Air Conditioning: I took a gamble and...

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GRWeldon

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In February of this year (2024) when I bought my less-than 100K miles, non-running white 96 4.6 Bird with ONLY* a bad fuel pump, the outdoor temps were in the 25-50 degree range. Well, now in June, the temps are in the 85-95 degree range, today being the hottest day yet this year at around 98, I really wanted to get my air conditioning running.

I knew it was a gamble, but I thought it might be a good one. I pulled a vacuum on it last week and it only dropped 2 inches of mercury over 2 days. I figured I'd charge it up and see what happened. I bought a full 36-oz charge of R-134a today at the auto-parts store for a grand total of $34 and some change. I promptly drive home and pull it up on the concrete and once again pull a vacuum on the system. I then proceeded to empty the 3 cans of freon into the low-pressure port. After the first can I hear the AC compressor clutch start to quick-cycle on and off. After the second can it stopped cycling. I emptied the last can and disconnected the gauges.

I moved around to the open window and expected to find ice-cold air coming out of the vents. I was quite disappointed to feel that I had the heat on instead of the air conditioning! I'm pretty bummed that I just wasted $35 but I knew it was a gamble. I lost.

So I have another 4.6 parts bird that I pulled a vacuum on last week as well. Over three days it lost 10" of mercury. I was thinking of swapping this compressor in to the white 96 and see if it works. It would be nice to be able to test it out before I install it but I do not know if it is possible. Is it? Also, I have a V6 parts car, I was wondering if the AC compressors are the same between the V6 cars and the V8 cars since the V6 compressor is much easier to get to. Any helpful ideas would be welcomed. I'm just wondering if I should just buy another compressor, condenser and drier and just redo the entire system, but I'm not going to pull the dash to replace the evaporator coil so I'm wondering if I should really even consider installing new stuff...

* Not only did I replace the fuel pump but the intake was leaking, the power steering rack and pump were shot, also the upper and lower ball joint and outer tie-rod ends. So much for a good deal...
 
The 4.6 and 3.8 compressors are both the FS-10, but they mount differently so they don't interchange. With A/C it's often best to tackle it all at once, so you don't find yourself kicking yourself for skipping something and it backfiring on you in the near-term.
 
Totally agree, especially since the problem in your system isn’t even necessarily the compressor it seems like.
 
I'd suggest tearing it apart and taking a look at the orifice tube; if there's no crud and it's not clogged, your compressor is probably fine. You could also drain the oil from the accumulator (or cut it open) to be sure it's clean (dye non-withstanding) for confirmation.
 
The only way to diagnose an ac system is with a proper charge - so your $35 wasn't a total waste of money. Losing 2 inches isnt much - its either a slow leak or non condensible gases still in the system. I pull a vacuum on one side and put my micron gauge on the other to get a better indication of the overall system. That would make something like an obstruction in the orifice tube quickly evident.

What are the pressure readings on the high / low side ?
 
Assuming you do have proper pressures based on your ambient, I'd verify that the blend door is moving correctly.
 
If it only goes to 150nmillitorr, t's full of water. Clean with acetone, on a hot day and it's dry.
 
I want to address all responders comments because some items I don't understand. First thing first. I found out on the return leg of a very hot 400-mile round trip that my AC worked. Very well in fact. This morning went to a restaurant 5 miles away. It worked there and back with an hour pause in between. Then just 2 hours later after driving another 25 miles home it did not work at all after multiple switch on-off cycles. I got home. Shut off the car for 15 minutes. Attached the gauges. Started and turned on AC. Didn't have to look at the gauges to know the AC was working again...

TheTerminator93: Trying to save a buck or hundred by trying to salvage what I have. Might end up saving nothing but I'm going to try. Since I know now the AC works sometimes, I'm encouraged. Also, if I have to vent the system, I'll definitely try to find/replace the orifice tube. If it's it the back under where the LP tube is, forget it!

dDUBb: I don't know what a micron gauge is, or what to do with the information it tells me. Pressures while running and cooling are LP:45 HP:250 ambient:90

racecougar: I thought about this on my very hot 25-mile drive today, although I'm not exactly sure how to do this. While it was working this last time when I put on the gauges, I did turn the mix control from full cold to full hot. It started blowing hot air then back to cold again when I turned it. It works, but that doesn't guarantee it's working all the time.

