Z-car Air Conditioning FAQ

References:

Haynes Automotive AC manual (sometimes confusing, typical of Haynes)

Section in Nissan 280Z shop manual for 1977 (Watch out for japanese to english translation problems)

"Modern Air Conditioning, Heating, and Ventilating", by Carrier, Cherne, Grant, and Roberts (1959) (A good combination of theory and practical applications, but nothing specific to automotive applications in general or Z-cars in particular)

"Refrigeration and Air Conditioning", by Stoecker and Jones (1982) (A classic college text, but heavy theoretical analysis-if the terms "isenthalpic expansion" or "isentropic compression" are unfamiliar, this text may be difficult to read)

A1. Evaporator freezing occurs when the moisture present in the air as humidity freezes in the airside passages of the evaporator and blocks the flow of air. When airflow is blocked, the thermoexpansion valve controlling the flow of refrigerant into the evaporator will close, decreasing the pressure and thus the refrigerant temperature, which then makes matters worse. Moisture is not the real culprit, although of course if there were no moisture present in the air, no freezing could occur. Moisture will always be present in the air flowing across the evaporator, either from the outside air or from sweating and breathing. A good evaporator design will condense the moisture, and reject the liquid outside the car.

Two possible methods of avoiding freeze-up are to ensure adequate airflow across the evaporator so that the air temperature never drops below freezing, or control the refrigerant pressure so that the refrigerant temperature does not drop below 32F, thus ensuring that the air temperature doesn't drop below freezing. Also, if the expansion valve has a cap tube, it needs to be tight up against the suction line at the evaporator refrigerant outlet.

For the first method, a thermostat bulb is placed in the evaporator with the setpoint above freezing, so that the compressor clutch is disengaged when the evaporator air temperature drops below freezing, while the fan is left on to circulate air and warm the evaporator. That is the way most Z A/Cs work. One problem is that the thermostat can be set below freezing. The solution is to back the temperature setpoint back to above freezing. Make sure the probe is in the coil and not pinched. Another problem is that the thermostat fails altogether, where replacement is the cure. A practical, but not elegant solution would be to put a switch (or relay controlled by a switch) to cycle the compressor clutch manually while leaving the fan on to circulate air and melt any frost.

For the second method of controlling the evaporator pressure, either a suction throttling device or a simple pressure switch can be installed on the suction line connecting the evaporator outlet to the compressor inlet. A suction throttling device works by restricting the flow to the compressor so that the refrigerant pressure in the evaporator (and thus the temperature) is greater than freezing. GM calls their suction throttling device a POA (pilot operated absolute) valve on their older models -- they are built into the suction accumulator on later model units. The Ford suction throttling valve, because it is longer and thinner, is easier to install. This valve will absolutely prevent freeze-up under any condition. A simple pressure switch installed on the suction line that cuts out the compressor clutch when the refrigerant pressure drops below the pressure corresponding to freezing would also work. Installing a suction throttling device or refrigerant pressure switch can be considered as major surgery on the refrigerant lines, and perhaps other fixes may be easier.

Now to answer your question about a 72 Z-car. First off, Z-cars did not have factory AC until 74, so the AC on a 72 is an aftermarket unit that mounts under the fan on the passenger side. These things are designed to fit tight against the normal air inlet to the heater fan. The evaporator is sucked through. Suck-through evaporators are generally bad news. Especially so in Zs. A major problem is that the evaporator rarely fits tightly against the fan housing. With age the weatherstripping seal deteriorates. This situation will result in inadequate air flow through the evaporator, since some air will not have flowed through the evaporator, but flowed through the gaps. This is a common problem, and the first thing to do is seal the air flow leaks. The most satisfactory solution is to pull the evaporator, gom the sealing surface with either duct putty or RTV and reassemble. This will seal the interface. One other suggested fix is to install a 280Z blower motor and fan in place of the 72 OEM model. According to one source, a 280Z blower will move more air, but another source refutes this. YMMV. John DeArmond (JGD) installed a voltage inverter that puts 16 volts across the blower that increased the airflow, and also increased the noise. Another suggested plausible but unproven idea is to install a helper fan at the evaporator air inlet, electrically connected to the existing blower.

A2. I had the same thing last fall on my 78. I listened around the engine, but could never pinpoint it. Really annoying, but finally found that some of the vacuum lines over against the passenger side strut housing has some split ends, causing the strange noises. Sounded more like it was coming from inside the engine. As the hoses get old, (heated, cooled, etc) they crack at the stress points, where it is stretched over the connections. This made the strange vibration sound.

