Well, after tons of research on the subject (and resolving an unbelievable amount of conflicting/incorrect information), as well as some trial and error (mostly error), I think I've figured out the correct procedure for properly evacuating/charging the A/C system on my FD. FWIW, it's a 93 converted to R134a.

The FSM "body electric manual" covers much of the procedures, but is woefully incomplete and/or vague about a lot of it. I'm sure it simply assumes a deep familiarity with this stuff, which a lot of us enthusiasts don't have.

My procedure also differs a bit from the FSM procedures on a couple points, mostly thanks to my trial-and-error research, which showed that some of the FSM steps/conditions were simply unnecessary. I may also be a bit more comfortable doing things seat-of-the-pants than some. My attitude has been, get the A/C working, failing which it's getting yanked out of the car. Thus, I'm not too concerned with things being absolutely perfect, as long as it works and isn't in danger of failing 2 months from now. I hope for at least 5 years of working A/C before needing to do any more work on it.

Also, don't bother doing any of this unless ambient temps are at least 80F. Pressure readings seem to get pretty wacky, and I had a hard time getting the compressor to run constantly at lower temps.

I'm starting from an open system here, where some A/C system component(s) needed to be repaired/replaced, and the system was open for an indeterminate amount of time. Assuming replacement of the dryer as a pre-requisite...

1) Connect manifold gauge set. Self-explanatory, except mine was tricky - the service port adapters were a little off on manufacturing tolerances, so the couplers didn't want to fit. I was able to force the low-side to fit, but the high-side simply would not keep a seal, which I later found was due to the adapter o-ring being damaged. I replaced them with new adapters which I picked up at the local Advance Auto Parts (Interdynamics brand). These were only a few bucks, and now the couplers slide right on/off and keep a good seal. Also make sure the adapters are threaded onto the original R12 schrader valves tightly.

2) Evacuation. Don't waste your time with the air-powered vacuum pumps - they take forever to draw the entire system to a vacuum, and they draw a lot of air, so the compressor is running constantly for 20-30 minutes. The electric vacuum pumps, though expensive, are well worth it IMO. Connect the pump to the yellow center service hose on the manifold gauge set using whatever fittings are needed. I've found that these pumps vary widely in what types of fittings they provide, but most seem to have 1/4" and 1/2" ACME male fittings. The yellow service hose usually has a 1/4" ACME female fitting, which connects up to most pumps without an adapter. Turn on the pump, and open both the low- and high-side manifold valves, and let it run for at least 15 minutes. Then close the manifold valves, turn off the pump, and check the low-side gauge, which should read about 29" Hg vacuum. If it doesn't, you have a leak, either somewhere in the system itself, or in the manifold gauge service hose connections (check/tighten these first). Otherwise, let it sit for 5-10 minutes, then re-check the gauge, and the reading should not have changed. If it hasn't, you've successfully evacuated the system. Go ahead and disconnect the vacuum pump from the yellow center service hose.

3) Oil. The most convenient way I've found to add oil is with one of these "PAG Oil Charge" canisters, all of which seem to contain 8.5oz of PAG oil and 2oz of R134a. Using a can dispensing valve, connect the oil charge can to the yellow center service hose. Shake the can thoroughly, tap it with the valve, and then, while holding the can upside down, open the dispensing valve, and then open the low-side manifold valve. The vacuum in the system, combined with the positive pressure in the can, will draw the oil into the system. When the can's empty, close the manifold valve and the dispensing valve, and disconnect/discard the can. You now have 8.5oz of PAG oil and 2oz of R134a in the system.

4) Refrigerant. Go ahead and fire up the car now, and let it warm up completely. When it's warm, turn on the A/C and recirc switches, and set the fan switch to its highest position. Despite being low on refrigerant, the compressor clutch should have engaged, and the compressor should be running constantly. Let it run like this for 3-5 minutes - in the meantime, using the can dispensing valve, connect a can of R134a to the yellow center service hose and tap it. Keep the can in an upright position, despite the various suggestions I've seen on the refrigerant cans themselves, which generally state that it should be held upside down. I set it on the UIM, so the heat of the engine will keep the can warmer as the refrigerant is transferred.

When the compressor's been running for 3-5 minutes, go ahead and open the can dispensing valve, and then open the low-side manifold valve. Keep an eye on the manifold gauges, but the first can won't fill the system, so don't expect it to reach your target operating pressures yet. Charging as a gas (can upright) takes a lot longer than charging as a liquid (can inverted), so give it several minutes. When the can's empty, close the manifold valve and can dispensing valve, and then disconnect/discard the can.

