1970s 1976 ATC questions

[Pghcc2006]

Good evening gentlemen.

I replaced all the mechanical/under hood components with rebuilt/new items by old air And original air products. New evaporator/condenser, rebuilt A6 compressor, barrier hoses and rebuilt VIR with new valves.

Vacuumed down to 880 microns which held for 40 minutes and recharged with vintage National  Refrigerants R12 . 60ish ounces as verified by accuteck scale 245 oz to 180.

Pressures are as follows@ 80 to 90 degree small garage:

1.) Static cold, engine off: 70L/75H

2.) 600 RPM A/C on: 55L/260H

3.) 2000RPM A/C on: 33L/262H

4.) 2000 RPM A/C on very hot in garage over 100: 35L/290H

5.) Static off hot: 110L/110H

Idle vent temp won't go below 50 some. When driving will dip to low 40s occasionally. VIR sweats like an Only Fans Girl being asked at a family reunion what she does for a living

Is this normal as a function of the ATC? Pressures charge too high/low? Performance seems underwhelming compared to my 1979 Lincoln. 

[Chopper1942]

If the vehicle has been sitting and is cold (not started within the last hour), the static high and low side pressures should be the same as the ambient temperature. Also, this is a clutch cycling system, so the low side is pulled low to the switch set point and then the clutch is cycled off until the low side reaches it high pressure setting and then it engages the AC clutch and the compressor runs.

Below is an AC performance chart for a 2008 Chevrolet Trailblazer. This is a R-1234A system and is not as efficent as an R-12 system,so the outlet temps are a little higher than usually with R-12.


Ambient Temperature       Relative Humidity        Low Side                 High Side                Outlet Temp

13–18°C (55–65°F)              0-100%       175–207 kPa (25–30 psi)    1030–1220 kPa (135–175 psi)   7°C (45°F)

19–24°C (66–75°F)             Below 40%      175–254 kPa (25–37 psi)   1200–1430 kPa (160–200 psi)   11°C (52°F)

                              Above 40%      175–262 kPa (25–38 psi)    1170–1400 kPa (170–210 psi)  11°C (52°F)

25–29°C (76–85°F)             Below 35%      208–288 kPa (30–42 psi)    1370–1570 kPa (150–190 psi)  14°C (57°F)

                              35–50%         213–292 kPa (31–42 psi)    1350–1570 kPa (160-200 psi)  14°C (57°F)

                              Above 50%      216–300 kPa (31–44 psi)    1340–1550 kPa (160–200 psi)  15°C (59°F)

30–35°C (86–95°F)             Below 30%      242-328 kPa (35–48 psi)    1510–1750 kPa (200–240 psi)  17°C (63°F)

                                30–50%       246–335 kPa (36–49 psi)    1500–1740 kPa (200–240 psi)  17°C (63°F)

                              Above 50%      252-346 kPa (37–50 psi)    1490–1730 kPa (200-240 psi)  19°C (66°F)

36–41°C (96–105°F)            Below 20%      281–366 kPa (41–53 psi)   1680–1920 kPa (240–280 psi)   19°C (66°F)

                              20–40%         285–374 kPa (41–54 psi)   1670–1920 kPa (245–285 psi)   21°C (70°F)

                              Above 40%      292–383 kPa (42–56 psi)   1670–1910 kPa (250–265 psi)   22°C (72°F)

42–46°C (106–115°F)           Below 20%      322–400 kPa (47–58 psi)   1850–2070 kPa (250–290 psi)   22°C (72°F)

                              Above 20%      326–410 kPa (47–60 psi)   1840–2060 kPa (260–300 psi)   23°C (73°F)

47–49°C (116–120°F)           Below 30%      360–426 kPa (52–62 psi)   1990–2150 kPa (290–330 psi)   24°C (75°F)

Based on the pressure readings, I would say it is doing very well.  If the low side goes any lower it will definitely freeze up the evaporator.  Make sure that the evap drain hose is clear and water is dripping from it. I always cut a cross cut on the end of the hose to make sure it drains easily. If the outlet temp is varying from the 40's to the 50"s, the evaporator may be freezing.

When are you getting 32 from your outlet? If your outlet temp is going to 32 at the outlet, I would definitely suspect the evap is freezing over.  Have you noticed a change is the airflow when the temp goes up?

[V63]

FWIW I found ford products with same A6 Harrison compressor as superior performance moreover the compressors last longer on that application, most still have their original compressors.

One of the best air conditioners I experienced was a 1975 LTD, absolutely incredible FRIDID problem free air.

