Automotive Air Conditioning Information Forum

I have a Vintage Air system in my 55 Thunderbird. I installed it last year and it worked very well. This summer, the air goes from cold to warm, intermittently. It does not seem to be related to any RPM or MPH

I measured static pressure with car off Low = 95, High= 100.

Specs from Vintage air

Acceptable operating pressure ranges for Vintage Air systems:II.
R-134a type:A.
High-side pressure: 160 to 250 PSI. NOTE: A general rule of thumb is two
times the ambient (daytime) temperature, plus 15 to 20%.
Low-side pressure: 18 to 22 PSI in a steady state.
Center duct temperature: 38 to 46° F.

Using these guidelines it seems to be OK. If I use a pressure chart for 134 the high/low readings look low

Temperature: 78

When the system is running Low side = 23, High side 185.

AC duct temp 30 (is this too cold? What could cause that?)

Any suggestions on what else I should check?

Is the evaporator icing-up and blocking airflow after running the system for a while? Evaporator icing could explain the cold-to-warm-to-cold cycling.

At a 30°F vent temperature (too cold) coupled with the low, low-side pressure, the system appears to be undercharged.

JohnHere wrote: Fri Jun 06, 2025 8:51 pm Is the evaporator icing-up and blocking airflow after running the system for a while? Evaporator icing could explain the cold-to-warm-to-cold cycling.

At a 30°F vent temperature (too cold) coupled with the low, low-side pressure, the system appears to be undercharged.

Should I try adding a little R134?

Would this condition cause the condenser to give off more heat? I've been dealing with a vapor lock condition I never had last year

These systems typically require a specific amount of refrigerant. Check the instructions or contact them for the current capacity. Does this system have a preset thermostat? Is the probe inserted correctly to the proper depth?

At 18-22 PSI on the low side, and 23 PSI now, you'd be running the evaporator at about 19-25°F, promoting evaporator freeze-up and blocking airflow, unless you have some kind of mechanism to cycle the compressor.

In this instance, possible causes include the system developed a leak, undercharging after installation, or the thermostat or thermister (if present) is defective or improperly installed, as suggested above.

Contrary to popular belief, an undercharge (and hence, low LS pressures) will promote evaporator freeze-up.

I wouldn't add any refrigerant until we know how many ounces (net weight) of refrigerant is recommended for this system and how much was put in.

Originally, did you charge it by weight or by pressures?

You mentioned vapor lock. Is that in the fuel system?

JohnHere wrote: Sat Jun 07, 2025 8:52 am
I wouldn't add any refrigerant until we know how many ounces (net weight) of refrigerant is recommended for this system and how much was put in.

Originally, did you charge it by weight or by pressures?

You mentioned vapor lock. Is that in the fuel system?

Vapor lock reference is fuel system, it seems to be triggered by the extra load when the AC is on.

I weighed the cans when I charged it. Unfortunately I didn't write down the pressure readings. Vintage Air System: 1.8 lbs. (28.8 oz.) or 816 grams of R134a, charged by weight with a quality charging station or scale. No additional oil needed (Compressor is shipped with proper oil charge)

As I drove the car I measured the difference in vent temps when it was cold 30 degrees, when it felt warmer it was about 40 degrees

Another symptom, I sometimes hear a high pitched sound in the evaporator.

The evaporator has a drain on either side, I "teed" them together but I'm not 100% sure both sides are totally draining. I do see condensation under the car when the AC is on

Vintage Air returned my call today. They said the high/low numbers are good. They suggested I turn down the temperature control to try to maintain 38/40 degrees out of the vents.

RossL wrote: Sat Jun 07, 2025 12:34 pm I weighed the cans when I charged it.

Does your scale "zero" and "count up"—that is, weigh only the refrigerant going into the system and not the refrigerant plus the cans themselves? If not, then the system might be undercharged.

RossL wrote: Sat Jun 07, 2025 12:34 pm Another symptom, I sometimes hear a high pitched sound in the evaporator.

You probably have both refrigerant gas and refrigerant liquid flowing through the TXV into the evaporator, resulting in the noise, which might be caused by an undercharge. Also, if you didn't draw-in the refrigerant left in the hoses on completion of charging (a relatively small amount depending on the length of the hoses), then it's probably "down" that additional amount.

RossL wrote: Sat Jun 07, 2025 12:34 pm The evaporator has a drain on either side, I "teed" them together but I'm not 100% sure both sides are totally draining. I do see condensation under the car when the AC is on

I don't believe that combining the two condensate outlets into one makes any difference, as long as you're seeing drainage of the condensate, which you are.

RossL wrote: Mon Jun 09, 2025 7:16 am Vintage Air returned my call today. They said the high/low numbers are good. They suggested I turn down the temperature control to try to maintain 38/40 degrees out of the vents.

Definitely try their suggestion about adjusting the thermostat to see whether it resolves your original concern.

I asked the other day if it was a preset thermostat. The rotary dial, when turned completely clockwise, will not cycle.