Low Side Pressure Shoots Up

[shamaal]

I've changed out the condensor, compressor, orifice and refrigerant lines and have loaded up with 30 oz of R134a. After 26 oz the additional 4 oz had no effect so I stopped at 30 oz. Vent temperature is 60 deg on an 87 deg day. I left the safety clip off the high side condensor connection (it was soaking in cleaner) and the connection blew apart while I was loading freon - Goggles - Goggles - Goggles. After immediate reconnection I ran the vacuum for 15 minutes. Prior to the accident it was on for 2 hours. Perhaps I should let it run longer. 60 deg seems a bit high.

At any rate operating pressures are 56 X 150. After I turn the engine off there is a loud hissing sound for 5 seconds near the evaporator and the low side pressure shoots up to 95 psi. I don't appear to be losing freon, perhaps some internal stabilization in the system? Once the car is started it returns to its steady state of 56 psi.

Any insight would be appreciated

 

[BrooklynBay]

If the orifice valve (expansion valve) is not clogged, then you might be getting this reading because your compressor clutch is cycling.

 

[shamaal]

It's a brand new orifice. I'm thinking that perhaps the R12 orifice and R134a orice may be different, this might account for the change in efficiency.
As to the hissing and rise in pressure I'm at a loss. A blockage sounds reasonable, something that's in one position when the compressor's running and loosens up when it's not acting like a ball valve. It would have to be in the evaporator though, it's the only part that hasn't been changed.
Thanks for the reply.

 

[BrooklynBay]

The orifice valve is the same for both systems. It is basically an inline filter. I've seen A/C systems vary in pressure when the compressor cycled. Did you find any debris caught in your old orifice valve when you last changed it?

 

[BrooklynBay]

Here is a picture of an orifice tube courtesy of V8BoatBuilder:

 

[shamaal]

Attached is a picture of my orifice that caused me to replace the condensor, compressor and refrigerant lines.
Mines the bottom one. I backflushed the evaporator with alcohol four times, there was a bunch of green/yellow stuff (dye?) that came out.

As I undertsand it the orifice is not only a filter, but it is the mechanical point where the liquid is physically changed to a vapor and then sucking up the heat as it expands in the evaporator. I've got a writeup somewhere that I'll post sometime.
If I understand you correctly you are saying the orifice size for the R12 and R134a is the same.
Thank you for the reply and interest.

 

[BrooklynBay]

I've converted my system from R12 to R134A, and it was the same part. There was no other reference in the parts book. I remember seeing your picture of the orifice tubes in another thread. It looks like you have a lot of debris in your system. I'm just thinking that you might have another clogged orifice tube that is causing your problems.

 

[BrooklynBay]

Could you upload that picture of the orifice tube comparison to the photo gallery?

 

[shamaal]

Could you upload that picture of the orifice tube comparison to the photo gallery?

Done, it's in the general photo gallery.
I'll wait till it gets really hot, then check thhe orifice and evacuate for a couple hours. When you converted, any chance you recall how much freon you put in?

 

[BrooklynBay]

I didn't have the tank on a scale, so it's hard to say an exact number of pounds. I have the big tank that looks like a barbecue propane tank.

 

[zhanx]

shamaal (no clue on your a/c knowledge level but it never hurts for the basics)- best guess on the hiss pressures equalizing. (need to run a leak test in said area try soapy water in a spray bottle to check for the leaks, note: will not alway get leaks.)

Next you need to grab a pressure temp chart off the net and base your pressures off the temp. Also while you had it part i would have gone the extra mile (and it is that) to clean the evap coil. Yours is a 91 so you know its got to be aleast a little dirty and that was reflect in the exchange of heat also.

here is the low down on r134 - its not as good as r12 - i.e doesnt absorb heat as well as r12. which will make your vent temps higher.

 

[BrooklynBay]

I've heard of crooked mechanics using propane instead of freon. In an accident, the vehicle caught fire when the condenser ruptured.

 

[zhanx]

propane works decent. but your right it "could happen" liquid propane and fire = bad. anmonia is another old school freon type used still on the other continents.

 

[shamaal]

Zhanx, thanks for the suggestions. I held a vacuum for 15 minutes so I think I got the leaks covered. The soapy water helped me locate the leak from the Pressure Control Valve on the refrigerant lines and a leak from a bad R-12 to R134a adapter on the low side. The schrader depression doo-dad was in to low and had to be unscrewed a couple turns to keep the schrader valve from being permanently depressed.
When it gets unbearably hot I'll clean the evaporator, vacuum and repress the freon. I'm familiar with the charts I just haven't seen ones that provide anything other than a general range. When I messed about in air conditioning on an USAF contract twenty years ago there was a chart that provided the exact differential for the type of freon and ambient conditions measured two inches in front of the condensor. I'll do a google chart. I'm basing my expectaions on others claims of 40 degee air after conversion. Incidentally ammonia is what we use on the International Space Station as a coolant.

