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Deputy Director, Region 1
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Discussion Starter #1
Trying to get the A/C working in my 78. First some background info. New 350/290 HP crate engine. New 4 row aluminum radiator, new carb, intake, heater core, radiator and heater hoses, new alt, new p/s pump & box, new water pump,new belts, new heater control valve, new fuel pump, rebuilt TH350 trans.
Rebuilt 4seasons A6 compressor, new A/C hoses, new accumulator, new evap.

So only things related to A/C and heat that weren't changed are the condenser and the fan clutch.

Vacuumed system added oil and R12. Used 12oz cans of R12 and it took about 3 1/2 - 4 cans.
Did this work when outside temps were in the upper 60's low 70's & low humidity.
System blew ice cold air. Only problem was after being driven about 15 minutes, the accumulator, evap, and suction lines are iced up. So bad cycling switch, right? The 78 has the switch with the sensing bulb on the long aluminum wire. Previous owner had this in a horse shoe bend mounted with a small clamp on the inlet tube of the evap. That's the tube with the orifice.
Last few days it has been in the 90's with very high humidity. A/C blows cool not cold. No freeze-up. Low Side Pressure 49psi High Side Pressure 250psi. Rev the engine to about 2000rpm and hold it there., the pressures are Low Side 37 psi and High Side 275 psi.
Cools off in the evening. Temps drop to low to mid 80's still humid but not as bad. Low side pressure 45psi and High Side 225psi. Blows colder air out the vents.

Today temps in the low 70's and very low humidity. Freezing up again. Pressures are Low side 37psi High Side 225psi.

So aside from the cycling switch being bad, why is it not cooling in higher ambient temps and high humidity? Is it overcharged? Under charged? Not enough air across the consenser?
 

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Bobby, sounds like it is a little overcharged. I do not have my capacity chart in front of me but the pressures seem a tad high. I would also suggest you verify the recirculation door is working in the max mode, (run your tests in "max".) Check you fan clutch too.
And again, the thermostatic switch bulb/tube should be mounted to the outlet of the evaporator (big tube) as close to the evaporator as possible. Also mount it on the evap tube at about 11 o'clock. Make sure your heater control valve is shutting the flow of coolant to the heater core in max and norm modes.
 

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I would suggest changing over to an adjustable pressure switch, instead of the temp switch.
 

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Discussion Starter #4
OK all these pressure readings and symptoms are on HIGH FAN and MAX A/C. Heater valve works, both heater core pipes are cold.
But to me neither of those should matter. Think about, it freezes up unless it's high 80's - 90's and very high humidty. So it is capable of getting cold. If hot coolant was travelling thru the heater core and warming the vent temps, that would happen every day. If a door wasn't closing that would happen every day too. I can hear the door moving. It is possible that it's not making a good seal. Can't tell w/o tearing things apart.
As for the cycling switch being bad, yeah I know it needs to be changed. But all that tells us is that the system can get cold enough under the right conditions. Those conditions being cool outside air temps and moderate to low humidty. Changing that switch is not going to give me cold air on hot humid days. So let's put that aside for the time being.
I changed the position and location of the bulb last week and it made no difference so that pretty much confirms bad switch. Ok but the question remains, why won't it cool when it's hot and humid?
Next plan of attack is a new cycling switch which at this point will only affect cool weather operation. A new fan clutch. BTW, any suggestions for a clutch? regular duty, heavy duty or severe duty are the choices. I'm leaning toward "severe" duty.
After that maybe make some baffles from the grille to the condenser to try and concentrate air flow across the condenser. Then tear into the A/C box, check the door movement and see why the condensation is leaking into the car even though the drain holes are free and clear. My theory on the condensation is that one of two things are happening. One, maybe the new 4 seaons evap is a bit taller than the OEM it replaced and it's trapping condensate on the other side of the evap. Two, maybe because there is so much condensation on the lines and evap, the fan is just blowing it into the car and it's finding a way out.
If all of that doesn't help, then what?
Replace the condenser, accumulator(again) orifice tube(again), flush the rest of the system and recharge. I don't have a scale so I'm going by the ounces of each can based on the system capacity. But in reality there's always some left in the cans that won't get sucked in.
As far as being overcharged, what's the solution? Recover some slowly until the pressures drop? Where should they be? Or does the entire system need to be evacuated and then charged to certain pressures and stop at that point?
It also seems the pressures change based on outside temp and relative humidity. So then what? One day good the next day bad?
 

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Bobby, @90* ambient temp you would like to see around 38 on the low side and around 190-200 on the high (add around 100 or so to the outside temp as a general rule of thumb). That should yeild you around 36* vent temps. You're probably overcharged or possibly have a bad fan clutch, (is the shroud installed?) The reason it is working better when it is cool out is the system does not have to work as hard to remove heat and it is freezing because the system never cycles due to a faulty thermostatic switch or bulb mounting location.

Bleed some refrigerant into a recovery machine and recheck giving vents temps as well as gauge readings.
 

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Discussion Starter #6
OK. I understand it works better when cooler out because it doesn't have to work as hard. That's the obvious part. My question/problem or whatever is why it doesn't work when it's hot and humid. So you think bleeding off some freon and changing the clutch.
Which clutch would recommend? Regular, HD or severe duty? I guess before I remove any gas I should change the clutch.
 

