I am trying to adjust the pressure on the low side of a R134 converted system..
What should the proper LOW side pressure be with the system running?

I am trying to maximize cooling to get a lower vent temperature.

Thanks in advance

Barry

 

There is not an easy answer to this question.

Whats your average cruise RPM?

You should adjust charge with the doors closed, AC on MAX, fan on high and RPM at or around 1500 to 1800 with a very good (high velocity) fan blowing into the front of the condenser. You really need to have a thermometer to know what the cabin temp and vent temps are. You also need to know the temp of the large line leaving the evaporator going to the compressor. When doing this you need (very important) to know the what the pressure is on the high side also.

You can get close with just knowing the pressures if the system is in good working order (every part is working as its suppose to).

With the car setup as stated above look for around 25 to 30 psi on the suction side (low) and around 250 to 275 on the discharge side (high).

As the cabin tmeps come down the suction pressure will drop also. The low pressure cut off is normally set around 18 to 20 psi. Shot for the suction line at the compressor to be sweating when your close but don't put anymore R134 in once to that point or you'll over charge the system.

You really should shot for the recommended amount as indicated on the tag under the hood.

There is allot to look for but you can get it close, if you see something out of the ordinary post here so someone can help.

Neal

 

Thanks for your response.

I should have said the car is an early 77 with the R134 conversion completed.
I have run the system and have gotten vent temps as low as 45 degrees.

I just wanted to know a ballpark low side pressure to make sure the system was not overcharged..

I have run water over the condenser while engine was at 1500 rpm and the line from the compressor formed an ice cover..the temperature of the line was 30 degrees.

I also found the less pressure at the low side line the colder the system ran..

 

Barry
It sounds like you are doing very good. I have seen less than 55 mentioned for R134 C3 (not 77s).

W/ R12 it will go into the 30s. I am still using R12. For R134, I would use a smaller orifice tube like the Ford Red .062 in place of the GM White .072.

 

You are correct with the lower pressure = lower temps. If you look on your charging gauge set on the low pressure gauge there is a pressure/ temp chart. The idea is to get the lowest pressure at cruise but stay above the low pressure cut off point. The coil temps can easily get below 32* and freeze up if you let the pressure get to low.

You will be able to run a lower pressure than whats indicated on the gauge set due to the air being above freezing in the car cabin. This air keeps the coil above 32*, this is why the air at the vents will be colder when the fan is on low.

Gotta go, working on my daughters AC in her Saturn

Neal

 

om my gauges from snapon, both R12 and R134 i almost always saw near 30 psi with the car running. it can go up with temp and if your cooling fan is working well but thats the area 30-40 psi

 

Thanks to all who helped...

I'll leave the suction side pressure at 30 pounds and see how it cools..

If necessary, I'll make the pressure lower to see how low I can get the

vent temps without freezing up the system.

By the way, my system does not cycle on and off........the compressor

runs all the time....it does have a pressure switch which functions when

the R134 drops below a certain point.

 

I posted earlier today to help another form member and he was also working on an early 77. He indicated that his car had a VIR, if yours has a VIR valve than it may not cycle.

I don't remember if this system had a low pressure switch. I'll have to look that up and see. It should have a high pressure switch though.

Neal

 

I have a late 77 which has a low pressure switch & did not cycle new.