Ijust had another thought, my heater has suddenly stopped working. Perhaps the temperature mixer/damper is not working correctly and I'm getting mixed air? I have a busted vacuum line that disappears into the firewall that I thought had something to do with max a/c. Now I know next weeks project.
Everyone thanks for the assistance. I have an accumulator, muffler and a Dremel saw. I'll post inside pics later.

 

[zhanx]

Zhanx, thanks for the suggestions. I held a vacuum for 15 minutes so I think I got the leaks covered. The soapy water helped me locate the leak from the Pressure Control Valve on the refrigerant lines and a leak from a bad R-12 to R134a adapter on the low side. The schrader depression doo-dad was in to low and had to be unscrewed a couple turns to keep the schrader valve from being permanently depressed.
When it gets unbearably hot I'll clean the evaporator, vacuum and repress the freon. I'm familiar with the charts I just haven't seen ones that provide anything other than a general range. When I messed about in air conditioning on an USAF contract twenty years ago there was a chart that provided the exact differential for the type of freon and ambient conditions measured two inches in front of the condensor. I'll do a google chart. I'm basing my expectaions on others claims of 40 degee air after conversion. Incidentally ammonia is what we use on the International Space Station as a coolant.
.

the charts are old there and sometimes old school helps.. ammonia is deadly but works great.

 

[BrooklynBay]

There was a building in my old neighborhood that had one of the early versions of central air conditioning. It used ammonia, and had a gas line with a pilot light to heat it up. That is how it's able to turn into a gaseous state. Somebody once showed me a diagram in a book of a refrigerator that worked on ammonia.

 

[Glacier991]

First off, on engine shutdown, the hissing and the low side rise is perfectly normal. It is the system high and low side equalizing. (See the Recharging thread in the stickies.... why I wait to disconnect the high side fitting).

Next off 56PSI Is high for the low side during operation, making me wonder about charge state (overcharge). You never mentioned but does your system ever cycle? Do you have a gauge set? The best advice can be given when I know 4 things.... High side pressure, low side pressure, ambient (outside) air temperature and vent temps.... (I am guessing your vent temps are about 60 degrees).

[To correct a misinterpretation - the Orifice Valve is more than an inline filter. It provides controlled metering of refrigerant for expansion and proper cooling. Yes it provides filtering, but that is NOT what it is - a filter. The filtering is secondary to its main function.]

 

[shamaal]

Low Side: 56
High Side: 150
Ambient 87 deg F
Vent: 60 deg F

I agree with your assessment about overcharge, I have a large amount of condensate at the evaporator output and some on the accumulator. I think this happens when there's too much Freon for the evaporator to evaporate. The condensate was dripping on the manifold and causing the hissing sound. Also, as you divined, the system does not cycle.

I mentioned before that I no longer have heat. The heater hoses are hot and as I move the thermostat the cool goes away but no heat. I don't think I can trust the vent temperatures are a real indicator until I fix the mixer. I also have a broken vaccum line that disappears into the firewall that I think is associated somehow with the max A/C.

I have some research to do to understand how the mixing works and what max a/c is. I'll start another thread with any questions.

The Haynes automotive heating and air conditioning book I have says that the presuures should be 22-50 X 130-230, so I'm clearly over. I didn't stop to stabilize when filling. Brooklyn Bay mentioned that Freon tables still exist, I still have more research to do yet.

Thanks for the interest.

 

[Glacier991]

I stopped using tables a long time ago. Too many other variables exist.... airflow over the condensor for one.... When you convert start by adding about 60-70% of the R-12 charge. So if the R-12 charge was 1.75 lbs (as in the case of the 1st Gens...) that is 28 oz... 60% of that = 17 oz; 70% = 20 oz. Fine tune it from there by vent temps... put a fan in front of the condensor as you do.

By the way....Your clutch should cycle... up to about 100 degrees ambient anyway

At 56 psi your vent temps SHOULD be around 60 degrees, so I doubt your are mixing heat with cold, you may be stuck on cold.

With an overcharge I usually expect high low pressures and high high pressures.... we now enter the realm of a possibly failing compressor with your pressures. (Or a stuck open Orifice valve)

Recover the charge, replace the orifice tube and readjust your charge and see what you get. Keep ypur fingers crossed. I am fearing your compressor may be dying.

 

[shamaal]

Thanks for the rapid reply. My earlier numbers for the high side were erroneous - bad setup, I forgot to open the port valve.

I also had a slow leak on the low side port adapter, the R134a schrader valve extension was too low and slightly depressed the schrader valve. It's fixed.

New numbers are:
Low side: 43
High side 225
Ambient 78
Vent: 62

I also have condensation on the accumulator with some at the compressor suction. I'll follow your recommendation to recover and reload this weekend with 20 oz Thanks for the advice!