Grog06: I don't know what this means and I don't know what to clean with acetone...
 
GRWeldon said:
dDUBb: I don't know what a micron gauge is, or what to do with the information it tells me. Pressures while running and cooling are LP:45 HP:250 ambient:90
Click to expand...

Micron gauge is a vacuum gauge. One Micron is one millionth of a meter on the metric equivalency of a Mercury gauge when you pull a deep vacuum and need to be super accurate - it's a better indicator of any moisture or non condensible gases still in the system. Common practice for warranty is to pull below 500 microns ( equivalent to 29.9 inches of Mercury) and if it holds below and does not rise above 500m for one hour, it's considered "moisture free".

Your pressures are within the normal range on a pressure / temperature chart for R134A refrigerant. I build commercial and industrial AC systems for a living by the way. 😉
 
Yeah; it won't pump below 150 microns IF it's full of water. Which is a 150millitorr, due to the "partial pressure of water at that pressure level. PV=nRT, lol.
 
Thanks for that explanation guys. I appreciate it. My cheap-ass harbor freight vacuum pump won't pull below 28in anyway..hehehe!

Any guess as to what is causing the intermittent go/no-go?
 
I got a Leybold Vacuum pump in a repair needed state; t was full of rtv. We used it for degassing rtv, but some idiots... I took it apart, cleaned it out, put a rebuild kit n it, and have used it for carbon fiber baggng for years.
 
GRWeldon said:
My cheap-ass harbor freight vacuum pump won't pull below 28in anyway..hehehe!

Any guess as to what is causing the intermittent go/no-go?
Click to expand...

Your mechanical gauges may not be accurate at that range - I am sure your HB pump is capable of pulling below 28" no problem .. the biggest hindrance to pulling down that low is if there is moisture in the system.

As for intermittent problems - need to be able to duplicate the conditions and monitor pressures ( with mechanical gauges and electrical signal off the sensor ) - if your pressures are in range and the low pressure pipe off the evaporator is cold - it's working properly; the compressor turns on and off based on the pressure sensor.

If the system is working based on the conditions above but not blowing cold air out of the vents - it would more likely be a blend door or blower motor issue.
 
dDUBb said:
Your mechanical gauges may not be accurate at that range - I am sure your HB pump is capable of pulling below 28" no problem .. the biggest hindrance to pulling down that low is if there is moisture in the system.

As for intermittent problems - need to be able to duplicate the conditions and monitor pressures ( with mechanical gauges and electrical signal off the sensor ) - if your pressures are in range and the low pressure pipe off the evaporator is cold - it's working properly; the compressor turns on and off based on the pressure sensor.

If the system is working based on the conditions above but not blowing cold air out of the vents - it would more likely be a blend door or blower motor issue.
Click to expand...
I can visually tell when the compressor clutch is engaging. I would assume that this means the compressor scroll plate is rotating? Since I know it was rotating and blowing hot air after charging and I know that it will indeed cool intermittently, this does indeed point to a blend door or actuator motor. Can this condition be verified without yanking the dash?

To anybody who may know this, please tell me how it is to be done. I have plenty of parts cars but if I have to pull the dash, I might as well replace it with one I have that isn't cracked. Although I n all honesty, I don't cherish the thought of doing so...
 
Check if the low pressure pipe (larger one ) off the evaporator in the engine bay is cold to the touch.
 
I know which is the LP pipe,; you have to to hook up the hoses, but isn't the LP pipe the smaller one in the back near the passenger side firewall? Perhaps you are not talking small/large in regard to the hose coupler size? Obviously I won't be checking tonight but when I check in the morning I need to verify which pipe you mean... My gage set has blue for LP, red for HP. LP port is near the firewall, HP port is close to radiator. Please state again which I should check for coldness?
 
