Spent some time reading through this for the first time. Way back in post 5,
@Dissent posted his ac pressures 25/265 @ 106 ambient.
Im sure he wasn't told what rpm they tested at but, the low side looked a bit high to me and the high side was pretty close.
My 96 with a serpentine condenser at 100 degree ambient I get 20/285 with 43 degree vent temp. This is tested at high fan, 1500 rpm, eng at full temp.
Let me give you a few basics so when you replace things you can look at your pressures and dial things in.
The condenser controls the high side. If the condenser was not moving heat, you will see higher high side. This includes any airflow issues.
Your basic high side formula (R134) is @1500(ish) rpm, 2.54 X ambient for min and 2.4 X ambient +50 max. 100 degree day high side should be 254-290. humidity will move these numbers slightly. I got this formula from an engineer at Dupont some years back.
The txv (expansion valve) controls the low side. The txv monitors the evap outlet. It tries to keep the outlet just over freezing
The normal across the board low side spec on a txv is 10-30 psi. Their normally real happy 20-22 psi (except Mercedes for some reason).
Basic diagnostic. No testing is done at idle. test 1500-1700 rpm, high fan. Run system at this speed until vent temp is lowest point. Write down the numbers. If the system was turning on and off during this time (cycling) write down lowest low/highest high.
Next when running system look at low side pressure and grab suction line (big line running from firewall to compressor). Grab the aluminum not rubber hose. How does it feel?
If I have a 20 psi low, that better feel like 20 degrees or RFC (real friggen cold). If it does the issue isn't the refrigeration system.
Cold suction line, bad vent temp. Heater door out of adjustment or water valve allowing water past.
High, high side pressure (significantly out of spec). Air flow, condenser dirty (airflow), too much oil, too much refrigerant and my favorite, wrong condenser (you had a parallel flow and the auto part store gave you a tube and fin (less heat transfer) because that's what their vendor gave them.
Low, high side pressure. Low refrigerant and or oil.
Low side can be tricky because the TXV wants to compensate the evap. If you have low refrigerant it starts to open (low side pressure goes up) to flood the evap and make it colder. it can also close off to bring temp up (lower pressure). If the system has been low on oil for a long time, the txv can be sticky and not meter correctly. During your testing, you should see the txv fluctuate some until it finds its happy spot.
I'm in favor of running a better condenser. That will bring the temps down during low speeds. The basic thing to remember is what an auto ac is designed for. Move max BTU (heat) from inside to outside.
The system better be able to handle all the heat. Max BTU into evap (high fan) carried in the refrigerant to the front where the condenser dumps it out. if you don't have enough refrigerant, you cant carry max heat, too much raises pressure which raises temp.
Think of it like this. Radiator, can you drive your car on the highway with only 70% of your coolant? Sure, as long as its cool out and you have no hills. The system has enough coolant to transfer the BTU (heat) generated to outside. Now its 105 and 80% humidity, vehicle is almost maxed out on weight and your driving over a hill. More load=more heat and outside temp/humidity is working against you. With the cooling system full you have enough coolant to move the heat but you dont, now car overheats.
If your ac system is low on refrigerant it will never pass the stress test (high fan, 1500 rpm). It will give you good vent temps on lower fan speeds (think less btu carrying capacity).
Wow I cant believe I typed all this out. Somewhere in there I said I was in favor of running a better condenser. I'd like to see what you come up with in the way of type and dimensions. The only issue will be, the more efficient parallel flow will also be more restrictive to airflow.
Scott
