Mac78and2K

My AC on my 78 SA quit cooling. The clutch does not engage. I converted it to R134 3 years ago and since I drive it about 3-4 times a year I try to run it each couple of months to keep seals and things lubricated. I checked the power to the low pressure sensor and it was there. I then bypassed the low pressure sensor (paper clip)and the compressor kicked on. It made a clicking sound but did not appear to be in a bind or belt squealing. My assumption is it leaked down some this year. I would like to see if I can put a charge on it thinking it may just be low of Freon. If I get one of those charge units from Oreilly's that has a hose and gauge, can I use it to check and see if I have some Freon in the system and just need topping off? I was thinking to connect the end of the hose on the low side and see if it shows Freon pressure or does the compressor needs to be running to check for pressure and to add Freon?

thanks for any help

MelWff

The car has to be running to add the R134 to the low side and your paper clip method will engage the compressor.

REELAV8R

What it means if the compressor will not run is that the pressure is too low to overcome the low pressure switch. So it's somewhere between that threshold and empty.
To answer your question, yes you can just get the charge hose with the gauge to check and fill it to the proper level. Try to find one that also comes with recommended low side pressures to achieve. Low side pressure for full varies with temperature.
You do not need to jumper your low side pressure switch to fill it. Once you put a little freon in and the pressure gets high enough the clutch will kick on. To get it filled as mentioned you will need the engine idling the A/C on max with fan on high.
I also like to use a digital kitchen thermometer put into the outlet as a second indication of when it's full. You should achieve at least 50* at the duct when it's operating correctly, probably lower, depends on the outside air temp at the time and how much air your pulling through the condensor at the radiator. Set your idle up to 1000-1200 if you have a mechanical fan for more accurate readings.
It is possible to overfill the system using only low pressure indications. So shoot for the low side of the indicated pressures for full at your temperature. Don't invert the can. You don't want to introduce liquid freon to the pump just gas. It is ok to rock it from horizontal to upright briefly and repeatedly to help empty the can.
Here is a pressure chart posted in this forum post #6.
http://www.alfabb.com/bb/forums/alfe...den-507-a.html

Before you start it is desirable to see some pressure still in the system. If it indicates no pressure then you would be better off vacuuming it back down before charging. However if you discover it's empty and you don't have access to vacuum or don't want to pay to have it done there is an alternative method should you be interested.
Also can get a vacuum pump from harbor freight that uses a compressor that works good enough.
http://www.harborfreight.com/air-vac...ors-96677.html

DUB

GREAT advice from 'REELAV8R'.


PERSONALLY/PROFESSIONALLY.

Although adding more 134a Freon is 'do-able'. The system DOES NOT take the same amount as what it took with R-12 Freon. If I am not mistaken it is 80% of what the R-12 system would have taken.

And it would be best to take it to someone who can check it out for you. They can evacuate it and measure the amount of freon they pull out....so when they charge it up...they add what the system can take because you are using 134a freon. They can open up the system and check the orifice tube...which will allow them to see if you have crap in the screen which can direct them to let you know you have more of a problem than just low freon. Also...when the engine is running....they can watch the gauges to see what the high side is doing....which has a lot to do with air movement across the condenser.

The reason I am writing this is because I get Corvettes in from time to time where someone added freon and over charged the system greatly....thinking that more freon is BETTER....and sometimes it is NOT.

DUB

1Hotrodz

An easy way to identify which line is high pressure and which line is low pressure is;
Low side is always the larger line because it carries the vapor. High side is always the smaller line because it carries the liquid. In a closed system, refrigerant mass flow must be equal, so vapor line has to be larger than liquid line in order for the flow of refrigerant to be the same in both lines. This is true with any refrigerant system.

lakerider57

Since you suspect you have a leak and you have no tools, buy the 134a freon with the dye in it so when it leaks again, you will be able to see which fitting it is leaking from.

Richard

lars

You guys are responding to a thread from over 6 years ago... he probably has it figured out by now..

1Hotrodz

My mistake, don't recall how I stumbled on this thread.

vettebuyer6369

Response to post from 6 years ago.