A3. You have a vacuum leak in the damper system. Check the vacuum hoses from the manifold to the solenoids on the right fender through the firewall and under the dash. Common for the small rubber hoses to crack in the engine compartment where the heat gets to them.

A4. What you have to do depends on whether the system is still closed, intact and at least partially charged. If it still has some charge in it, simply topping it off is a good first step. If it is empty or is open, it will need at a minimum, a good evacuation and purge. Check for leaks by looking for oily, dusty spots around the system components, particularly the hoses and fittings. The condenser can have leaks caused by impacting road debris, such as gravel. Refrigerant leaks will also carry some compressor lubricant, and dust will collect at the oily leak site. It also probably ought to get a new receiver/dryer. Check the oil level in the compressor.

A5. These cars have a temp switch which keeps the a/c compressor from engaging if it is too cold outside. The fan will still run but no cold air. The normal temp switch which lets the car warm up before starting the heater also controls the fan.

A6. It is very important to pull a good vacuum for two reasons: moisture in the system will freeze and block refrigerant flow, probably in the expansion valve, and even the smallest amount of residual air will decrease the heat transfer in the condenser and cause poor system performance. Pulling a good vacuum (29 in. Hg minimum) will dry the system by causing the moisture to evaporate at room temperature, and will remove the non-condensible residual air from the system.

I have rented a good refrigeration vacuum pump (two stage) from a Redi-Rental a few years ago, but I'm not sure if anyone can rent them now (the rental clerk did ask if my refrigeration system was open, which it was, before he would rent it to me ($20/weekend). New ones are $200+ the last time I looked. Others (JGD) have made their own homemade vacuum pump from a salvaged home refrigerator compressor. The refrigerator compressor is handy because after you attach a couple of 1/4" SAE flare fittings to it, you can also use it to recover your R-12 to a storage tank if you ever have to open the system again. Harbor Freight sells a venturi pump for about $30, claiming 29.5 in. Hg vacuum at 4.5 SCFM and 90 psi. I bought one of these, and according to my refrigeration gages, only gets to 27 in. Hg and really keeps my 5 hp 2 stage compressor busy. IMHO, this venturi pump is not good enough for automotive air conditioning applications. However, JGD has one of these venturi pumps, and gets 28.5 in. Hg vacuum with it. He finds it useful if there is the possibility of contaminated refrigerant in the system, preferring to not expose a more expensive refrigeration vacuum pump to unknown and possibly unfriendly chemicals.

A7. You could put the receiver/dryer in the oven (conventional not micro- wave) and bake it out over a few hours. When I did my retrofit of a 280Z compressor and condenser to the aftermarket system with suck- through evaporator in my 72 240Z, the new receiver/dryer was the cheapest part ($10). This was just too cheap to do anything but discard the old one. The new receiver/dryer was a generic model, and not a Nissan dealer part. I imagine that a Nissan OEM dryer would have been more expensive. Good refrigeration system practise is to replace the dryer if the system has been open for any period of time.

Testimonial #1 - Does window tinting help decrease the heat load?

I am also using window tinting to try to lower the heat load on the A/C. I have a 35% gray film (3M brand) on the side windows (this is the maximum legal tint for side windows in my area. Some parts of the country do not allow any tint on the side windows adjacent to the driver or passenger), louvers on the hatch glass and titanium film on the windshield (tint on the windshield is not technically legal around here but just about every window tint installer I talked to was more than willing to do the job). The window tint helps the heat, but is not a cure all. The dealer add-on A/C still doesn't cut it. According to friends living in the DC area, window tint on the driver, passenger, or windshield glass is not legal in your area (or at least in Virginia). Check into it before getting it done. If you want to maximize the light transmission through the car, go for the titanium window films. They are more expensive than gray tints, but they are worth it. There is little light loss through this type of film, but it blocks 90+% of the UV. The heat reduction is quite noticable. The windshield is the greatest source of heat, so I recommend tinting it also. I have not noticed any visibility problems with the windshield tinted. Just DON'T use anything but titanium film on the windshield. Gray films do cause some visibility problems at night so save them for the side windows only. My last recommendation on this subject to to have a professional installer do the job. If you have ever installed window film before, you will know why. It takes alot of practice to do a clean job with no trapped debris, air bubbles, and clean cut lines. If you do it yourself, you will probably go through several attempts until you are happy with the job. In the long run, you probably won't save much money. Also, the rear side windows are very hard to do. They are convex shaped and most installers will want to use two pieces of film (seam in the middle). They can be done in one piece, but it takes alot of work. The film has to be worked smooth while the adhesive dries. This took about 1 hour per window on my Z. I doubt my installer will ever volunteer to do one of those windows with one piece of film ever again.

