1979 Mark V AC Questions & Updates….

Hi Bill,
Alasdair in Australia here. I have a 1979 Mark V and I am preparing to do an upgrade of the air conditioner. We use R134a refrigerant.
I have looked at your parts listing and am preparing to order a suction throttling valve (calibrated for R134a), an expansion valve and a drier from you.
I have already got a new compressor here in Australia compatible with R134a as the same type were used on local GM cars “back in the day”.
I have studied the picture of the suction throttling valve on you parts listing and I have even found a copy of the instruction page on the Internet that goes with it. I see that the replacement suction throttling valve that you sell has a port for an external equalised expansion valve equaliser line along with a port for the liquid bleed line. My car has an internal equalised bypass orifice expansion valve without the equaliser line. See page 36-32-3 & 4 of Workshop Manual Volume 3, Figures 1,2 & 3. My car system is as is presented in figure 3.
My question: If I fit a new R134a suction throttling valve (that I intend to buy from you guys), do I have to fit a new external equalised expansion valve or do I stick with a new (same as existing) internal equalised bypass orifice expansion valve and blank off the port on the new suction throttling valve ? I am curious about this as I want to make sure I order the correct parts.
I have all manuals, diagrams (electrical and vacuum) and books on auto air conditioning.
I am going to try and get the rubber hoses redone to be compatible with R134a.
Do you have any other suggestions on an air conditioner rebuild ? The air conditioner has been in constant use for quite sometime and has not been sitting idle. It blows cold only at low engine revs. Speed up and it warms up. All switching functions of the ATC work fine (electrical and vacuum).
Thanks for any help you can give.
Regards,
Alasdair
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Hi Alasdair –
For now this response is to your last paragraph. Excellent that you have all of the necessary manuals. Your last two sentences strongly indicate to me that your a/c performance suffers from a low charge of refrigerant. This has happened to myself years ago and was very interesting. This system and others can operate as normal with loss of refrigerant to a point and then with only a loss of a few ounces more the cooling collapses. At idle however cooling appears to be fine until the engine is accelerated or the car driven and the condenser is cooled to a point where the cooling collapses because of slight further shrinkage of the remaining refrigerant. This can be tested at idle with the system operating and a thermometer in the outlet vent. While it is cooling the interior at this time spray cool water on the condenser and watch the air outlet vent temperature for a rise. If you have a/c gauges attached, the a/c pressures should also change somewhat. If this is so, a loss of refrigerant may have occurred over time and adding 1/2 to 1lb of refrigerant should restore the cooling to operate again at higher rpms.
The above is only to be sure that you have all of the possibilities to assist in your decision. If you still wish to change your system as you describe, please let us know and we will send you a list of what we have used in the past to complete a change over.
Sincerely,
Bill
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Hi Bill,
Thank you for the advice. I will do what you suggested and let you know the outcome.
Regards,
Alasdair
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Hi Bill,
Thanks for your advice and for taking an interest.
I just ran my car and did as you suggested with a thermometer in one of the centre cabin vents.
I had the system running on A/C thus recirculating the cabin air.
At idle the cabin vent was sitting at 50 degrees F. It is summer here and is very hot.
There was ice on the suction throttling valve(STV), the pipe on the compressor side of the STV and on the steel pipe that goes into the compressor.
I ran some water over the condenser and the vent temp increased to around 56 to 58 degrees F. The ice on the pipes remained.
I increased the revs for several minutes and no real change in vent temp or ice covering on the pipes occurred.
I then changed the A/C setting to HiLo (outside air) and the ice disappeared almost immediately. I assume this is because the cooling load across the evaporator changed with the introduction of warm air across the evaporator. The cabin vent temp increased immediately as well to over 60 degrees. Again, it is summer here and is over 100 degrees F most days. It was about 95 when I did these tests. My car is a weekend cruiser and is not used everyday. Too nice for daily work in the heat.
When you suggested a refrigerant leak along with your diagnosis methods, I also had a good look around for system leaks. I found some minor oil moisture around where the compressor line joins the condenser (very minor) and a minor oil leak at one of the drier connections to the condenser. If oil can leak, I assume so too can refrigerant.
What do you think ?
I will send you an email later regarding my interest in changing the STV to a new valve calibrated to R134a.
Regards,
Alasdair
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Hi Alasdair –
Your figures indicate to me a low refrigerant charge as discussed. The actual refrigerant pressures would tell us more. You could if possible where you are located and have access to tools and refrigerant, safely remove the present 134a, save it and add it and enough by weight of fresh 134a for a full charge. The charge should be 80% of an r12 charge ( Possibly ) 3-3 1/2 lbs. of 134a. Then do your tests again and watch for an improvement. If you have a positive improvement, you now need to find the leak and correct this issue as per most refrigeration laws. Proper knowledge and equipment for testing would be necessary. Anywhere refrigerant is routed are suspect. The evaporators on these models are very popular leak areas as well. We drill a special hole in the plastic evaporator casing ( but not very near the evaporator itself ) to gain access with our test equipment if a leak is suspected in that area. Extreme caution is required to avoid puncturing the evaporator. A body plug will seal the drilled hole when finished. The radiator cooling fan must be the correct one for the vehicle and its clutch ( if it has a clutch fan ) must be operative.
Sincerely,
Bill
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Thanks Bill,
I know a very experienced auto electrician who has state of the art equipment for refrigerant handling at his business. I will do as you suggest and get back to you. May be a couple of weeks by the time I can book it in.
Whereabouts do you drill the hole in the plastic evaporator casing ? Can I remove the blower resistors and use that hole to probe for leaks ?
Radiator fan is correct.
Thanks again.
Regards,
Alasdair
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Alasdair –
Yes..,, you can use the blower speed resistor duct insert for an access. If there is one located in the duct under the hood between the blower and the evaporator that is a good safe place to access. I do not have a manual to verify access locations of components here where I am today. A leak detector tube can be inserted and the evaporator can be carefully tested electronically. A leaking evaporator will usually display visual oil leaks but may be difficult to see in some locations. Good luck with the testing. Did adding refrigerant (as a test only) improve the cooling?
Sincerely,
Bill
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Hi Bill,
Thanks for your answers and information.
It will be a few days before I can get to book my car in at the auto electricians workshop and do the refrigerant top up along with the leak test. I will let you know the outcome as soon as I get it done.
Regards,
Alasdair