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OK. I understand it works better when cooler out because it doesn't have to work as hard. That's the obvious part. My question/problem or whatever is why it doesn't work when it's hot and humid. So you think bleeding off some freon and changing the clutch.
Which clutch would recommend? Regular, HD or severe duty? I guess before I remove any gas I should change the clutch.
I would go severe or HD but I'm in TX, expect them to be noisier than standard.
 

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Discussion Starter #8
I'll never hear it over the flowmaster 40's LOL!:nanawrench:
Oh yeah, to answer your question about the shroud, it's an NOS GM top and bottom. I had to trim the edge that goes against the radiator to fit the new 4 row aluminum radiator but it fits close w/o rubbing on the radiator.
thanks:texas:
 

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Cool, just make sure the blade is half in and half out of the shroud.
 

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Discussion Starter #10
Cool, just make sure the blade is half in and half out of the shroud.
Hmmm.... That's something I never really paid attention to. Just took for granted the engine was the same size and the shroud the same. Now I'm curious.
 

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Discussion Starter #11
OK half in half out. Any way to really check the fan clutch? For $50 doesn't really matter I'll get the severe duty one anyway. As a body guy I usually just pull up and down on them to see if there's any play. This one is very tight. Spins stiffly with engine off.
 

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Discussion Starter #12
OK now even more confused. Didn't remove any freon. Temps today 80* sunny low humidity.
Fan on HIGH, selector on MAX. LS 29-30 psi HS 150 psi. These are at idle. Raise the rpm's LS drops to 25 psi, HS 160 psi.
So unless I'm just not understanding this whole pressure/temp thing today the readings seem to be OK?
 

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Cool, just make sure the blade is half in and half out of the shroud.
Robert, in all the threads on AC that I've read here, that's the first time I've read that remark, and you are absolutely correct (but you knew that).:smileyb: Other forums I belong to, STRESS the placement of the fan in the shroud. OR, maybe everyone takes that as a "known" objective. Whatever. I've really glad to see you state that.:You_Rock:
 

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OK now even more confused. Didn't remove any freon. Temps today 80* sunny low humidity.
Fan on HIGH, selector on MAX. LS 29-30 psi HS 150 psi. These are at idle. Raise the rpm's LS drops to 25 psi, HS 160 psi.
So unless I'm just not understanding this whole pressure/temp thing today the readings seem to be OK?
Looks ok at 80* ambient, still doesn't rule out a fan clutch, what was your vent temp?

Remember higher ambient temps put a greater heat load on the system, don't over think the solution. Systems tend to behave until they are stressed.
 

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As a body guy I usually just pull up and down on them to see if there's any play. This one is very tight. Spins stiffly with engine off.
With the engine hot and engine off, try to spin the the blades by hand, if it spins more than two or three blades replace it.
Or, if you have the engine running and hot, observe the fan as you shut the engine down. If the fan does not stop and continues to spin after you shut the engine down, replace it.
 

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Discussion Starter #16
Installed a new "severe duty" thermal fan clutch today. Outside temps here are still cool so can't tell if it helped. but it'll get hot soon enough. Still waiting on the cycling switch. Should be in tomorrow, Thursday.
 

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bobby just how much oil did you put in system? when lines start frezing you got a restriction or no air flow yes the cycling switch could be bad but you look inbetween the prongs and there is a screw to adjust turn slowly.hope this . helps
 

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Discussion Starter #18
I don't remember how much oil I added but I followed the directions that came with the compressor.

I don't know if this switch is adjustable? It's not the newer style switch that mounts on the schraeder valve of the accumulator. It's a square box that looks like a relay mounted to the front of the A/C box down behind the accumulator. It has a long aluminum wire with a sensing bulb at the end of it.
When it freezes it only does it after being on the road for IDK 20-30 minutes. I can tell from inside because air flow out the vents drops off. Then if I pull over and open the hood it's all iced up. So I guess if it is adjustable, I need to adjust it then so the compressor turns off. Let it thaw and keep repeating the process until it's adjusted correctly. It doesn't freeze in idle.
Should have a new one tomorrow for about $14. Cheap enough I guess and then at least it's one less old thing to question.
 

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As I said in the PM a couple of weeks ago Bobby, most thermostatic switches have one to two small adjustment screws on the bottom or backside. The adjustment is very fine, but for a wider range of adjustment use the kind with the large adjustment knob like the ones on the old under dash knee knocker units.

Also, if the system were oil-logged, it would probably not freeze up at lower ambient temps and your low and high side gauge readings would be higher than previously reported.
 
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Discussion Starter #20
Ok put the new switch in today. Left it set as it came. It has 2 small screws for cut in and cut out. Took it for a ride 15 minutes north on the highway at 60-80 mph and back at the same speed. Got off the highway and drove about 10 blocks to the house. Suction lines were sweating and suction side tube of evap was frosty but not like before. Orifice tube pipe looked OK and a little frost on parts of the accumulator. So I don't know if the switch is adjusted OK or if I didn't drive it long enough. But I think if it had froze up on the highway it would have still been frozen when I got back to the house. Don't see or feel the compressor cycling.
So suppose it does freeze up again. The adjustments are on the back of the switch. I'll have to remove it to adjust. What would be the amount to turn each screw? Looks like this will be a trial and error proceedure?
 
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