Automotive Air Conditioning Information Forum

Hi all,
I have pieced together an A/C system for my restomod truck and I am seeking some diagnostic help with pressure and fluid volume.
Here are the vehicle details:
1971 Ford F100 body on a 1999 Mercury Grand Margquis chassis and drivetrain
1971 FoMoCo Integral A/C & heat box with 1971 FoMoCo evaporator
1999 Grand Marquis compressor, condenser, receiver/drier & pressure switches
1999 Grand Marquis 2 speed electric fan (fan does switch to hi speed when A/C is engaged)
Custom A/C lines (around 8' total length of #10 flexible hose)
GM type Orifice tube repair kit modified to house orifice tube
White .072" orifice tube

The ford/Mercury manual states the following for "standard operating specs"
80 deg. ambient temp at 50-70% humidity
engine RPM 1200
Lowside pressure: 22-45
Highside pressure: 125-235

My current readings:
Cold static pressure: 110psi
at idle 800 rpm pressure: 39psi lowside 170psi highside
1200 RPM: 32psi lowside 200psi highside
2500 RPM: 25psi lowside 225psi highside
1500 RPM while driving 55mph: 25psi lowside 210psi highside
Evap inlet temp: 66 deg
Evap outlet temp: 69 deg

To achieve these pressure I had to add 68oz of R134a... I know I added a LOT of extra capacity with the large and very long hoses but it seems like a lot of freon! I tried driving the truck with 60oz but after a mile or so the lowside pressure would drop below 22psi and the pressure switch would disengage the compressor and begin a "short cycle" issue.

My questions:
1. Is there a way to accurately measure my system and come up with a mathematical solution for the amount of freon required? or is my method of using the evap inlet & outlet temps combined with idle and driving pressures the best route?
2. Does the lowside pressure drop seem "normal" from idle to driving?
3. Does the highside pressure increase seem "normal" from idle to driving?
4. Since the original install of this system I have been informed that the Grand Marquis came factory with an orange 0.57" orifice tube diameter, should I remove the .072" that is in there now and replace it with a .057" tube?
5. Do you have any advice on any changes I should make? (I.E. Orifice tube diameter, pressures, etc...)

Am I understanding you're using a 71 ford evaporator? I'm not sure that you will ever get that evaporator to work correctly with the newer stuff off the mercury. At this point you're probably going to have to experiment with a few different orifices, probably would not hurt to use an adjustable low pressure switch also.

Is the air temperature out of the vents in the 30s? Or am I reading wrong? If so, I wouldn't do anything right now.

Mr Bill wrote:Am I understanding you're using a 71 ford evaporator? I'm not sure that you will ever get that evaporator to work correctly with the newer stuff off the mercury. At this point you're probably going to have to experiment with a few different orifices, probably would not hurt to use an adjustable low pressure switch also.

Yes, I am using the 1971 Ford Evaporator. I don't mind experimenting with different orifice sizes, would you be able to explain what changes I should expect from the different sizes?

Cusser wrote:Is the air temperature out of the vents in the 30s? Or am I reading wrong? If so, I wouldn't do anything right now.

In the very beginning I was able to reach high 30's vent temps before our summer heat and humidity came in, I accidentally typed my initial evap temp readings when the ambient air was under 70 deg. I have corrected the original post to show my current evap temp readings.

Current vent temps vary from mid 60's to low 70's

A smaller orifice should increase the suction pressure, an increased suction will increase coil temperature. If your vent temp is 60° you probably need to increase the orifice tube size "meaning the hole in the tube need to be larger". A lot of things increase vent temps. fan blowing to hard, humidity in the cab, low on charge. Your really going to have to do some tweaking with the orifice tubes and the charge to get this thing cooling better, using the mixmached system.

Mr Bill wrote:A smaller orifice should increase the suction pressure, an increased suction will increase coil temperature. If your vent temp is 60° you probably need to increase the orifice tube size "meaning the hole in the tube need to be larger". A lot of things increase vent temps. fan blowing to hard, humidity in the cab, low on charge. Your really going to have to do some tweaking with the orifice tubes and the charge to get this thing cooling better, using the mixmached system.

Thank you very much for the info!
I did some more testing today and I think I have drawn a proper conclusion on whats happening, please guide me here too!

I set the truck up to test in the best driving conditions I could
Idle increased to 1700 RPM
Large fan placed directly in front of condenser
Doors closed, windows up, fan setting on medium and ensured the blend door was fully set for cool air (I have temporarily disconnected the heater core lines to eliminate any heat transfer possibilities)

76 deg ambient 56% humidity
Initial readings:
Lowside 25psi
Highside 145psi
Vent temp 58 deg
The compressor will short cycle with any lower amount of refrigerant...

I removed 8oz of refrigerant and BYPASSED the lowside cycle switch to keep the clutch engaged for a quick test
Lowside 20psi
Highside 143psi
Vent temp 53 deg (5 degree drop with less refrigerant and less lowside pressure)

With the lowside at 20psi for 30 seconds the accumulator froze over, the evap outlet line and the evap did NOT freeze, this leads me to believe that I am flooding the evaporator when I add enough refrigerant to "properly" cycle the compressor. This means I need to decrease the refrigerant amount and mechanically increase the pressures on both low and high sides. a SMALLER orifice tube diameter would allow me to decrease refrigerant and maintain pressures correct?

Your input and output of the evap should be about the same temp. This will tell you the evap is fully flooded. The output can be a few degrees colder. Gm used to state that to do a partial charge to fill system until both of these lines are the same temp and then add a few more oz of refrigerant.

Dougflas wrote:Your input and output of the evap should be about the same temp. This will tell you the evap is fully flooded. The output can be a few degrees colder. Gm used to state that to do a partial charge to fill system until both of these lines are the same temp and then add a few more oz of refrigerant.

I can get the evap inlet and outlet temps within a few degrees of each other but it's not cold, low to mid 50's on a mild day (75 deg ambient) and mid to high 60's on a hot day (85 deg) with 68oz of refrigerant. Any more refrigerant and the vent temps get hotter, less refrigerant and the compressor short cycles.
What step should I take next? smaller orifice and less refrigerant?

Again, thanks for all the help!!

Hard to beat you listed pressures.

IS the capillary tube on the exp valve attached to the suction line and wrapped with moisture resistant tape?