Does anyone know what the low and high side pressures should be on the Fiero's A/C system? I have purchased a set of gauges, and I am going to charge my system with R-134a, but don't know how much I should put in or what the gauges should read. Do any of you have this information?

I charged it with R-134a after most of the R-12 had leaked out over several years or more (previous owner did not maintain the A/C). It worked well with the initial charge of R-134a, but it is now losing some of its cooling capacity after a few years. I am going to evacuate the entire system, put in new R-134a with ester oil. I have considered replacing the compressor, orifice, and the dryer, but I have heard both sides to this story. On one hand I have heard helpful people in this forum saying that you have to replace everything, and on the other mechanics I know personally that have done 100's of change-overs from R-12 to R-134a say you don't have to replace a single item in the system.

A good rule of thumb is 30 on the low 200 on the high but this is a general on all a/c not a specfic number. Just keep in mind if you change over the system from R12 you need to change the fittings, oil in the compresser, the reciver/dryer. vac down the system befor you put R134a in, If you dont you can possable clog the lines and or the orfice tube. Its always good to change the reciver/ dryer any time you open the lines. As far as the oil goes R12 used a different type of oil in the compresser then R134a. Give me a second an I will look up specs to see what the actual numbers on the low and high side are.

The pressures in the system depend upon the ambient temperature. Some AC techs have gauges that have a temperature scale for the refrigerant in use, others use temp/press tables.

Here's a link to a PDF temp/press document: http://www.hantech.com/supp...TemperatureChart.pdf

My gauge set has a temperature scale; very handy, I just charge until it reads the ambient temperature and I'm done.

But just charging to a certain pressure without considering the temperature - may as well put a blindfold on and guess. Look at the chart and you'll see why...

quote Originally posted by pontiacman63383: Ok it should take around 35-40oz of R134a and it should be on the low 22-29 and on the high 165-205.

Thanks for the info, I'm in the same boat. As told in another thread I have had success with $30 a/c conversions and dramatic failure with $1500 professional conversions. I have bought a set of gauges and I'm going to attempt to do it myself the right way this time.

Whuffo is right on concerning the importance of both temperature and pressure. That said, you can't reliably fill an A/C system using temperature and pressure alone. To do it really "right," you have to evacuate the system and charge with a known amount of refrigerant. (On some large industrial systems, proper charge can be determined by pressure, "superheat" temperature, and "subcooling" temperature ... but that is beyond the scope of this discussion.)

The reason is that the refrigerant in a properly charged system exists in both liquid and vapor (gaseous) phases. But you cannot tell how much liquid and how much vapor from temperature and pressure alone. What will happen is that the pressures will stabilize as soon as the minimum amount of refrigerant for acceptable operation is introduced into the system, even though it's still undercharged, and the working pressures will remain pretty much the same as you add more refrigerant, right up to the point that the system is seriously overcharged.

[This message has been edited by Marvin McInnis (edited 07-18-2007).]

Since doing it the "right" way means measuring the amount of R134, how accurate do you have to be? Will my bathroom scales work? They have a resolution of 0.2 lb.

OK,

I connected a set of gauges to the high and low sides of the AC system. Ran the car (87 GT V6) with the air on, and initially the low side gauge read about 54 and the high side read about 215. After running for a few minutes, the air out of the vents blew about 62. The low side settled at about 48, and the high side settled at 175. I still believe it is not cooling as well as it did when I put the first charge of R-134a in.

Any suggestions on what to check?

Does it need more R-134a?

Should I check the orifice screen?

Should I evacuate the system, blow out all the lines to clear as much of the old oils as possible, and start over?

TIA for any help, suggestions, and opinions.

quote Originally posted by Formula Owner: Since doing it the "right" way means measuring the amount of R134, how accurate do you have to be? Will my bathroom scales work? They have a resolution of 0.2 lb.

Scales are usually a good way to go, but it's hard to get accurate weights when using 12 ounce (0.75 pound) cans, due to the hose flopping around. And 0.2 pounds resolution may not be very practical when using small cans of refrigerant.

Fortunately, the exact refrigerant charge isn't that critical. Three full 12 ounce cans will (conveniently) be 36 ounces ... which is about right for R134a in a Fiero, even with slightly more than 8 ounces of refrigerant oil in the system.