The fittings are large/small. So you don't hook them up backwards. I dk about the tubes, never paid attention.
 
Feel them all. 🤔 I'd want to touch the evaporator itself but that's not possible. It's the one that goes to the accumulator.
 
As far as the blend door goes you can see the actuator with the glove box removed. So if you switch between hot and cold you can see the shaft of the actuator move. When it is hard to see things like that I will put a small dot from a marker or paint. That way you can see your mark move, or not move depending on the situation at hand.

The liquid line is the small diameter line that is attached to the condenser and goes the the evaporator at the firewall. The liquid line should be cold at the firewall. The suction (the larger tube) line should be cool at the accumulator.
 
Grog6 said:
It's a trip that refrigeration works. :) Again, PV=NRT. :)
en.wikipedia.org

Ideal gas law - Wikipedia

en.wikipedia.org en.wikipedia.org
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Yeah dude. That is way over my head. I'm more of a practical implementer. I am a retired precision metalworker. Much of the time I understand how things work by the mechanism employed. I'm gifted in natural physics and mechanics. Theories and such aren't my bag, but I admire those who are immersed in it.
 
It isn't clear to me from your posts. When the A/C works, obviously the compressor clutch is engaging. When it is blowing hot is the A/C clutch engaging or not?
 
When it's not working, you need to check the state of the blend door, the high pressure switch, the low pressure switch, And see if the clutch is running. I can see the ac clutch on the voltage gage. It drops a bit when it comes on.
 
So here is the latest update. I believe the problem is with the blend door. When it wasn't working the last time, the tube outside was cold. I didn't remove the glovebox, but reached up under it and whatever duct or plastic piece I touched was cold. I have found that if I turn off the AC before I shut down the car, it seems the AC will blow cold air the next time I go somewhere. If I leave the AC on when I shut the car off, it won't work when I next start the car. This may be coincidence. I will continue observation. I've also observed that when the car is superheated inside after sitting in the sun, that is when I have issues with the AC not blowing cold air. I suppose I'll have to remove the glovebox at some point and see if I can determine anything. Right now, the car is on the lift with the front subframe dropped... more about that in another thread...
 
The low pressure vapor lines are larger diameter and the high pressure vapor or liquid lines are small diameter. In order to pull the same mass of refrigerant through the compressor's inlet as the high pressure gas coming out its outlet, a larger diameter pipe is required on the lower pressure side. Otherwise the pipe itself would become a restriction.
 
Blend door actuator is doable with some minor contortions. I've seen much worse on some newer cars. Mine got stuck on cold in the middle of February with my son and I about 6 hours from home several years ago. That was a cold ride home.
 
KevinVarnes said:
Blend door actuator is doable with some minor contortions. I've seen much worse on some newer cars. Mine got stuck on cold in the middle of February with my son and I about 6 hours from home several years ago. That was a cold ride home.
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My AC issue only happens when the interior is hot as hot can be...exactly when you need the AC most. In the mornings and evenings it works fine. I just made a 25-mile drive home in 97deg temps turning the AC on, having it feel like a blowtorch, turning the AC off. Did this about 5 times. When I get to within a half mile of my home, I try again and it works. The compressor is working and generating cold air, it's just not making it to the cabin. I'm thinking that maybe the blend door expands (don't know exactly how it's configured, wish I had a manual or diagram) and when it expands it gets stuck. Sounds like a good idea at first but I would think the box/duct would also expand with extreme heat.

Kevin, maybe you could give a few more details on how to pull the actuator? I pulled the glove box down and I see a box to the left of the fan. I'm assuming this is the blend door module. I see a couple of small hex head screws, on on the right, one on the left. The left one seems like it might hold a bracket/clamp that holds things in place? I figured I would experiment on one of my parts cars to see what I can remove, trying not to break anything.

Sure would like this puppy to blow cold air into the cabin in the extreme heat of mid-day...
 
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