A/C issues - icing up

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losendos
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Joined: Sun Jun 26, 2016 2:01 pm

A/C issues - icing up

Post by losendos » Sat Jul 13, 2019 4:00 pm

When I bought my car, the factory a/c wasn't working, so I had the system checked. There was no refrigerant, so they recharged it and ran the car for a long time to make sure there were no leaks before giving it back to me. Since then, the system will blow nice and cold sometimes, but as I drive the car, it generally blows less and less cool. The last time I used the a/c, after blowing cold initially, it eventually was just blowing ambient temperature air. I stopped and opened the hood, and found that the a/c line that contains the sight glass was completely frosted over. Once the frost melted, the system was working better again ... for a while.

Any ideas of what would cause this?
-Patrick
1966 Town Hardtop
VTCI #12691
Past Bird: 1986 Turbo Coupe

Cliff Rankin
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Re: A/C issues - icing up

Post by Cliff Rankin » Sat Jul 13, 2019 4:48 pm

Basic a/c would say it is low on Freon or the orfice /expansion valve is bad. But I do not have enough experience with the old compressors that have the adjustment on top of that is what you have. Think of it
Like a balloon and pinching it to get that squick noise.

Cliff Rankin
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Re: A/C issues - icing up

Post by Cliff Rankin » Sat Jul 13, 2019 4:51 pm

Cliff Rankin wrote:
Sat Jul 13, 2019 4:48 pm
Basic a/c would say it is low on Freon or the orfice /expansion valve is bad. But I do not have enough experience with the old compressors that have the adjustment on top , if that is what you have. Think of it
Like a balloon and pinching it to get that squick noise.

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losendos
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Re: A/C issues - icing up

Post by losendos » Sat Jul 13, 2019 5:15 pm

Sorry, but I don't understand the balloon analogy. This is the first time I've tried to troubleshoot an a/c system and I'm learning as I go.
-Patrick
1966 Town Hardtop
VTCI #12691
Past Bird: 1986 Turbo Coupe

stubbie
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Re: A/C issues - icing up

Post by stubbie » Sat Jul 13, 2019 9:47 pm

Just did a bit of reading. According to some, low refrigerant or a leak that will introduce moisture into your system causing the moisture to freeze. Moisture can be sucked into your system by vacuum if you have a leak or the system left open for any length of time. Freezing can also be caused by your evaporator being blocked by debris or the lack of airflow over the evaporator.

Peddller
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Location: Bel Air, MD

Re: A/C issues - icing up

Post by Peddller » Sat Jul 13, 2019 10:04 pm

Can you see ice if you look through the outlet to the evaporator (I think) while it is running and starts blowing tepid air. Also the compressor should be cycling of and on as the car get cooler.
Bill Clark
Bel Air, Maryland
1957 F code
2005 50th anniversary
1963 Avanti R2
1956 GMC Suburban Carrier pickup
1966 Corvair Corsa Turbo
Prior Birds
1963 M code SR
1964 Convertable
1965 convertible

jtschug
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Re: A/C issues - icing up

Post by jtschug » Mon Jul 15, 2019 11:27 am

The way this system is designed, I don't think low refrigerant charge would cause the evaporator to freeze. The basic problem is the compressor is staying on too long. The problem could be a broken probe tube off the clutch cycling switch. These are very delicate and easy to break. Another possibility is hacked up wiring that leaves the compressor on all the time.

The first question is: does the compressor clutch cycle on and off?

marknat
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Re: A/C issues - icing up

Post by marknat » Mon Jul 15, 2019 1:43 pm

I agree with jtschug, sounds like the compressor may not be cycling. Just had a similar problem and found the temperature control relay under the dash near the steering column was not working. This is the switch that is attached to the probe in your evaporator. The probe tells the switch the evaporator is at a certain temp and to turn the compressor on or off.
On my 66 there is a vacuum line that runs to the temp control on the center console.

