Real Fiero Tech

How many ounces (or pounds) of R134a refrigerant does a Fiero system take? Any one have the service manual handy to tell me what the normal system pressure (PSI) is on the low side?

20-30ish psi. Don't go much more above that.

I'd guess maybe 2 cans. Normal cars take ~1 but the fiero has extra long tubes

I did the math once based on the Fiero's capacity (found it in the service manual) and what the recharge kit said, and for the cans in that kit, it came out to like 2 1/3.

Now the bigger question... on an R134A converted Fiero, which oil... PAG 46, PAG 100, Ester 100, ...?

My A/C system is needing a complete overhaul anyways. When I replaced my engine, I had to cut the hoses because they would simply not come off the compressor, which is also dead anyways.

What funky kind of things do I need to do to upgrade to the new oil, aside from replacing the compressor and hoses?

Just those things. All R-12 seals/o-rings are compatible with R-134. The EPA requires non-removable R-134 specific fittings to be added to the ports so that the system can never be refilled with anything but R-134.

Although R12 O-rings in general will not leak, I wouldnt go so far as to consider them compatable.... If you want to be technical, neither are the hoses.

Youll need to install the fittings, high and low side, and use a set of 134 hoses, make sure to pull vacuum on the system for at least 30 minutes to be sure there is no air or more importantly, moisture left, then use your R-134 canister upright to charge the high and low sides, or just the lowside.... once the pressure is equal... roughly 50/60 psi or so.... start it up and charge ONLY FROM THE LOW SIDE, flip the canister upside down and slowly charge liquid in until you get a comfortable 25-30psi lowside pressure.... then you should be blowing cold...

There should be a sticker affixed to the HVAC box in the front trunk compartment that tells you exactly how much R-12 refrigerant the system takes. All of the Fieros I have worked on said 2.50 lbs of R-12 (40 oz).

When converting an R-12 system to R-134a, the general rule of thumb is to only charge the system with 90% amount of spec for the R-12. This means on a 40oz R-12 Fiero system, you would only charge with 36oz of R-134a; which makes it really nice because R-134a is still sold in 12oz cans. This means you only need to charge the Fiero with 3x 12oz cans of R-134a.

BEFORE you charge/covert your system, you MUST do the following.

1) Reclaim any R-12 refrigerant that is still in the system using approved EPA equipment and proceedures.

2) Remove the A/C accumulator, oriface tube, and A/C compressor from the vehicle. Flush the A/C lines, condensor, and evaporator using approved A/C flush solvent. Follow flush container label instructions for removing any flush residue from the system before re-assembly.

3) If reusing the A/C compressor, drain all oil possible from it. DO NOT attempt to flush the compressor or clean it out with solvents.

4) Purchase a NEW A/C Accumulator and install it. Lube the O-rings using ESTER oil and tighten the fittings to factory specs. (The A/C accumulator contains desecant that becomes contaminated by moisture and oil; it cannot be cleaned or flushed and must be replaced).

5) Purchase a NEW A/C oriface tube and install it. You can use an OE-replacement oriface tube (cheap) or get the new automatically adjusting variable oriface tubes which increase system performance. Regardless of which type used, make sure you lube up the O-rings on it with ESTER oil and install it correctly.

6) A/C oil type. VERY IMPORTANT! If you are re-using an A/C compressor that once had R-12/mineral oil in it, you MUST only use ESTER oil in the new system. PAG oils react with mineral oil residue and create a glue-like substance that can clog up the system. If you are installing a new compressor, or a compressor that came out of a R-134a system, then you can use PAG oil. I suggest using 80% PAG 46 and 20% ESTER 100 mixture. The lighter PAG 46 moves thru the system better than ESTER, but ESTER oil has better lubrication properties. PAG 100 can also be used if PAG 46 isn't available (the number indicates viscosity).

7) A/C oil total amount. Total system oil charge should be about 8oz. Pour 4oz into the compressor directly before installation and the rest into the accumulator (or charge thru A/C hose connections).

Dye. I suggest adding a small amount (follow label instructions and recommendations) of UV dye (green). This will aid in finding any future leaks if any occur. DO NOT USE THE RED DYE! It has been known to clog up A/C systems.

