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Refrigerant overcharge

By Richard Hawkins, MACS contributor

Let’s take a look at the subject of refrigerant overcharge.

This is the fourth in a series of blogs about refrigerant recovery. You can read the preceding blogs here. Blog 1,

Blog 2, Blog 3

Last week’s article ended with the following: “A shop could end up with an overcharge of refrigerant in a system and a resulting performance problem and waste a lot of time trying to diagnose what the problem is. Check back in next week for a recap of a tech call conversation with a technician who encountered that very problem.”

I do not recall what the application was, but it was an expansion valve system with a 24-ounce refrigerant charge and 6-ounce oil charge. This call occurred in the spring when you can get some large ambient temperature swings.

The refrigerant overcharge scenario conversation went something like this:

Me:  Hello. This is Richard with A/C tech support. I had a message to give you a call about an A/C issue.

Technician:  Thanks for the call. Yea, I’ve got one here that wants to ruin my day.

Me:  What’s going on with it? (He provided the application, but as mentioned previously, I do not remember what it was.)

Technician:  Well, it acts like it’s got an overcharge, but I’ve charged it 3 times now. The first two times I used my recovery/recycling machine. The last time I used a scale.

Me:  What is the ambient temperature and humidity there and what are your pressure readings? Also, what are the vent temperatures?

Technician:  It’s about 92 degrees here.  I don’t know what the humidity is, let me check the internet. My search says the humidity is 78%.  The high side is running 325 psi and the low side is running 45 psi.  The vent temperature is about 55 degrees.

Me:  What was the engine speed those pressure readings were taken at?

Technician:   A curb idle.  At 1500 RPM the high side goes up to about 360 psi and the low side creeps up to 50 psi.

Me:  Have you checked the cooling fan operation?

Technician: Yes, it is working fine.  Also, driving it down the road above 35 MPH where you have plenty of ram air doesn’t make much difference.

Me: OK. Can you fill me in on the work that has been done?

Technician: Sure.  It came in about 10 days ago and I replaced the compressor and the drier.

Me:  Why was the compressor replaced?

Technician:  The front shaft seal was leaking.

Me:  Did you do an oil balance or flush the system and install a complete oil charge?

Technician:  I flushed the system and put a full oil charge in the system.  With the leak, the system had lost some oil and I didn’t want to do any guessing about how much oil might end up in it.  The expansion valve is located on the firewall, so it was easy to remove it to flush the evaporator.

Me:  How much oil did you put in that system?

Technician:  6 ounces, just like the specification calls for.

Me:  Did you drain the oil that was shipped in the compressor?

Technician:  Yes, I always drain compressors before I install them.

Me:  That is a variable displacement compressor that has a drain plug. Did you remove the drain plug and drain it from the drain plug hole?

Technician:  Yes, I did and I filled the compressor through the drain plug hole. 

Technician:  OK. Sounds like the system has the correct amount of oil in it and you were spot on with your procedure.  So, we can rule out an oil overcharge causing the high pressures.

Me:  How much refrigerant did you charge it with?

Technician:  24 ounces, just like the specification calls for.   It was working perfect the day the customer picked it up.

Me:  Do you recall what the ambient temperature and humidity might have been and what the pressure readings and pressures were and the temperatures from the vents were?

Technician:  It was in the low 90s and the humidity was probably about where it is today.  I don’t recall the exact pressures, but the high side was about 200 psi to 225 psi and the low side was around 30 psi or so. It was cooling great, in the low 40s out of the vents.

Me:  Ok, so when did it start acting up?

Technician:  About a week later, 2 days after we replaced the discharge hose.

Me:  Why was the discharge hose replaced?

Technician:  There was a damaged spot on it.  It looked like it had gotten hit with an air chisel or hacksaw.  It wasn’t leaking, but we called it to the customers attention and the plan was to replace it when we installed the compressor.  Unfortunately, there was a delay in getting the hose and the customer had to have the car, so we installed the old hose. The customer brought the car back in and I installed the new hose about a week later.  

Me: So,that was when the high-pressure problems started?

Technician:  No, the system was working fine.  The pressure problems started a couple of days later.  

Me:  Do you recall what the ambient temperature and humidity was when the hose was replaced?

Technician:  It was mild. Probably in the upper 60s or low 70s and moderate humidity.

Me:  What about the ambient temperature and humidity when the pressure problem occurred?

Technician:  About like today.  Lower 90s and about the same humidity.

Picture #1:  If it had been possible to replace the discharge hose when the compressor was replaced, the high pressure problem would not have occurred.  More on that next week.

Calls investigating a problem like refrigerant overcharge can get involved and lengthy and this is a good place to stop for this week. 

We will pick up where we are leaving off next week.

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