[ubernoob51]

Thanks for the tips Kyler! I’ll be sure to check on the bolts after it runs for a bit. Not currently driving the GT on the road, it still needs bodywork and paint.

 

[ubernoob51]

Any ideas on what kind of pressure to expect on the high/low sides once it’s filled?

 

[ubernoob51]

Minor update: borrowed some AC tools from a friend, aaand..... the system holds vacuum!!! Gonna fill it tomorrow when it gets warmer!

 

[Knorm65]

No I never paid much attention to that as long as it was close to the center of the gauge. I recommend using a thermometer and stop filling whenever you get the recommended temperature differential for a car AC (which I also can’t remember lol).

 

[The Cub]

Pull a good vacuum down to 200 microns or so (leave on the vacuum pump overnight) to be sure there’s no moisture in the system. I know they say to get it lower but with the typical A/C refrigeration gauges that’s about as good as it gets. Low side between 25-40 PSIG high side between 225-250 PSIG for 134a depending on ambient temperature. You want your ambient temperature to be 70 to 90°F. Don’t charge with a jug of refrigerant that’s been sitting outdoors at this time of year it will be too cold and lower the pressures in the container. Store it in the house overnight too, you want it around 70° and well..
The recommended weight volume if available should get you right in there at those pressures. You’ll notice the pressures stabilize more as you reach the proper charge. Better to undercharge than overcharge. HTH

 

[ubernoob51]

I vacuumed the system again today, although once I put the lines on again it showed that it held vacuum overnight. Then I slowly added a total of 1.88 lbs. of r134a. while running the AC. The inside of the GT was ~85 degrees, ambient temperature outside was about 72-75. The temperature of the vents on full cold setting was from 49-55 degrees Fahrenheit. Idle dropped from 1200 to 1000 when the compressor kicked on, so about a 200 rpm drop. Engine temp stayed below halfway.

High side while running was much lower than your readings, just under 150 psi according to the gauges I used; as was the low side, at 25 psi. I don’t recall what the pressures were with the ac off. It seems the high pressure drops and the low goes up when the compressor turned off.

Is my scale off and I undercharged it? It seems to cool ok.

 

[Knorm65]

I vacuumed the system again today, although once I put the lines on again it showed that it held vacuum overnight. Then I slowly added a total of 1.88 lbs. of r134a. while running the AC. The inside of the GT was ~85 degrees, ambient temperature outside was about 72-75. The temperature of the vents on full cold setting was from 49-55 degrees Fahrenheit. Idle dropped from 1200 to 1000 when the compressor kicked on, so about a 200 rpm drop. Engine temp stayed below halfway.

High side while running was much lower than your readings, just under 150 psi according to the gauges I used; as was the low side, at 25 psi. I don’t recall what the pressures were with the ac off. It seems the high pressure drops and the low goes up when the compressor turned off.

Is my scale off and I undercharged it? It seems to cool ok.

Mine cools down to about 35-40° when it’s 80° outside.
I’d add more. If you haven’t already added more than 2lbs

 

[Knorm65]

Also you didn’t mention if you revved the car while you were filling and testing. The GT will not cool very well at idle. Very little air moves through that condensor. Rev the car while filling and test the outlet air temp after a short 2500rpm+ run

 

[ubernoob51]

I’ll try revving it tomorrow once it warms up. All my testing was at a 1000-1200 rpm idle.

 

[The Cub]

Your charge is a little low still. Try getting the low side pressure in the 30’s, your discharge (air) temperatures should come down. Your evap temperature is only 23° that’s a bit low your refrigerant is boiling off too early in the evaporator.

Knorm also has a good point, your discharge temperature will drop as you raise the rpms, even when they drop back down look for everything to remain pretty stable you want pressures around 35 / 150 ballpark

 

[ubernoob51]

Pressures with AC on, at idle. Around 33/140 psi. At 2500rpm the suction side goes to 20 psi and discharge side goes to 155 or so. This is after adding .160 lbs of r134a.

 

[The Cub]

Pressures with AC on, at idle. Around 33/140 psi. At 2500rpm the suction side goes to 20 psi and discharge side goes to 155 or so. This is after adding .160 lbs of r134a. View attachment 424838

I would still add more, unless you have another cooling fan your head pressure might be a little higher than usual due to the GT’s less than spectacular radiator fan. As I said when you get close to the optimum charge you’ll stop seeing the pressures vary so much. My guess with that low low side pressure is the compressor is short cycling too?
Here’s a good chart I found, you still have room to add more. See if you can get the low side to stabilize more and stay above 30-35 PSIG.

Be sure that you purge your hoses free of all air before you introduce refrigerant into the system. I usually charge with a liquid and blast away until I see liquid at the manifold, you can also just release refrigerant pressures from both, the system and the container until both hoses are free of atmospheric contaminants IE moisture.

When you get there document the total weight volume of your refrigerant charge as best you can so you can have a reference for future servicing to the system if you have to take it down again for any reason. Usually I count about 3 ounces of waiste from purging a standard set of hoses like you have. You’ve charged twice so subtract 6 ounces from your total charge at this point. That’s the amount of refrigerant that gets trapped in the hoses and never makes it into the system. Don’t be afraid to add more you’re almost there.

Diagrams: System Pressure Chart

R-134a pressure chart and other instructions on car A/C repairs.

rechargeac.com

 

[ubernoob51]

Thanks for the replies Mr. Cub. I forgot to factor in the three-ounce loss, I think 3 ozs as a decimal is 0.187, which means I actually lost more than I added the last time. I will give you an update tomorrow once it gets warmer. At night where I am the temps drop to the mid-40s, too low for AC work I think.

