Since Fieros were never designed to use R-134A, factory charging information is not available. The Fiero does not have an R-134A system but can be easily converted by flushing the lines, adding Ester oil, replacing the accumulator, then evacuating and charging with R-134A.
Assuming that the R-134A conversion has already been completed. I have found the ideal pressure points for optimum cooling and I thought that I'd share these with the forum.
1. Attach your manifold gauge set. 2. Start the engine and put the A/C system on maximum, with the fan on max. 3. Hook up the charging line (yellow to your R-134A source) Bleed the line of air by unscrewing the refrig connection at the gauge set, open the source, listen for gas to esape and then screw the fitting tight. .4. Look at the gauges. The high side should be reading TWICE the outside temp (F) PLUS 20. This morning it was 80*F so I charged back up to 180 psi. The low side settled in at 30 psi. Measured the outlet temperature and it was blowing ice cold air at that point. No need to measure temps as it was very cold in the passenger compartment.
Point is that you can have a very cold R-134A system if the conversion and rechaging is done PROPERLY Tthe system must be charged to the max or near the max for optimal cooling.
Note: . For the system to be converted and charged properly ALL of the old R-12 type oil must be removed before recharging. This means opening the lines, blowing them out and getting all the old oil out of the compressor. If you don't want to remove the compressor and dump out the old oil you can break the line at the firewall, put the ends of the hoses in a bucket and momentarily energize the compressor (only for 2-3 seconds) so the old R-12 oil blows out. Put everything together, use new O rings, add a new accumulator and you are ready to add the PAG oil and recharge w R-134A .

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, 3.4" Pulley, N* TB, LS1 MAF, Flotech Exhaust Autolite 104's Custom CAI 4T65eHD w. custom axles, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

Thanks Dennis!

I'd give you a +, but I already have..

Maybe we can have Ogre add this to his cave info?

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Currently in the middle of my 88 + 3800NA swap

While this method may work "well enough," you cannot determine the proper charge level in an AC system by pressures alone. That said, this method is still probably better than the usual "I just put in a can of freon ..." method.

It is possible to determine the charge level in an AC system using refrigerant pressures plus temperatures (i.e. by calculating superheat and/or subcooling), but I've never seen such data published for any automotive AC system ... much less for a Fiero.

[This message has been edited by Marvin McInnis (edited 06-24-2010).]

Sorry...

I agree you should replace acc/dryer, O-ring, bad hoses, to keep from leaking etc...

but...

Pressure test to charge the system is a iffy method for HR6/DA6 system. Charge by Weight instead. 3 12oz can - lost = 80-85% weight of 40oz Freon that Fiero uses.
Lost? Because you lost some Refrigerate in gage and switching can. Every time you switch can, you lost 1-2oz and if it a "good" switch...

Also... V5, V7, and other variable displacement compressors, needs charge by weight only. Can't be sure if V5 is max displacement mean High side pressure is unreliable. GM has a method to make sure the V5/V7 is a max displacement but it not 100%.

Flush... Don't flush unless compressor is hosed and left crap in line. And then use a filter and/or intake screen to try to protect the new compressor. Left over oil in hoses will act like barrier hoses. So just blow it out with dry clean air to get the major leftover oil out.

Oil... Most maker recommend PAG in all conversion. Thing 1 is you can't flush some parts like evaporator and condenser.

Some is in cave... interior, HVAC. tho I should move it....

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)

To the people who doubt my method. Come by my shop and bring along a thermometer. If my Fiero A/C system doesn't blow 36*F air, I'll eat my hat. Its so cold in fact that you can't keep the system on max for very long. If there is a way to charge more efficiently its not needed as the system is already ice cold. BTW you can only charge by volume/weight on the very first fill and you still need to watch the gauges. If you have a miniscule leak like I do, I need to recharge once a year and the gauge method works and delivers great performance. I always use Ester oil as its the oil recommended for retrofits. PAG is normally used in untouched or new R-134A systems as it will degrade when exposed to the slightest amount of 625 mineral oil used in R-12 systems. I agree with Ogres flushing method in that during a R-134A retrofit, the lines only need to be blown out not washed out with solvents. BTW I am using the GM CV7C 2006 GTP compressor.

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, 3.4" Pulley, N* TB, LS1 MAF, Flotech Exhaust Autolite 104's Custom CAI 4T65eHD w. custom axles, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

[This message has been edited by Dennis LaGrua (edited 06-24-2010).]

Real good information as there have been so many inquiring about repairing their AC system as of late. Thanks Dennis for the input.