Cliff Rankin
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Re: A/C issues - icing up

Post by Cliff Rankin » Mon Jul 15, 2019 6:18 pm

Ok , if I may ask. Let’s get to basics. Running a 12 system
Converted to 134 ? Are you running the stock compressor?
How much does it hold by 12 specs. and how much is in there by weight / scale. What are
The high and low side readings on the gages with the system running and what is the temperature outside when you do this ? Do you have a low side pressure switch on the car ? Thanks Cliff

jtschug
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Re: A/C issues - icing up

Post by jtschug » Tue Jul 16, 2019 11:22 am

These cars do not have a low side switch unless someone has modified them.

Temperature control of the evap core is done by modifying the clutch cycling set point with a vacuum diaphram. The pressure in the diaphram is altered in A/C modes with the temperature lever. A probe tube from the collapsible dome goes into the evaporator and should cycle off the compressor when the temperature of the evaporator approaches freezing, but clearly something isn't working quite right.

Cliff Rankin
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Re: A/C issues - icing up

Post by Cliff Rankin » Wed Jul 17, 2019 7:15 pm

Ok fair enough. Still , are you running 134 ? 12 ? 1234 ? In a stock system ? What are your gage readings ? At what temp outside by the car ?

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losendos
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Re: A/C issues - icing up

Post by losendos » Sat Jul 20, 2019 9:41 am

It is a stock system, stock compressor, with R12. I don't know the gauge readings - I had a shop recharge the system. It was probably around 75 degrees outside the day I noticed this. In the past, the compressor would cylcle on and off, but I didn't notice if it was doing this or not that day. I haven't driven the car since, so I don't have any more data right now.

Another odd thing - when I put the control to REC, the system blows warmer air than in the FRESH setting. Like the vents aren't operating properly. Don't know if this is related or a separate issue.
-Patrick
1966 Town Hardtop
VTCI #12691
Past Bird: 1986 Turbo Coupe

Cliff Rankin
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Re: A/C issues - icing up

Post by Cliff Rankin » Sat Jul 20, 2019 3:19 pm

Pat , sorry can’t be much help without some basics. Off
The cuff , if it is low on Freon it can freeze up if there is a restriction it can freeze up and usually at the restriction.
I think it would be unlikely to freeze because of the setting on the switch.

stubbie
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Re: A/C issues - icing up

Post by stubbie » Sat Jul 20, 2019 10:00 pm

If in Rec it is recycling the air already in the cab so your air will be at that same temp. If the fresh air vet is open you are bringing in the air from outside so if you are moving then the air will be cooler.

jtschug
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Re: A/C issues - icing up

Post by jtschug » Sun Jul 21, 2019 4:10 pm

Cliff Rankin wrote:
Sat Jul 20, 2019 3:19 pm
The cuff , if it is low on Freon it can freeze up if there is a restriction it can freeze up and usually at the restriction.
All refrigeration systems have a 'restriction' to create a pressure drop which causes the liquid to boil into vapor. The heat required to boil the liquid causes the cooling effect. The combination of the restriction and the suction of the compressor causes the pressure drop. When liquid refrigerant enters this low pressure area, it starts to boil, as it boils, it absorbs the "heat of vaporization" from the surroundings, and this cools the evaporator.

The metal probe that extends from the switch to the evaporator is intended to switch the compressor clutch off when the temperature of the evaporator approaches freezing. The metal tube is filled with a gas, as the gas cools, the pressure inside the tube is reduced, and when the pressure is low enough, a dome inside the switch collapses and turns off the power to the compressor clutch.

If the compressor keeps running, when the temperature of the evaporator goes below freezing, moisture in the air will quickly start to freeze on to the fins of the evaporator and it will very quickly become a solid block of ice. The ice stops the air from flowing through the evaporator and suddenly, you are no longer getting cold air. If you have ever seen a old freezer that is not "frost free", imagine that kind of ice forming in the matter of a few minutes.

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