Quick A/C Recharge Question
 

My 82 240d needs an ac recharge - at first had 5 psi low side. Had already been converted to r134. So I picked up some refrigerant, started the car, jumped the compressor, and got it up to 30 PSI low side. My car has very low idle (another issue I am dealing with) and have seen that when recharging, you want the engine up to 1500 RPM. So I get in the car, and now have cold AC. Go back and check the pressures, and its showing 45 PSI. Since the car is about to die, I revved it up a little (my car does not have a Tachometer) and it went down to 36 PSI. If I rev it up a lot, it goes down to 25-ish PSI.

My 2 part question is, should the pressures change that much by revving the engine? And since I have no tachometer on my car, can I just assume when I hit 1500 rpm and be safe at 35 PSI?

Details of the system:

Ambient Temperature is 85 degrees - my chart shows that I should have a 30-40 PSI low side charge at that temperature. My car reads:

About to die 45 PSI
Revved up to what I assume is 1500 RPM is 35-36 PSI
Revving the engine to higher RPM's goes down to 20-25 PSI
AC off completly shoots up to 100-110 PSI

Thanks for the feedback.

Yes, the pressure will vary with speed of the compressor. The low side is the suction side. When the compressor is turned off, the high and low side should read about the same. So the low side will go up and the high side will go down.

You need to adjust your idle up a bit, as IIRC, the readings are checked with the engine at idle. You may need to bleed off some freon, but since you say it stared w/5 psi, the leak will bleed it off for you.

the bad thing is you don't know what the high side is reading. yes low side pressures can vary widely depending on compressor speeds, and temps inside and outside. also vary widely with a clean vs. a dirty evaporator and condenser.
it's best to have everything evacuated and weigh in the exact charge according to a chart. it is usually slightly over 80% of the R12 weight.
John

Why on earth would you jump out the compressor? It would have started on it's own as soon as the pressures came up.
That is meaningless without knowing the temperature of the discharge air.
What's the temperature on your chart at this pressure? What temperature does water freeze at? Are you running the risk of making your evaporator a big ice block at this pressure???
Your not comparing apples to apples here. The outside temperature has nothing to do with the discharge temperature inside the car. It would help a lot if you knew what that temperature was.
It sounds to me like your undercharged but you can't really tell without knowing the head pressure and discharge air temp inside the car.

Also be warned that if you leaked refrigerant you leaked oil too. At this point you don't have any idea how much oil you have in the system.

You also know you have a leak and you did nothing to find and fix it.

There's a right way and a wrong way to do this. The right way would be to find the leak and fix it. Then flush out the system and put a known amount of oil and refrigerant in.

Danny

Thanks for the replies so far.

Danny - I jumped the compressor because it did not start when the pressures went up. Got the compressor started and it went on its own from there.

As far as the pressure chart, I picked it up (a printout) from a mechanic locally. He stated that it goes by outside ambient temperature - that is why I was giving outside temperatures.

As far as the oil, dont worry, I added the 2 oz of oil per 12 oz of refrigerant.

As for the leak, there is no known leak that I know of yet. I recharged because there was no pressure in the system. I dont know the last time the AC was run because I just recently got the car. The refrigerant I purchased has dye, so if there is in fact a leak, I will be able to see via a UV flashlight.

just to let you know. if it has ever run and cooled, and it now has low charge, you have a leak.
and just adding 2oz of oil per can of refrigerant is not a good thing. it's best to know exactly what you have. flushing out the lines, evacuating the system, and putting the correct oil and refrigerant charge is best.
John

Nothing turns forum posters more into bullies than AC questions....:D :D
Anyone else ever noticed this? ;) ;)

**{no specific reference to above posts, just a general thought}**

The dye is a good thing temporarily. If you have the POS R4 compressor they are known to leak atthe front seal. You will need one of those led flashlights and the yellow goggles to see where the dye "lights" up. Even though it is a pain to work in low light, you can find the leaks easier with the "flashlight and goggles", and then save your repair work for daylight hours. What ever you do, do not use the stop leak stuff.

Waydayamean BULLY?
just do it right.
or don't do it.
period!
John

Just out of curiosity, what is wrong with the stop leak stuff? I went to two automotive stores and could not find any pure refrigerant.

It was all refrigerant + leak sealer + oil.

And I beg you, don't flame me for the question! haha.

The stop leak stuff can and does plug up evaporators, expansion valves, and other small openings. Plus it voids the warranty on new or remaned compressors. I find pure freon at NAPA, O'reilly's, Parts Plus, Car Quest, etc. True, some chains do not carry pure freon. A lot of indies do though.

K-Mart sells the stuff around here. I think I saw some at Wal-Mart recently too, but it might have been the stuff with the "conditioners."

I only buy the pure stuff.

No, You are quite right....Auto-Stores ONLY supply the 'Ready-Mixed' stuff....

Last year, I had to sort the AC out on that hideous BMW. The Condenser was faulty, so that was replaced, and the 'receiver/dryer' thing. I stripped and cleaned out the compressor, which I filled with the correct PAG100 oil. and used an old 'Fridge' compressor as a Vac-Pump, pumped it down to near full-vac, after confirming all hoses/unions were leak-free and then filled it with R134a I got from a refrigeration supply-house, I filled the system enough to eliminate the vac, then re-vac'ed that out to sort of 'flush' the last remaining air. I then charged it with the correct weight of gas

The BMW contains 1.5Kg of gas approximately if memory serves correctly, easily measured by the weight of the gas-bottles. I was able to buy the gas in disposable 1Kg bottles, which made things nice and easy. I used the fittings from the 'Auto-Shop' re-charge can to fill the system

The two bottles of R134A cost around the same as the 500g container of stuff I first bought in a auto-store, when I first investigated why the AC didnt work.....

It seems the Only place you'll get just the gas, and just the oil separate is at a specialist refrigeration/A.C. supply-house.....

If you need to fill with gas, you'll need to fill with oil too....Too much oil will make the system inefficient, Too little will cause compressor failure/seizure, so its important to guage the quantity in the system There Should be a chart somewhere for the M.B, saying the 'expected' quantity of oil in each major component if the system has been discharged properly for other work.

PAG oil, is rather like Brake-Fluid, Meaning it will absorb water from the air. IF the system was Completely down to air, or has been 'open' for some time, then it really will need 'flushing' through with a solvent before re-charging with the correct amount of oil and gas....

Check Exactly what sort of oil Your Compressor uses before getting it. Its NOT that cheap!--Most systems using 134A seem to use PAG of one type or another

Grrrr this thing is frustrating me.

Two days ago, the AC would only work decent when at high speeds, did not really cool extremely well even when at high speeds, and when in stop and go traffic or putting around town it works but not well.

Yesterday, after making NO changes whatsoever, I started my car up, ac worked very well. Drove in town, very cool ac. Stop and go traffic, very good ac. Entire day the ac worked great.

What would cause the ac to perform below expectations, or almost not at all in slow moving conditions, and the next day run great at all times?

The oil in the system had not had the chance to get evenly distributed through the system entirely! Too much oil and not enough refirgerant will cause this, but if it is cold at all times I would leave it alone for now since you got it working. Just remember to check for your leakage and make repairs where necessary.;)