9) Once all components have been installed using NEW, R-134a-compatible O-rings, the system must be EVACUATED using an A/C evac pump for a MINIMUM of 45 minutes. The system should pull down to 28-30 in/hg vacuum, depending on barometric pressure at your location. After 45min of evacuation, stop the pump, close off the connection to the pump and allow the system to sit for 15min. During this time the vacuum level in the system SHOULD NOT change. If the pressure in the system rises (vac decreases), this could indicate a leak or excessive moisture present in the system. If moisture is still present in the system, it might be able to be removed using further evacuation. On A/C systems that sat open (exposed to atmosphere) for long periods of time, I have left the evap pump hooked up and running continuously for as long as 3 hours to make sure the system was a dry as possible.

10) Once the system is ready, charge using recommended amount of refrigerant. If you still need to add oil, you can do so before adding refrigerant (the vac in the system will draw the oil in). Just make sure you don't allow air to enter the system. Charge only thru the low side port and with the engine running and a/c system on. On cycling clutch systems, the A/C compressor should activate once the pressure in the low side reaches 50-55psi. The compressor will deactivate if the low side pressure goes below 28-25 psi.

COMPRESSOR TYPES: All 2.8L V6 Fieros had cycling clutch systems. The later 2.5L I4 Fieros came with constant-run, variable displacement compressors. Likewise, many later model compressors are variable displacement and don't require a cycling switch (ex: all Series 2 3800's). Variable displacement compressors will vary in the amount of low side pressure you will see on the gauge. I usually see them run consistantly at 25-30 psi, and this might not be compatible with the stock Fiero's low-side cycling switch.

If you remove or disable the low-side cycling switch, make sure there is another device in the system that will prevent the compressor from running if the system runs low on refrigerant (most newer PCM's monitor A/C system pressure directly via a hi-side pressure sensor and won't allow A/C compressor operation if system charge/pressure is low). If you have one of these newer engine management systems, you can remove/disable the stock Fiero A/C cycling switch. If not, you should replace this switch with a low-pressure cutout switch from another GM vehicle.

Concerning Hi-side pressure, you shouldn't normally see it get above 300psi, and certainly never see it go above 350psi in a Fiero. The stock Fiero HVAC wiring activates the coolant fan any time A/C is selected on the HVAC control panel. (DEFOG also activates the A/C, but doesn't turn on the coolant fan).

In engine swaps utilizing exclusively computer-controlled fans (I disconnect the coolant fan connection at the HVAC head in my swaps with computer controlled fans), the ECM/PCM should activate the coolant fan exclusively anytime the A/C compressor is activated, or when the hi-side A/C pressure goes above 225-275 psi (depending on calibration or A/C hi-side press switch setting). On systems where the PCM isn't monitoring the hi-side via a pressure SENSOR, there should be a hi-side a/c clutch cutoff switch installed. The function of such as switch is to protect the compressor if the hi-side pressure goes to high (usually above 350psi depending on switch). Such cutoff switches can be found on 96-99 GM trucks.


I would like to make one thing very clear. Only properly trained/qualified persons should work on A/C systems. The pressures that exist and the gases used in A/C systems can be deadly if not serviced/handled properly.

-ryan

That kind of reply makes me think you might be such a qualified person. How much can I expect to pay to have the compressor and the hoses to it replaced, and have it converted to 134?

Ryan, excellent write-up!

Once you've filled the AC system, some time goes by and some R134 leaks out. How can you measure how much you need to add to bring it back up to "full"?

Here's what I'm seeing for pressure measurements. When the system is off, high and low are about 55 PSI. When the system is running, high goes up to about 130 PSI and low goes down to 28 PSI. This is a 3800 Series II engine/compressor in an 88 Fiero with R134. Are these numbes normal, or do they indicate too little or too much refrigerant?

Starlite528 wrote:That kind of reply makes me think you might be such a qualified person. How much can I expect to pay to have the compressor and the hoses to it replaced, and have it converted to 134?

I have been trained, certified in and hold a license to service automotive air conditioning systems. Concerning the cost of compressor and hoses, I have no idea how much stock Fiero hoses cost but I think you can get a reman compressor for about $100 or so. The cost of converting from R-12 to R-134a depends on the shop that is doing the work. If you are asking me what I charge for this service, drop me a PM and we can discuss it.

Red wrote:Once you've filled the AC system, some time goes by and some R134 leaks out. How can you measure how much you need to add to bring it back up to "full"?

Here's what I'm seeing for pressure measurements. When the system is off, high and low are about 55 PSI. When the system is running, high goes up to about 130 PSI and low goes down to 28 PSI. This is a 3800 Series II engine/compressor in an 88 Fiero with R134. Are these numbes normal, or do they indicate too little or too much refrigerant?

