switches which actually cycle a compressor off and on to prevent evaporator freeze up.  And if we go back to the old days, there would have been no need for STV, POA or EPR valves which were used on those old systems to control evaporator pressure and temperature.

Also on systems which utilize computer control, evaporator temperature sensors  provide information to the computer which is used in determining when to cycle the compressor off and on to prevent evaporator freeze up. That system you are working on has an evaporator temperature sensor. If the evaporator is freezing up, then that means that the compressor is not being cycled properly.

Technician:  So, are you saying there is a problem with the evaporator temperature sensor? 

Me:  Not necessarily. I just mentioned that to indicate that this is a cycling system you are dealing with. Yes, there could be a problem with it, but there could be something else going on.  A relay could be sticking. There could be a wiring or connector problem with the circuit the evaporator temperature is connected to that is causing incorrect voltage readings to be sent to the computer.  I have even encountered a situation where a new compressor was installed on a vehicle and there was no gap between the clutch hub and pulley and the compressor was running all the time as a result.  If the gap on the clutch had been checked prior to installation of the compressor, that could have been avoided.

Technician: It sounds like I’ve wasted a lot of time replacing the expansion valve.

Me:  Well, yes, but you are not the first person that has had this experience. There are a lot of guys out there who have the same idea that you had about expansion valve function. Unfortunately, a lot of expansion valves have been replaced unnecessarily as a result.

Technician:  Yes, had is correct. I have a better understanding about that now. I’ll need to dig into this to find out what is going on.  Thanks for the call.

Me:  You’re welcome.

As I’ve often said in clinics when discussing this, the moral of the story here is: Do not chase an evaporator freeze up problem with an expansion valve replacement.

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