Prepare another can of R134a using the same procedure, and then open the can dispensing valve and the low-side manifold valve again. This time, keep a close eye on the manifold gauges. It can be a little tricky to recognize when it's full, but some simple math helps here - system capacity for a 93 is 21oz of R12, but with R134a, you'll want about 17-18oz (around 85% of the R12 capacity). So far, we've added 2oz of R134a with the oil charge, and another 12oz with the first can of refrigerant, so we're at about 14oz at this point - only about 3-4oz away from being full, so we need another 1/4 - 1/3 of a can. You can weigh the can with an accurate scale to get this right, or just keep an eye on pressures and feel the weight of the can for a rough measurement. Pressures vary with ambient temperature, but at about 80F ambient, low-side should be around 25-35PSI, and high-side should be around 200-220PSI.

Sight glass condition is the least accurate way to determine when the system's full - when the refrigerant appears clear of any bubbles while the compressor is running, it has at least the correct amount of refrigerant, but you have to stop the compressor and observe the sight glass immediately after to tell whether you've overfilled. Aside from making sure the sight glass reads correctly when I'm all finished charging, I don't use it to determine how much refrigerant to add to the system.

When you're satisfied that you've added enough refrigerant, close the manifold valve and the can dispensing valve. Double-check pressure readings again, and check the sight glass for absence of bubbles. If the A/C is blowing nice and cold, you're done - disconnect the manifold gauge set from the system and replace the service port caps.

5) Beer. I've found that it helps a lot at this point...

Well, I've probably missed a couple details, or gotten a figure or two wrong from memory, so please let me know if you see anything wrong with this. Like I said, a lot of research and trial-and-error went into this, and some of it may have just happened to work on my car for whatever reason, so if you see any discrepancies with your experience, please point it out. I'd like to refine this procedure, and eventually add photos to make a step-by-step guide out of it, to hopefully prevent others from going through the same research/misinformation nightmare I went through to get to this point.

 

Oh yeah, one more thing - when adding the refrigerant, make sure you're using cans of straight R134a, with NO OIL ADDED. Most cans of "R134a" actually contain a mostly-correct ratio of oil, but every one I've seen had polyol ester (POE) oil, rather than the polyalene glycol (PAG) oil our systems were designed for. Don't mix oils!

 

Anybody see anything I missed?

 

Almost forgot about purging the yellow center service hose...

When adding the contents of the oil charge and refrigerant cans to the system, after you fit the can to the dispensing valve and tap it, you should purge any air that's present in the center service hose before opening the manifold valve.

Most of the manifold gauge sets I've seen have a schrader valve teed into the fitting where the yellow center service hose connects to the manifold body - this is the purge valve. After tapping the can, open the dispensing valve and hold the can upside down. Use something to push the purge valve pin down. At first, you'll hear hissing as gas escapes, but hold it open for a few seconds, and liquid will reach the valve and start to spray out - when it does, release the purge valve pin to close it. At this point, there may still be a tiny amount of air in the manifold body above the purge valve, but at least you've cleared most of the volume between the can and the refrigerant system.

Other manifold gauge sets rely on a purge valve adapter which fits into the low- or high-side quick-disconnect coupling (the ones that connect to the service ports on the refrigerant system). For these, you would have to remove the low-side service hose from the service port on your car, then open the dispensing valve and low-side manifold valve, and then insert the purge valve adapter into the quick-disconnect coupling on the low-side service hose. Now it's the same as before - at first, you'll hear gas escaping, but after a few seconds, liquid will reach the coupling and start to spray out, at which point you remove the purge valve adapter from the coupling, and reconnect the low-side service hose to the service port.

The second approach is more thorough, since you're purging the entire volume between the can and the refrigerant system. Using the built-in purge valve, you're actually only purging the center service hose - not the manifold body and low-side service hose as well.

 

For pulling the vac I've always rigged a fitting between the intake of my air compressor and the low side port rather than rent or buy an actual pump. Most compressors have simple screw in filters on their inlets. Remove the filter and come up with a suitable set of adaptors so you can attach a hose from the low pressure port there instead.

I have a nice set of R12 gauges which won't work with R134 ports. Nobody sells adaptors to allow me to use the R12 gauges on 134 systems; they force you to buy a completely new set of gauges since all the fitting sizes are different. Even the hoses themselves from an R134 gauge set won't screw onto the manifold of an R12 gauge. Screw that. I cut the hoses of my R12 gauges off and inserted barbed brass fittings in them. Now I either fit the R12 ends or the 134 ends on which I robbed from an el cheapo refill kit. Hose clamps make sure the barbs don't leak.

 

Good stuff! AC is one area that just isn't documented enough on the FD. Sad thing is, with a properly functioning AC, it's an excellent system - gets VERY cold, and actually has very little load on the motor.

Since you're in Virginia, going 134a is a good idea. For hot Florida, it's another story . 134a, if done right, will never get as cold as R12. My AC is charged up with R12, just like factory, and I get 34-35 degree air out of the vents.

The big trick with 134a conversions is the oil and the O-rings. Dunno if you'll run into trouble with the O-rings over time - supposedly, older R12 compatible o-rings don't like 134a and can break down. Hard thing is so few RX-7 people have really experimented with AC, and it's not helpful to use general statements that applies to ALL auto AC systems. What causes a problem on one car works fine on another.

