A/C Question 300D

[dieselpilot]

I have an `81 300D running on R12. I have read various posts reporting "38 degrees temperature air at the vents". My questions:
1. Is the temperature an A/C system capable of producing at the vents static, or is it a function of outside air temp? I have read/heard that an A/C system can only produce a differential temperature at the vents vs OAT (30-40 degree difference). Thus a 38 degree temp at the vents might be possible at 75 degrees F , but not at 100 degrees F. Is this correct?

2. Leathermang, Matt L, and Larry Bible have mentioned high and low side temp readings that vary with OAT (at least I think they did!). Is there a chart for this available online?

Thanks!

Yes, a properly functioning 300D A/C system. still on R12 should be capable of 38 degree vent temp in 75 degree ambient, but it will not happen immediately upon start up. It would be several miles down the road at highway speed.

[kingdoc1]

1. Trying to associate a specific temperature drop with proper performance is difficult due to variables such as inlet air temperature, moisture content (relative humidity) and airflow. On a day with extremely high humidity, up to 70% of evaporator capacity may be utilized for dehumidification, leaving only about 30% to actually lower the temperature of the air. So, on an extremely humid day, you might only measure a 10 degree temperature drop, while on a day with low relative humidity the temperature drop might be 30 degrees. Looking for a specific amount of degrees in temperature could lead you to believe that a system is not functioning properly when in fact it is.

2. Suction and discharge pressures also are dynamic, and vary with outside air temperature, relative humidity,cabin air temperature, and refrigerant charge. They also vary while the system is running--the longer the system runs the pressures will decrease because of the decreased heat load--some of the heat has already been removed from the air. There is no specific chart, you are looking more for a range of pressures.

[dieselpilot]

Thanks Kingdoc- thats the answer I was looking for. 38 degrees on a 75 degree day is probable. 38 degrees on a 100 degree/ 50% humidity Texas summer day is not. Makes sense.