There is no way to tell just how much refrigerant has leaked out of the system by pressures alone. The only way to know how much refrigerant is in the system is to reclaim it with a machine that tells you how much was in there.

Concerning the pressures you are seeing, I can't make an accurate guess of what you have going on without first knowing what the ambient temp was on the day and location where you took these readings. Outdoor temps will affect the pressures you see on the gauges as there is a pressure/temp relationship with refrigerant. As ambient temps gets colder, the pressure in the system will get lower. Vise-versa for warmer temps. Your readings sound "normal" for what I would expect to see on a 50 deg day (hi-side pressures won't go very high because there isn't much cooling load on the system). If it was an 80 deg day, I would expect to see the low side between 30-40psi and the high side above 175psi (depending on how good the condensor and coolant fan are).

The BEST test you can do is to stick a thermometer into the vents inside the car, set the A/C to MAX, and fan to med-low. A good Fiero A/C system should be blowing out at 40-45 deg F or less on an 80 deg day depending on humidity (on some of my swaps I have seen temps as low as 35deg). However, don't take what you find here to heart and assume that your system is undercharged if your vent temps aren't this low. There are many factors that determine vent discharge temp. Overcharging an A/C system can result in poor performance or even damage.

I took the readings while sitting in my driveway on a 65 degree day. Thermometer in the vent was showing 41 degrees.

I may have to pirate this thread for my own nefarious purposes.

I'd like to come up with an A/C system for Northstar Fieros that has a minimal number of harness & equipment configurations for different recipient chassis.

This is coming from memory, so forgive an incorrect detail or two...

The V6 Fiero uses a pressure cycling switch and high & low pressure cutout safety switches with a cycling compressor. The high pressure cutout switch is built into the compressor case.

The 4cyl Fiero uses a variable compressor with high & low pressure safety switches. It does NOT have a pressure cycling switch or wiring for one.

The Cadillac uses a cycling compressor with high & low pressure switches read by the PCM and high & low temp sensors read by the climate control module. The PCM controls the compressor, but the climate control module communicates compressor requests to the PCM via serial data. It does NOT have a high pressure cutout switch built into the case.

IOW, using the Cadillac compressor with the Fiero V6 system means that there is no high pressure cutout switch without the addition of an external switch and extra expense on the custom lines that have to be made.

Using the Caddy compressor with the Fiero 4cyl system means that a pressure cycling switch has to be added to the system, which again is more complexity than the minimum required.

I'm conjecturing that there is a compressor useable with the Cadillac mounting pattern that has enough ports to have integral HP cutout switch, but I haven't been able to do the research necessary to find out yet.

Is there a variable compressor that will bolt to the Caddy?

The Dark Side of Will wrote:
I'd like to come up with an A/C system for Northstar Fieros that has a minimal number of harness & equipment configurations for different recipient chassis.

IOW, using the Cadillac compressor with the Fiero V6 system means that there is no high pressure cutout switch without the addition of an external switch and extra expense on the custom lines that have to be made.

Using the Caddy compressor with the Fiero 4cyl system means that a pressure cycling switch has to be added to the system, which again is more complexity than the minimum required.

I'm conjecturing that there is a compressor useable with the Cadillac mounting pattern that has enough ports to have integral HP cutout switch, but I haven't been able to do the research necessary to find out yet.

Is there a variable compressor that will bolt to the Caddy?

I don't know if there is a variable displacement, constant run compressor that is available that will directly bolt up to the caddy N*. You might want to take a look at a compressor off of a Series II 3800 and see if it could be made to work.

Now assuming that you are stuck with a fixed-displacement, cycling clutch A/C compressor, here is what I would do concerning the wiring...

-Add a cycling switch (mount on accumulator) and wire it in if necessary. Wiring shouldn't be a big deal just wire it up like stock Fiero V6. This cycling switch will take care of the cycling and low-pressure cutoff functions.

-Concerning the hi-pressure cutout switch, one can be mounted up front on the hi-side test port; although it is recommended that the hi-press C/O sw is mounted as close to the compressor as possible. Most A/C shops that have the ability to TIG weld aluminum should be able to weld a hi-side fitting on the custom lines you are going to need them to make for you if you use a non-stock Fiero A/C compressor. You can cut one of these fittings off a junkyard car if you need them.

-Concerning coolant fan and PCM wiring; you can do one of two things: You can either wire all required devices up to the PCM, or take the PCM out of the loop. If you choose the latter, there are hi-side pressure switches available for both R-12 and R-134a type test port fittings that can be installed to activate the coolant fan independant of the PCM or HVAC control head.