Of course, all of this assumes that you are starting with an empty, evacuated system. That's the only way to really know your starting point.

[This message has been edited by Marvin McInnis (edited 07-20-2007).]

quote Originally posted by Marvin McInnis: Fortunately, the exact refrigerant charge isn't that critical. Three full 12 ounce cans will (conveniently) be 36 ounces ... which is about right for R134a in a Fiero, even with slightly more than 8 ounces of refrigerant oil in the system. Of course, all of this assumes that you are starting with an empty, evacuated system. That's the only way to really know your starting point.

Refrigerant capacity for a Fiero using R12 is 2.5 pounds; 40 ounces. When using R134A in a R12 system, start with 80% of the R12 charge - 32 ounces.

This is only a starting point - the correct amount will depend on how much of the old R12 oil is still in the system. There's always some of the original 8 ounces left; while it doesn't interfere with R134A, it does take up space in the system and changes the true refrigerant capacity of the system accordingly. Even if you evacuate and pump the system down that oil will still be there.

Since the true capacity of the converted system is unknown, we need to determine the amount of refrigerant to add in a different way if we want the system to perform optimally. The tools are simple; a couple of accurate dial thermometers (those little ones at the grocery store are fine), a set of gauges, and our temp / press chart.

One thermometer goes in an AC outlet, the other sits on the cowl air intake. Start the car, set the AC to Norm and the fan to high. Let the system run until the temperature at the AC outlet stabilizes (this'll take a few minutes) and the system will now have reached equilibrium. Read the thermometer at the cowl air intake and look up the pressure on the temp / press chart. That's your target low side pressure; now add refrigerant slowly and "sneak up" on that target. Keep an eye on the AC outlet thermometer as you slowly trickle the refrigerant in - it should bottom out well below 40 degrees. If you see the AC outlet temperature start to rise while charging, stop charging, let the system stabilize and check your cowl inlet temp / temp-press chart again. Now adjust the charge level as needed. This is the way we used to do it in the days before those fancy AC service machines came along.

This procedure will get the optimum amount of R134A into the system. You'll probably be surprised to discover that it takes more refrigerant than expected to reach this point - and you'll also probably be pleasantly surprised by what the thermometer in the AC vent is reading. You might also notice that the compressor is running more smoothly / quietly.

I've warned about the pitfalls of retrofitting the Fiero AC system. Here's the good news: if you take the time to convert it right and get the right amount of R134A into the system it'll work great. R134A may not cool as efficiently as R12, but you won't be able to tell the difference in real life. The only difference you may notice is that the compressor doesn't cycle as often as it did using R12.

Can anyone tell me how to read the gauge dials on the set that comes from Harbor Freight? I can easily tell there are PSI scales on both dials, but there also seems to be temperature scales on the dials as well in degrees C. How do you correlate the temperature scale with the PSI scales, or is the point that you shouldn't? If the refrigerant is at 50 degrees C, then it should be at ??? PSI - is that how it should be used?

I know Whuffo has said that you should just put refrigerant into the system until the dial reads the ambient air temp, but there seems like there should be more to it than that. I mean, 200 PSI didn't seem to line up with the air temperature that I got those readings with (I think it as about 80F). Maybe I am wrong, as I don't have the gauges with me know.

quote Originally posted by computer_engineer: I know Whuffo has said that you should just put refrigerant into the system until the dial reads the ambient air temp, but there seems like there should be more to it than that. I mean, 200 PSI didn't seem to line up with the air temperature that I got those readings with (I think it as about 80F). Maybe I am wrong, as I don't have the gauges with me know.

Not all gauges have the appropriate temperature scale marked on them - it's a great convenience when they do, but it's somewhat rare to find. I suspect those Harbor Freight gauges probably don't have the markings - or if they do they might be incorrect.

You could check the temp marking on the gauges against a temp / pressure chart to see if they're right...

Well,

I picked 200 PSI on the gauge and it lined up pretty close to the 55 degrees C marking on the outer edge of the guage - this line was marked as the "R134a" line. I converted the 55 degrees C to about 130 degrees F, and looked it up on the chart you posted a link to. The chart says 198.7 PSI, so that seems like it matches up to me. How do I use the temperature scale, now that we know it matches the chart?