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I’ve noticed my compressor cycling on and off more than I think it should. I was worried I might have over-filled it, so I rented the gauges set and decided to test it out. Here are my notes:External Air temp-81F
Idle-80 psi
Low-25 to 40 (pauses and sits at 35)
High-140 to 250 (drops as soon as it his 250, back to 140, fan comes on, pressure starts building again)
compressor cycles on and off until ~170psi, then stays on steady until ~250, then drops.
Air blows cold inside. I do not know enough to know what expected behavior is for this system. Car is a 1985 Corvette.
High side should be in the 180-200 range. Simple calculation 100 + ambient in F it will be close enough most of the time.
low side 25-35. Outlet temp 40* or less.
you want to test at 2k rpm. Low side may rise to maybe 40 at idle and be normal. 1 or 2 fans?
If car sitting 8 or more hours a/c pressure should be close to ambient in F. There are charts that have exact numbers.
I would let it sit overnight see what pressures are compared to ambient. Then adjust pressure to pressure/ temp correlation a little high better than a little too low.
How much refrigerant did you add? Was it a top off or a clean fill? It should be about 36oz assuming r134a. In general, the total amount of refrigerant matters more than the pressures unless there are other system concerns. Lastly, ambient is generally more in the engine compartment than true ambient. If it's cooling well, I'd be inclined to leave it alone.
The issue is that the high pressure hits 250 psi and immediately drops (compressor does NOT shut off) it’s like a release valve lets go. Then the pressure builds from ~100 again with compressor cycling on and off during the process. I let some out since high side was getting too much pressure but now the low side is getting too low.
i think I’m going to vacuum the system and start from scratch tomorrow.
Something is out of whack from the way you described it. I am anxious to hear what an experienced A/C technician would tell us. I have never let out excess pressure from a system before.
Is you system a R12 system or has it been converted to R134? That is a major issue I believe as your 1985 would have been an R12 Corvette originally. There are a lot of people who claim the original R12 systems blow cooler than converted 134 systems. For me the R134 works well enough and I have a lot of cans of it. HF has a 2 stage vacuum pump for less than $100 and it works great on cars and Carbon fiber vacuum molding.
Having a clean condenser is important. Every time they service your Home A/C unit they clean the coils for maximum efficiency allowing the system to get nice and cold. Why don't we clean our condenser on our cars? I found the coil cleaner at Home Depot designed for Home A/C systems. Now I clean my condensers every few years as it removes all the corrosion and that impeded the transfer of the heat. The trick is not to get it on your painted surfaces.
I hope that you and your family continue to stay healthy!
Mabey your high pressure cut off switch is bad. On my 90 OLDS the compressor will expel if the pressure gets over 430 psi. I replaced all my sensors(as well as
dryer/orifice tube/orings/compressor/) when I did my 85 Corvette A/C .
Last edited by xrav22; Jun 8, 2020 at 11:21 AM.
Reason: information
I was actually able to speak to an experienced A/C technician online, and he's told me it's ok to share his advice here on the forum (with the caveat that he's never actually worked on a 1985 Corvette before, but that it looks similar to other cars he has done from that era). I'm going to take his advice and work on my system, I've ordered the parts off rockauto, when they get here I'll take it apart and see what I can do.
Basically, he thinks the orifice tube (which handles the transition from high side to low side) is clogged. On a 30+ year old car that wouldn't surprise me. I'm including a photo with the tube's location, as well as the full text from the A/C tech below. I'm hoping this will also help the next guy who has trouble with their system to find a solution.
If this is my car this is what I'd do:
Parts (I looked up 1985 corvette, feel free to research the parts yourself to make sure they're correct):
A/C flush
A few ways to go about this but you should really have access to compressed air with a rubber nozzle. You can use a flush gun, you can use an aerosol can like I linked above, or you can just use non-chlorinated brake cleaner (the green can) which also works fine. You need to flush to remove any contaminants but most importantly old oil. Just adding POE oil without removing the old mineral oil reduces system capacity and has a pretty significant impact on efficiency. You're going to flush the lines, the condenser and the evaporator. Disconnect both sides of each component, blow it out with air (this is going to make a mess), fill whatever flush solvent you end up with, let it sit, and then blow it out again until dry. Do it in both directions.
Remove the a/c compressor. Turn it upside down and let the oil drain out through the ports. Turn the compressor clutch until you have pretty much most of it out. You wont be able to get all of it out but do the best you can. Do NOT flush the compressor with solvent, they hate that. Some compressors have a drain plug on the bottom as well. Put everything back together using new green o-rings from your o-ring kit after oiling (described below).
- POE oil with dye
You did good by using POE oil to begin with as PAG oil is chemically incompatible with R12 mineral oil and causes something called "black death". Even after draining/flushing I would still use POE oil since you can't be sure you got all of the mineral oil out. Dye is a good practice because it will help diagnose leaks in the future. To add oil, find out what the total oil capacity of the system is, and add that amount using a measuring glass, or I actually like to use a graduated syringe. Split it up into multiple components. Add a couple of oz to the compressor, squirt a few oz into the accumulator. If you cant add it directly to the inlet port of the compressor because of how it's mounted (I have no idea how it's mounted on a Corvette) then feel free to squirt it into the low-side hose going to the compressor before you reattach it to the accumulator. All you want to ensure is that the compressor has ample oil on it's first startup. The oil will find it's way once the system is running.
Charging:
Vacuum the system. You already know how to do that. You already know how to charge too so no changes there. I would only caution not to overcharge. I'm fairly certain 2.25lbs is too much unless the Corvette has an unusually large a/c system. Ignore the sight glass if it has one, those don't work for r134a. The general rule of thumb for charging 134a is you want your high side pressure to be 2.2-2.4 times ambient temp. So if it's 80F you're looking for a high-side pressure of like 180-200. Low side of 35 is just about right. Run the a/c in recirc/max-ac with the driver's side window open while you do this. Also add slowly. It takes some time for the system to react and for the pressures to stabilize. It's very easy to overcharge and there's a somewhat narrow window with R134a between undercharged and overcharged. Just go slowly.
FWIW I did a R134a conversion on a '90 Mazda a long time ago before I was an automotive tech much in the same way you did yours and I was never happy with it. Pressures were too high and unstable, cooling wasn't that great (lousy actually), etc.. Eventually I broke down and did everything above.
Afterwards I was getting 28F coming out of the vents, on a 95F day in the blaring sun stuck in bumper to bumper traffic for 45 minutes. That's the difference doing it right makes.
That write up you provided us is wonderful! I really appreciate the education!
Everything I know about A/C equipment was taught me by a general mechanic. It is sorely lacking at best but I have been successful in switching out several R12 systems without any issues.
What I don't understand is why the car companies would let the interiors get sooo hot. It causes premature failure of any electronics and the interior cloth or leather materials. Have you ever seen a Land Rover with the second roof mounted about an inch above the roof of the vehicle? While living in Ethiopia we had a Land Rover with the second roof and it made the car so much cooler when sitting out in the Hot African Sun. It is virtually parking the car in the shade all the time and makes a significant difference. It is not visually appealing so that is why other car manufacturers don't make them.
Just wanted to update and say I followed the procedure above and man my AC is COLD now and working great. Definitely ready for the summer. It’s good to know it’s all been cleaned out and everything is like new in the system.
one gotcha I ran into was that new AC cans no longer use the puncture method-they have Schrader valves on them now! Man that was frustrating to figure out but I got it. Just had to buy a new fill gauge. Heads up if you haven’t done the job in a while.
A/C Pressure help
