Sponsored

A/C Diagnostic help requested.

TexasRebel

Gearshifter
Joined
Mar 19, 2016
Threads
27
Messages
2,500
Reaction score
835
Location
between the mustard and the mayo
Vehicle(s)
2016 YZ GTPP - PP2
Back in 2017 I had the dreaded failed evaporator.
Now on hot days my A/C will intermitentely blow warm (not hot) air and my compressor seems to be short-cycling. For the most part the system does blow cold, but not as cold as I believe it should.

What I find interesting is that my low-side pressure seems to be rising with the high-side when the compressor is on. (The following graph was built using OBDII data.) I have a set of R134a gauges, but the low side doesn't even seem to move on those. I was expecting the low-side to drop and the high-side to rise when the compressor kicks on.

This doesn't seem like a typical low-on-refrigerant issue.
Does the drop in PSI after the initial compressor-on indicate a weak compressor?

Is my expansion valve shot?

20210720103810.png
Sponsored

 

RichGT350R

Well-Known Member
Joined
Oct 16, 2020
Threads
4
Messages
533
Reaction score
737
Location
PA
First Name
Rich
Vehicle(s)
2020 Shelby GT350R HEP
something is way out of wack there. R134a pressures run pretty close to saturation temps. High side sat temp should be about 20-25 over ambient and suction saturation temp should be around 38-40 depending on the temp inside of the car.

Either grossly overcharged, bad TXV.

Disregard pressures, you have to look at the temps. With you high side should never be that high. Best place to start, is recover the system, evacuate and weigh in the correct amount of refrigerant.

Then you have a good place to start.
 
OP
OP
TexasRebel

TexasRebel

Gearshifter
Joined
Mar 19, 2016
Threads
27
Messages
2,500
Reaction score
835
Location
between the mustard and the mayo
Vehicle(s)
2016 YZ GTPP - PP2
something is way out of wack there. R134a pressures run pretty close to saturation temps. High side sat temp should be about 20-25 over ambient and suction saturation temp should be around 38-40 depending on the temp inside of the car.

Either grossly overcharged, bad TXV.

Disregard pressures, you have to look at the temps. With you high side should never be that high. Best place to start, is recover the system, evacuate and weigh in the correct amount of refrigerant.

Then you have a good place to start.
The mystery is:
The system has not been touched since the evap core was replaced 3 1/2 years ago. Overcharging doesn't seem like a possibility. I guess there is a very slim chance a vandal decided to add refrigerant to the system.

I agree, I expected the low side to be in the 30-50 range (~90 when the system is off on a 90F day).
However, the high side pressures are where I expect them to be. (225-275 psi on a 90F day)

The manifold gauge shows a low-side pressure closer to the 35-50 range, so it's possible the OBDII reading isn't getting converted correctly.
 

RichGT350R

Well-Known Member
Joined
Oct 16, 2020
Threads
4
Messages
533
Reaction score
737
Location
PA
First Name
Rich
Vehicle(s)
2020 Shelby GT350R HEP
The mystery is:
The system has not been touched since the evap core was replaced 3 1/2 years ago. Overcharging doesn't seem like a possibility. I guess there is a very slim chance a vandal decided to add refrigerant to the system.

I agree, I expected the low side to be in the 30-50 range (~90 when the system is off on a 90F day).
However, the high side pressures are where I expect them to be. (225-275 psi on a 90F day)

The manifold gauge shows a low-side pressure closer to the 35-50 range, so it's possible the OBDII reading isn't getting converted correctly.
It could be... the other thing it could be, is it wasnā€™t charged properly from the get go..

There are procedures to evacuating and charging a system and believe it or not, a lot of people like to shortcut them to save time.

I do industrial HVAC for a living (38 years now) I have came across so many systems that werenā€™t remotely charged correctly.

Looking at the graph, which takes a little ciphering.. thereā€™s a lot of fluctuations in the graph. Analog gauges can show you a lot more in this case. Seeing the fluctuations on an analog set is better then a digital set!

Fluctuations show you that there is ā€œNon-condenseablesā€ in the system, air nitrogen or even being charged with the incorrect refrigerant.

The best place to start is a recovery, evacuation down to 500 microns at least and weigh in the charge.

Now you have a good baseline of what your looking at.

Best Regards,
 
  • Like
Reactions: dx2
OP
OP
TexasRebel

TexasRebel

Gearshifter
Joined
Mar 19, 2016
Threads
27
Messages
2,500
Reaction score
835
Location
between the mustard and the mayo
Vehicle(s)
2016 YZ GTPP - PP2
It could be... the other thing it could be, is it wasnā€™t charged properly from the get go..

There are procedures to evacuating and charging a system and believe it or not, a lot of people like to shortcut them to save time.

I do industrial HVAC for a living (38 years now) I have came across so many systems that werenā€™t remotely charged correctly.

Looking at the graph, which takes a little ciphering.. thereā€™s a lot of fluctuations in the graph. Analog gauges can show you a lot more in this case. Seeing the fluctuations on an analog set is better then a digital set!

Fluctuations show you that there is ā€œNon-condenseablesā€ in the system, air nitrogen or even being charged with the incorrect refrigerant.