That said your car should be consistent at 40 degrees,  is the engine running cool enough? Engine fan pulling enough air?

[Dave Shepherd]

Got to take humidity level into account, when gaging outlet temps.

["Cadillac Kid" Greg Surfas 15364]

David,
The numbers you show here COULD be perfect, depending upon what the actual outside (of the car) temperatures are. The ATC with its sensors evaluates the outside temperature, the cabin temperature and the temperature control setting and controls the supply air temperature to meet the desired temperature to the best of the system. The system works by drawing all the air through the cooling coil first to dehumidify and cool the air down to between 35 and 38 degrees F. It is then partially (according to the desired cabin temperature) reheated through the heating coil to the proper temperature. Moisture removal requires substantially greater "effort" by the refrigeration system than the lowering of air temperature, so depending upon the outdoor temperature and humidity conditions, the temperature off the cooling coil can be higher.
As a point of reference, stationary AC systems such as commercial or residential AC are designed to have a supply air temperature of 55 degrees F. This ensures the supply air will mix with the "room" air and not just fall to the floor. With automotive systems it is the same. If the temperature is too low the supply air will just drop to the floor and the cabin will stay warm at the "breathing level".
The factory service manual has this chart showing what to expect from your system and I have found it to be right on for a correctly functioning system.
As a side note, here in Southern Texas we have had some warm weather recently (LOL) driving my 76 CdV, the outlet temperature is not "ice cold", but within a few minutes of driving the cabin is comfortable.

[Chopper1942]

The chart I posted from SI is vertually the same as "Cadillac Kid" Greg Surfas 15364 posted from his manual. My chart go a little scrambled when I posted it.

As a rule of thumb, if you get a 20-30 degree drop in temp from ambient to outlet the AC is working as designed.  The higher the ambient temp and humidity the less temperature drop across the evaporator.

[V63]

20-30 temperature drop would never cut it in Arizona, with ambient of 120 I'm still expecting at least 40 degrees (and get it) so that's an 80 degree spread. That's on max AC (recirc)

I had a 'new' 2006 premium vehicle that was at 55-60 degree discharge air (110 + ambient) and thats what the service department tried to impress upon me was industry standard, they ended further discussion.

I had to escalate remedy and I ultimately won that debate. 

["Cadillac Kid" Greg Surfas 15364]

A functional system with an A6 compressor has a capacity of 27000 BTUH. That's it. The leaving temperature is dependent upon the sum total of solar ambient temperature humidity and internal load. No majic. By the way here in San Antonio we have been having 105+ degree humid conditions and 50 degree outlet air temperature brings the cabin down to comfort conditions within 10 minutes from start up
Greg surfas

[Pghcc2006]

Good evening gentlemen, thanks for the responses.
I've attached some pics of the temp in front of the condenser/grille and vent temps. I did not have the ability to put the gauges on today  but it was probably 5 degrees cooler than in my first posts disregarding the temperature rise after running the car at 2000 rpm.

I don't believe car runs hot, fan is spinning and radiator and overflow tank are both at proper levels, but both my 70s cars seem to run hotter than modern stuff. VIR seems to sweet and a bunch of water drained out of the bottom when I drove it to the store 7 miles away over the weekend. Just wondering if you guys think the VIR and interior components are operating properly. Don't want to risk overcharging it.

[Chopper1942]

Do not just add refrigerant to the system. If you think the system is low, recover, evacuate, and then recharge the system with the amount specified for your system.

I guess that some of you don't under stand  what the temp drop across the evaporator means. Assuming a 30 degree drop, if the outside air coming into the evaporator is 70 degrees then on the outlet side of the evaportor it would be 40 degrees.  If the outside air coming into the evaporator is 105 then the outlet side would be 75.

When the system is in recirc mode, outside air does not enter the evatorator.  The air from inside of the vehicle is brought through the evaportor and cooled.  If the AC system lowers the inside air temp to 90, now you are cooling 90 degree air and the system will drop the outlet temp to 70.  If the inside air temp is stablized at 70, now the air coming into the evaporator is 70, Not 105, and the outlet temp would be 50, etc. This is why on a very hot day it takes awhile to get the inside temp to come down.  It is not instantly 105 to 40.

Now, to the question of why it doesn't get cool.