There are also R12 drop-in replacements, like Freeze-12, Hotshot, etc. These are supposed to be totally compatible with the old R12 o-rings and oil, and cool just like R12. But, you do need a license to buy it.

More on the topic later - I gotta run .

Dale

 

wow 8.5 oz of oil is alot to install

 

That's a good idea!

At first, I bought one of the air-powered venturi vac pumps, but it was painfully slow to draw the system to a deep vacuum. After 20 minutes, only about 20" Hg vacuum. It took 35 minutes to get it down to 29", with the compressor running the whole time!

The compressor inlet is a lot quicker, right? I know a lot of energy is wasted in those venturi-type pumps...

 

I never did find the actual system oil capacity in the FSM. The closest I got was to check the amount of oil the FSM says to add to the system when you replace each of the components, and add them all up. Here's what I got:

Compressor - 4.0 oz
Condensor - 1.2 oz
Dryer - 0.3 oz
Evaporator - 2.1 oz
Lines - 0.2 oz

Total - 7.8 oz

That approach may be incorrect, but I figured some extra oil wouldn't hurt. Does that number appear high for this system?

 

Thanks for taking the time to document this.

 

no in your other thread i posted similar specs. it is just that alot of the old oil is still in there. 8.5 is just more than i would do.

and damon Mac did or does sell some nice r12 to 134a adapters. i will get the part number tues at work if you are interested.

 

At what point does the oil begin to degrade performance? It displaces refrigerant, correct?

I've seen "A/C system flush" products, but haven't found much information about them, or how they're used. Is that how one removes all remaining oil from the system? Has anybody here used it, and if so, how does it work? Since I don't really know how much oil is already in the system, I'd like to start over from scratch to make sure I have it right.

It's 95F here today - the hottest it has gotten here in Virginia this year. The A/C just BARELY makes it liveable inside the car. Not blowing very cold, but as long as I keep the windows up and recirc on, and keep the car moving, I can stand it. And even though the fans are working, with the A/C going in this heat, the engine gets heat soaked very quickly if I'm sitting for a few minutes - temp gauge starts rising, and reaches about 210F before levelling off. Might go higher if I let it sit longer...

 

Yeah. Essentially you just turn your compressor into a vacuum pump by using the inlet. How much vacuum you can pull comes down to how healthy the compressor is but I routinely get more than 30".

Same issues I've had with the 134 conversion here in Texas. Since 134 is less efficient there is a point where the converted system hits the wall and won't cool well. On my car that seems to be somewhere between 90 and 95 degrees. Unless you're on the highway or the interior was already cool (parked in garage or shade) the converted system struggles. Recirc is a must. If the car was parked in the hot sun all day it will take it quite some time to cool the interior back off. I've found the quickest way to cool the car if it's been parked in the heat is to pop the hatch and let the a/c run for a couple minutes. This gets all the hot air out very quickly and cools the interior much faster.

I have a brand new compressor sitting in my garage. My a/c components are in good shape and leak free so I plan to install the new compressor and go back to R12. The system is currently just bareable when it's really hot out, but I want it cold, not merely bareable. The Texas summers are just too hot.

I tried Freeze12 and my results sucked. I followed all the directions and it cooled worse than 134a in my system.

Correct. Safe to say to merely run as much oil as you need. Running more will make the system less efficient.

The oil collects inside the compressor and the drier. If you need to remove the oil from the system you do so by removing the compressor and drier from the car and pouring it out. Purging the system or pulling a vacuum through the ports will not remove the oil, merely traces of it. You must physically pour the oil out to remove it. When adding oil from scratch just dump it into the compressor before installing it. Somewhere it will say how much oil the entire system holds. Pour that amount of oil right into the compressor after you have flushed everything.

I've always used lacquer thinner. With the compressor and drier removed fill the hoses full of thinner and then push it through the system with compressed air. I would not recommend allowing it into the compressor or drier.

 

I'd certainly be interested; thanks. The only ones I found before were fairly expensive. It would have made more sense to buy new gauges.

 

That's exactly how mine is, to a tee. At least it isn't just me!

What about the condensor and evaporator? Doesn't oil accumulate in them as well? The FSM does say to add some oil when replacing those components...

I did some research on the system flush, and it appears to be a lot of work to do it right - I'd like to avoid it if at all possible. I'm wondering how much oil may be left in the system after draining the compressor and drier, but without performing a flush. Then again, my system was open for more than a year, so it could probably use a flush anyways.

 

Actually, as far as flushing goes, can't you just disconnect one of the lines from the dryer, and blow compressed air into it? I could definitely see the condensor and evaporator not being flushed well by this, but most of the oil should be blown right out of the other line and into the dryer, right?

 

Anybody tried flushing the entire system with compressed air like this, or do you have to do it one component at a time? Is there anything wrong with blowing air through the compressor?