The best place to start is a recovery, evacuation down to 500 microns at least and weigh in the charge.

Now you have a good baseline of what your looking at.

Best Regards,
Thanks for your help.

I'm not sure the local Ford dealership would have messed up the charge when they changed the evap core out. The system had mostly exhausted to atmosphere (and my windshield) so they had to put a full, clean charge in, and they are pretty good at what they do.

The stair-step look of that plot is due to a sample rate of 5Hz on the app I'm using, 20Hz was generating HUGE files for my 30 min drive. It's not actually choppy.

I have no way to recover referigerant, so I set up an appt. at my dealership.
 

Sponsored

RichGT350R

Well-Known Member
Joined
Oct 16, 2020
Threads
4
Messages
533
Reaction score
737
Location
PA
First Name
Rich
Vehicle(s)
2020 Shelby GT350R HEP
Thanks for your help.

I'm not sure the local Ford dealership would have messed up the charge when they changed the evap core out. The system had mostly exhausted to atmosphere (and my windshield) so they had to put a full, clean charge in, and they are pretty good at what they do.

The stair-step look of that plot is due to a sample rate of 5Hz on the app I'm using, 20Hz was generating HUGE files for my 30 min drive. It's not actually choppy.

I have no way to recover referigerant, so I set up an appt. at my dealership.
Let me know how you make out. If a system leaks refrigerant, it can pull in air through the leak on the low side. If they just add refrigerant without finding and fixing the leak, doing a proper evacuation.. you can have the same issues your having.

Good luck and letā€™s hope they get you fixed! šŸ‘

Best Regards!
 

RichGT350R

Well-Known Member
Joined
Oct 16, 2020
Threads
4
Messages
533
Reaction score
737
Location
PA
First Name
Rich
Vehicle(s)
2020 Shelby GT350R HEP
Also, I just found a p-h diagram for R134a in imperial units from Danfoss... good stuff.
Download the Danfoss Refrigerant Slider. Everything you need is there.

Remember, your looking at condensing temps and evaporator temps. The pressures will correspond to that.
 
OP
OP
TexasRebel

TexasRebel

Gearshifter
Joined
Mar 19, 2016
Threads
27
Messages
2,500
Reaction score
835
Location
between the mustard and the mayo
Vehicle(s)
2016 YZ GTPP - PP2
Download the Danfoss Refrigerant Slider. Everything you need is there.

Remember, your looking at condensing temps and evaporator temps. The pressures will correspond to that.
Looks like I have a use for the Fluke thermocouple in my toolbox today. Evap temps would be close to cabin vent temps, correct?

I can use high side pressure at the saturated vapor line for the condenser temp (I sure hope I'm not superheating), or should I measure the temp of the line?

Condenser takes saturated vapor to saturated liquid?

It's been a while since I've looked at one of these.

You're a tremendous help, BTW. Thank you.
 

RichGT350R

Well-Known Member
Joined
Oct 16, 2020
Threads
4
Messages
533
Reaction score
737
Location
PA
First Name
Rich
Vehicle(s)
2020 Shelby GT350R HEP
Looks like I have a use for the Fluke thermocouple in my toolbox today. Evap temps would be close to cabin vent temps, correct?

I can use high side pressure at the saturated vapor line for the condenser temp (I sure hope I'm not superheating), or should I measure the temp of the line?

Condenser takes saturated vapor to saturated liquid?

It's been a while since I've looked at one of these.

You're a tremendous help, BTW. Thank you.
You donā€™t need a thermometer, the evaporator (Low side pressure)converted to a temperature is the evaporator temp in the middle of the evaporator. Same with the condenser (high side pressure).

Which yours is way too high!

The next thing to look at is ā€œsubcoolingā€! This is the heat rejected by the condenser. Cars are a TXV system.

So if you convert your low side to a temperature, thatā€™s the evaporator temperature in the middle of the evaporator. Same with the condenser.

Thatā€™s why I was saying, look at temperatures not pressures, yes you need the pressures, but you have to convert that to temperatures to see how the system is acting.
 

Sponsored

RichGT350R

Well-Known Member
Joined
Oct 16, 2020
Threads
4
Messages
533
Reaction score
737
Location
PA
First Name
Rich
Vehicle(s)
2020 Shelby GT350R HEP
Subcooling is the difference of what the pressure converts to temperature wise to the actual temperature of the liquid line before the TXV.
 
OP
OP
TexasRebel

TexasRebel

Gearshifter
Joined
Mar 19, 2016
Threads
27
Messages
2,500
Reaction score
835
Location
between the mustard and the mayo
Vehicle(s)
2016 YZ GTPP - PP2
Iā€˜m not sure that low-side pressure is reading where I think it isā€¦. or maybe thereā€™s a secondary obstruction?

center vent was around 38F on the drive home (which seems low, but also not 125 psi). Lowest temperature observed was 35F.
 

RichGT350R

Well-Known Member
Joined
Oct 16, 2020
Threads
4
Messages
533
Reaction score
737
Location
PA
First Name
Rich
Vehicle(s)
2020 Shelby GT350R HEP
The pressures listed donā€™t correspond to 39 & 45. Let ford check it out! The suction pressure is way too high for that.
 
 




Top