Installed new hoses, drier, evaporator core in my 71 car. Flushed condensor very good. Flushed compressor with mineral oil. Took it to local mechanic shop. The pulled a vacuum and let it set for 2 to 3 hours, no leaks. Charged it with 3lbs R-12.

On the drive home it blew cold for a couple of miles then started blowing warm. Got it home and checked my pressures.

High side 150
Low Side Dont Remember it was eitehr 35 or 25 psig.
Added 6ou R-12 started blowing cold again, vent temps 45F with 85F outside temp. Pressures then read 175psig high side and 30psig low side.

Left on a trip, drove about 150miles (2hours), ac working great. Then started blowing warm, about 70f, then 80f (outside temp).

Pressures now read:
High side 225
Low side 50
Vent Temp still ambient

Any ideas:

Could these pressures mean I have a leak and now have low freon.

Compressor is still running when Ac is on. Its the old original compressor and seeing that is putting out 225psig, I think its okay.

I bench checked the valves with 60psig air and measured 30psig downstream of the ac valve/switch. Does this switch have any moving parts that could be sticking?

Thanks

Roger

 

Correct pressures vary with outside temps. It doesn't sound like you are too far off though. I wouldn't add any more refrigerant.

If your compressor is running as you indicated it was, feel the pipe that leads to the condenser. It should be warm. The larger pipe that leads into the evaporator should be cold. If that is the case the compressor is doing it's job and you have enough refrigerant.

In that case, check your temp slide lever, cable and the vacuum actuator that operates the temp door in the distribution box. Same for the mode selector.


You are NOT suppose to flush the compressor or the accumulator/dryer. If you left any flush behind it will be a big problem. You can't remove the flush from the evaporator or the condenser by flowing air from your shop compressor. You have to let it build pressure in the evaporator and the condender and then release the pressure quickly as a burst of air.

Did you add the necesary oil to the compressor after flush? Too much oil or refrigerant is worse than too little. Did you replace the orifice tube in the evaporator pipe? If it gets clogged (known as "black death" 'cause it gets clogged with black gunkies) the high/low pressure differential will not be enough.

 

As an aside, I always wondered why the recommendation was to replace the orifice tube. Since it isn't a wear part, I never understood why. Now I know.

 

Here is what a clogged orifice tube looks like.

http://www.ackits.com/merchant.mvc?S...ode=BlackDeath

 

Roger3 you say you flushed the system very good -- what did you use? And how did you do it?

Also you said you flushed the compressor with mineral oil. What type, brand name, did you use?

 

71s don't use an orifice tube. They use a POA valve and expansion valve.

As for Rogers...are you sure the evaporator is not just freezing up? Does it get cold again after 30-40mins of no use? Do you notice reduced air flow from the vents?

Your pressures seem pretty good honestly so I'm thinking the evaporator may be freezing. My VA kit does this from time to time if I am getting 36-37* vent temps for too long...I manually regulate the vent temp to 40* and no freezing usually.

 

Sounds like your expansion valve is not regulating properly. Make sure the temp bulb is clamped snugly to the outlet tube of the evaporator and then insulated from ambient using the right stuff.
If that is ok then I'd be inclined to change the POA and expansion valve.

 

For r12 your low side should be in the low 30's
You're not freezing the evaporator at 50
First check the sight glass after running for awhile, if there are bubbles, slowly add r12 until the bubbles disappear.

Next would be a probably sticking poa valve.

CA-Legal-Vette
The oriface tube is also a filter and should be changed every 2 years, but no one does.

 

I'm on the road using my iPhone so please excuse spelling.

Flushed condensor with ac flush using shop air. Afterwards it sat in my shop for 2 months in 100f temps so I believe remaining residues dried out.

Flushed comp with generic mineral oil. Filled compressor and turned it using a speed handle. Fluid pumped out into a bowl so I could see how clear it was. At first it had redish brown color but after pumping 20 to 30 ounces through it it got very clear. Drained it and added 5 ounces of mineral oil and installed compressor,

Pipe leading to consensus is cold.

My temp sensing tube is connected to evaporator line and well insulated.

I never got vent temps below 45f.

After it stopped working I turned it off for 10 min. Turned it back on and it cooled to about 65f for about 5 min then warmed up to 80.

I added 5 ou min oil to discards line prior to pulling vacuum.

Expansion valve should drop low pressure to 30 paid. It wasworking properly when system was 1st charged. Now that low side reads 50 psig I am suspecting either the poa is malfuntioming or I have a slight leak. I tested poa before I installed it. 60 psig u
Air upsstream gave me 30psig down stream.

My sight glass is clear.

My gut feeling says

 

Just read that a bad expansion valve will cause "high" low side pressures.

Also found a wAy to test POA valve while still installed. Will give it a try on Sunday