1. Does the system have the proper charge?
2. Is the condenser, radiator, and the space between them clean?
3. Is there proper air flow through the condenser and radiator?
4. Is the engine overheating

If all of the above meet OE specs, then it may be an issue with the controls or actuators in the HVAC system. Some of the items to check:

1. Is it on recirc?
2. Are the mode and inlet doors moving or binding?
3. Are the seals for the mode and inlet doors good.
4. Is the condesant from the evaporator draining properly

Something else which your vehicle may or may not have is a hot water shut off valve. The purpose was to remove as much heat from inside the HVAC housin as possible so the AC would work better (lower outlet temps). GM installed this in the heater hose going to the heater core from the engine. The valve was vacuum operated and when they failed, they stick either open or closed. If closed great AC but no heat in the winter. If open, lousy AC but good heat in the winter.

I used to add these to customer's vehicles to improve operation of the AC.

["Cadillac Kid" Greg Surfas 15364]

David,
Where are you located? It sounds like it is a humid area by the condensate draining from the evaporator and the VIR sweating. Also,, what are the settings on your controls (should be on High) when you get the 50 degrees outlet temp (temperature dial should be at 65 degrees) and and is the car moving or stationary?
By the way, when you have the controls on these settings, the heater control valve closes and the outside air damper closes.
Greg Surfas

[Chopper1942]

The simple way to verify that the heater control valve closes is the heater hose from the engine will be hot and the hose out of the valve will be colder to the touch. If not, verify you have vacuum at the valve with the settings like Greg specified. If not, check to vacuum line for leaks or breaks.

[Pghcc2006]

David,
Where are you located? It sounds like it is a humid area by the condensate draining from the evaporator and the VIR sweating. Also,, what are the settings on your controls (should be on High) when you get the 50 degrees outlet temp (temperature dial should be at 65 degrees) and and is the car moving or stationary?
By the way, when you have the controls on these settings, the heater control valve closes and the outside air damper closes.
Greg Surfas

I'm just outside Pittsburgh.  When I drove it last Saturday it was 82 with 62% humidity. It was comfortable, but given everything I've heard regarding R12 and given the performance of my Lincoln (R134 btw) I guess my expectations were different. When tested with gauges and yesterday it fan speed was on high and 65/ full cold and car was stationary. Heater control valve appears to be original based on condition. Temperature and mode selector appears to work as intended. It will go from (very) hot to cool, heat blows on floor vent except on bi-level and defrost when selected. Defrost goes full speed etc, a/c speeds  modes work ( slow is incredibly slow btw)

["Cadillac Kid" Greg Surfas 15364]

If the cabin is "comfortable" then the system is doing its job. It is officially called "Automatic Climate Control", not frozen storage space.
Greg Surfas

[TJ Hopland]

If you have the selector set to High I would expect they system to start blending in some hot air to maintain the temp set by the dial since the blower will stay on high. 

If you set it to auto once it reaches the set point it would be more likely that it just slows down the blower to maintain the temp.  If you went to low it would probably always blow cold air because it can't move enough air to satisfy.   

[Chopper1942]

Is the hot water valve working correctly?

If it does not close, you will never get the cabin as cold as possible. If nothing else, clamp off the heater hose going to the heater core with hose clamp off pliers or long needle nose vice grips and see if the outlet temp is now colder.  If so, the hot water valve is not working.

[Seville Life]

This is a fascinating thread. I run a '78 Seville with fixed-orifice type refrigeration.

In Auto it goes into recirculate automatically, presumably based upon a pre-set temperature difference between ambient and requested interior settings?

I have, typically, vent temperatures of around 50 to 54F.

In hot weather the fan go to high-speed, the recirc door opens (noisy) and there it stays. It rarely closes the recirc door or drops the fan speed?

I started Cadillac life with a '70 and I always thought that was much more effective?

[Pghcc2006]

Replaced the water control valve with a part from Caddy Daddy and am still not getting anywhere close to the 40 degrees standard, much less the 32 to 35 I'm getting from my r134 1979 Lincoln at idle. To say I'm not thrilled is an understatement. The programmer/ control head both seem to work as the modes switch appropriately and the heat gets up to 146 degrees.  I'm considering installing a shut of valve on the heater core hose. Once again all new underhood components and running R12.

["Cadillac Kid" Greg Surfas 15364]

You don't say which of the dash temperatures is on full cooling.  . 40 to 50 degrees is normal. What is the outside temperature and what is the temperature in the car? 20 degrees between the cabin temperature and the outlet temperature i what these systems were designed to produce. They are designed to produce comfort conditions, not hang meat in.
Greg Surfas

[Seville Life]

Greg where do you get the 20 degree differential in temperature from?

The point I was making about my '78 Seville fixed orifice type was that it rarely comes off high fan and recirc door open? Grim though this is a pal's plastic Toyota car has much colder outlets?

Also that our '79 was ice-cold and very quick to get the temperature down and slow the fans down?