Joe

ACDelco TSB (2005) concerning R-134a conversions to GM vehicles.

[This message has been edited by Marvin McInnis (edited 06-24-2010).]

I have been "playing" with the AC on my Formula for quite awhile and this is what I have learned - applies to my Formula but it may work for others - Using a variable orifice tube, with ambient air @ 100*, my low pressure side is at 30 and the hi side is at 230. Sitting - idling, I can't get the temp down below 52*. If I raise the RPM to 1800 plus, the low side drops to 25 to 27 and the hi side will build to 250 - 260. The air temp will drop to 40*. Out on the highway at 70 plus (1800 plus rpm) I can maintain 38*. In city traffic - sitting at traffic lights, the temp will creep back up to 50 - 52. I believe that this is due to the vaiable orifice tube. I will be switching back to a standard fixed orifice tube in a few weeks to test it out. Bottom line for my Formula is that it seems to work best with 25 to 27 on the low and 250 to 260 on the high side - of course the ambient air, humidity, etc makes some difference too.

Pat

quote Originally posted by katatak: I have been "playing" with the AC on my Formula for quite awhile and this is what I have learned - applies to my Formula but it may work for others - Using a variable orifice tube, with ambient air @ 100*, my low pressure side is at 30 and the hi side is at 230. Sitting - idling, I can't get the temp down below 52*. If I raise the RPM to 1800 plus, the low side drops to 25 to 27 and the hi side will build to 250 - 260. The air temp will drop to 40*. Out on the highway at 70 plus (1800 plus rpm) I can maintain 38*. In city traffic - sitting at traffic lights, the temp will creep back up to 50 - 52. I believe that this is due to the vaiable orifice tube. I will be switching back to a standard fixed orifice tube in a few weeks to test it out. Bottom line for my Formula is that it seems to work best with 25 to 27 on the low and 250 to 260 on the high side - of course the ambient air, humidity, etc makes some difference too. Pat

Pat. The high idle pressure will vary with temperature and you are located in a hot climate area. . Looking back to my formula, if the ambient is 100*F then the High side pressure should be in the range that you are seeing. If you are experiencing insufficent cooling it might be that the system is actually overcharged and not allowing the gas -liquid , liquid-gas process that needs to occur for proper cooling. Its also possible that you might have too much refrigerent oil in the system taking up space for extra refrigerent. As for the variable orifice tube; I've never used them as I've acheived great results without one. Many retrofit technicians swear by them but since my A/C/s are blowing real cold I've never seen a need to change anything. When you change the tube again, make sure to evacuate to 29 " of HG over a period of at least 30 minutes. This is very important.

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, 3.4" Pulley, N* TB, LS1 MAF, Flotech Exhaust Autolite 104's Custom CAI 4T65eHD w. custom axles, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

I think you are 100% correct Dennis. I beleive that there is way too much oil in the system. The reason being is that I only put 2 cans of refrigerent in the system after evac and pulling vacuum for a few hours to get it to the pressure I have. I talked to a few AC guys here in town and they say that the variable works good if you have a weak compressor - helps keep the low side pressure up at lower RPMS then "varies" as rpms increase to maintain the low side pressure without letting it drop below the low pressure cut off. It sounds believeable!

In a few weeks, I am going to have the system power flushed to get as much of the oil out as I can. Then I'll take the compressor off to drain it and then start fresh. My son was driving this car for a few months and he thought the AC needed to be colder so he had it "serviced" at Pep boys. When I got the car back, it would not blow 60* air. After checking it, I found it to be overcharged and the orifice tube had tear in one of the screens - looked like it had blown apart. I am thinking that when Pep Boys "serviced" it, they added oil too! Thanks for your info.

Pat

I don't know of any 134a systems that cool well in hot temps at idle. In my little bit of experience this is normal and is the chief complaint when switching from r-12 to 134a. The VOV is designed to help this and switching back to a normal orifice tube may not make any difference or it may not work as well as the VOV but, I am very curious to see how Pat's experiment works out. The only issue is that when he goes to change the VOV for the standard orifice tube he will also be doing other things that could affect performance such as establishing the correct amount of oil in the system. If his system does indeed work better after he has made the swap we won't know for sure if it was the orifice tube or the oil quantity that made the difference. Currently my car freezes my butt in extreme heat but only after it has had sufficient time to cool down a heat soaked car and while it is rolling down the road. If the car is heat soaked and sitting at idle, the outlet air is cool but not cold. Current high temps ~ 110*.

Charlie