 

[opelspyder]

Run a digital tachometer too, if your idle is set at 1200, its way too high. You should be in the 850-900 range. But if you are going by the original tachometer, they are not that accurate. Also put a fan in front of the car to get some air flow.

 

[ubernoob51]

So I topped the system up a bit more, at about 70 degrees ambient temp the high pressure was 160, low was at 25. I know the low side is a bit low, but it cools well and I’m happy with it. So that’s pretty much it!

 

[The Cub]

So I topped the system up a bit more, at about 70 degrees ambient temp the high pressure was 160, low was at 25. I know the low side is a bit low, but it cools well and I’m happy with it. So that’s pretty much it!

Sounds okay and as long as you’re happy with it.

Note: 25 psig represents a 30 degree coil, your temperature at the vents should be in the 40’s. Preferably the low 40’s. You want no more than 10-15 degrees higher than the coil temperature for your air discharge temperature. Just Google pressure/temperature chart for 134a and you can find your evaporators temperature going by the pressure reading. If the refrigerant boils off too early you’ll have say a 15-25 degrees variance between the two indicating an undercharge, if you have a zero to 5 degrees difference you’ll likely be overcharged. You want the refrigerant to boil off right in the middle of the evaporator coil for best efficiency.
Of course there’s other variables that can cause similar conditions but yours is a brand new system and you are just adjusting the initial refrigerant charge not expecting failure with any of the components just installed.

What’s your discharge temperature at the vent?

 

[ubernoob51]

I don’t have a proper vent thermometer, but according to the cheapy Harbor Freight temp gun, I’m in the low 40s. That’s on the lowest temp setting on the system. I ordered a correct thermometer and will report back once it’s here.

Also according to the temp gun, the temperature of the air near the evaporator blower inlet was 75-80 degrees.

 

[ubernoob51]

If the evaporator coil temperature is 32 degrees or less, don’t you run the risk of condensation freezing up in the evaporator? I read about that happening in the GM AC service manual.

 

[The Cub]

If the evaporator coil temperature is 32 degrees or less, don’t you run the risk of condensation freezing up in the evaporator? I read about that happening in the GM AC service manual.

That’s the rule of thumb. That said, automotive A/C units typically have a protective pressure or externally mounted cut out switch on the low side that keeps this from happening hence the short cycling of the compressor. That’s why you almost never see an iced up coil when you gat a refrigerant leak, the efficiency just keeps getting worse until you get no cooling. I keep mentioning this, the compressor will keep clutching in & out if you don’t have enough pressure on the low side with this function.

Aftermarket A/C units don’t always have this pressure or temperature switch and if you’re not experiencing that symptom, the system you have installed may not have this function.

The switch I mentioned also protects the compressor, you want to allow enough refrigerant back to the compressor that’s what helps cool it down.
If your low side pressure is above 30 you’re out of the woods.

I just realize my first post here I did say 25 psig and that’s on the low side. I’d push it up in to the 30’s at idle just to be safe. I’ve seen evaporator pressures below 32 degrees that never freeze up, I cannot explain why, personally its not a number I consider ideal. Again while charging keep an eye out on the temperature coming out at the vents. It’s not uncommon to see it in the upper 30’s. It should always be at least 5 degrees higher than the evaporator temperature/pressure. You will not be overcharged if you keep an eye on that. Too much refrigerant will send too much liquid back to the compressor and can damage it that way too. You didn’t mention what you have temperature wise at the at your vents yet.

Here’s the steps I use, if you or anyone still has sub par cooling on a good working A/C system, this covers everything, if you skip any of them you can’t really be sure of what you’ve got.
1) Once the system is assembled and ready to go, pressure test system with about 300 psig of dry nitrogen and let it stand for about an hour, look for pressure drops, you shouldn’t have any. If so use soap bubbles to find the leak.
2) After it passes step #1 put the system in a deep vacuum overnight (this might be overkill but it’s foolproof).
3) I use a micron gauge to verify all of the moisture has been removed 200 or close on a running vacuum pump is good. This step isn’t necessary if you are successful with step 1 but I have the tool so I check it.
3) Best to use the larger container or jug of refrigerant with a scale. Valve off the refrigerant circuit in a deep vacuum at manifold gauge. Loosen the yellow charging hose at the manifold, run a very slow vapor charge (deminimus release) purging the yellow hose up to the manifold to be sure you’re introducing only refrigerant into the system. Then be sure all hoes connections are tight.
4) Carefully weigh in the proper charge, best if you can find the manufacturer’s recommended weight volume for starters. Top off from there to get the correct operating pressures at idle speed. Count or subtract about 3 ounces of refrigerant for loss in the ho The chart I posted earlier is a good ballpark of numbers that you should get close to.
I don’t use the little cans from the local automotive supply stores so I can’t comment on the use of them. Just be sure that you limit introducing atmospheric (air) into the refrigerant system. Your pressures will be erratic if you ignore this.

Remember when you start driving your head pressure is going to come down from the improved cooling of the condenser so keep the focus mostly on the low side when charging. Lower head pressure, lower evaporator pressure. The TXV will control the proper refrigerant feed to the evaporator on a proper charge so this really isn’t an area of concern regardless of what’s happening during your driving conditions. I’m just pointing this out to help you get a better understanding of generally what happens.
If you read, look at the chart posted earlier you can see how the outdoor temperatures affect the pressures, it sounds like you have a good grasp on that.

Once you’re satisfied (and that’s all that counts here, this is your A/C thread) please let us know what you’re getting temperature wise at the vent?

 

[ubernoob51]

My vent thermometer has shipped and comes in tomorrow! I will update you as soon as I check.