Testimonial #2 - Aftermarket 77 280Z swapout to a 75 280Z factory system

Unlike JGD, I replaced my aftermarket 'add-on' A/C in my 77 280Z with the factory air from a 75 280Z. I bought the 75 wrecked for the engine, and I thought it a waste to dump the A/C components. Like y'all, I was thoroughly disgusted with aftermarket A/C, especially the kludge job they did on your stock ventilation system to force air through the evaporator.

That aftermarket system just couldn't handle the Pensacola humidity and 90F plus temps. My previous experience with Datsun A/C systems is that the factory air is a super-simple modular-type installation; the factory bone stock wiring harness already has all the connectors for A/C, the firewall is already punched for A/C hoses, the alternator is the same, etc. The deltas are the radiator, condensor, ventilation control panel and the entire black plastic unit behind the radio and vent cluster. You will find that the whole mess just comes out in one piece, and the A/C- equipped system just drops in to replace it. I can give more details, but you can add factory air to any Datsun that originally had a factory option for air - the hooks are all in the car for you.

Total time? It's easier with the dash out, but you should be able to pull the old ventilation unit out and replace it within 6 hours, counting lost time in finding all the mounting studs, etc. Hookup in the engine compartment is a cinch, and the radiators interchange just fine (welded nuts and breakaway sheet metal already done for you). If you have your own vacuum pump and refrigerant supply, you can start in the AM and have A/C operating by sundown.

Steve Sovaiko, IZCC 105, Dayton OH

Testimonial #3 - Compressors and aftermarket system assembly

I worked my way through college (1974-1978) doing auto A/C installation and repair at a shop in Los Angeles. I've never seen Motorsports stuff, so some of this may be off base, but anyway here goes:

(question on the net) This is the guy who started it rolling w/the remarks RE: Motorsport radiators. The reason for wanting a big radiator was because I was also reinstalling A/C in the car. When we first got the car many years ago, it had a reciprocating compressor (dealer installed), which put cyclic loads on the nose of the crankshaft-and eventually widened out the keyway for the crank pulley. The only real cure is a new crank, and constant checking of the torque on the crank pulley assembly. At that point, we'd moved to Wisconsin and removed the A/C with installation of another engine.

We fixed a lot of Z's with this malady. I'm not sure the York compressors were to blame so much as poor aftermarket installation in most cases. The center bolt on a Z holds on the pulley, the slinger, the distributer drive gear and the timing gear, each of which is keyed and slides onto the crank. If all these aren't clean and the center bolt isn't really torqued down, the crank pulley will work loose. Our theory was that the installers slapping these things in were just hitting the crank bolt with their impact wrench rather than a torque wrench.

On the other hand, sitting on my desk here is a crank pulley from my own 260, with a small crack right down the keyway. This was a factory system with the "rotary" pump (isn't it a swashplate design a la GM?), yet it cracked also. I noticed the replacement pulley was machined a little bigger than the crank at the back (kinda like a counterbore effect). I guess that any crud on the crank would wind up in this space, rather than mashed in between the pulley and the oil slinger, maybe keeping things tighter. At the time, the parts counter guy couldn't explain it, he just told me that all the new ones were that way.

Anyway, our fix usually consisted of a new crank pulley (cast, not sheet metal), a new key, and sometimes a larger centerbolt. We bolted all this on, and then using a template drilled 3 set-screw holes right in between the crank and the pulley, and secured them with heavily loctited allen head screws. We charged a couple hundred to do this, mostly on cars brought to us by the local dealer (downtown L.A. motors). We never had one come back, so I guess it worked.

(question on the net) [ stuff about motorsports a/c deleted] Anyway, if you install A/C on an early Z, make sure you use a ROTARY compressor and NOT a recip. Your crank will love you for it.

The rotarys definitely vibrate less, making them less annoying to run and less likely to break things. But I think the rotary mount kits for the old z's required moving the smog pump, etc. More time-consuming and more things to take apart/put back together on that side of the motor. We never used them and rarely saw them, except as factory installs. Some shops of the era (aftermarket A/C was a pretty big business then) paid mechanics on commission, encouraging them to rush through installations as fast as possible.

Walter, walter@inmet.com, '74 260-Z undergoing restoration, IZCC #112

Testimonial #4 - John DeArmond comments on refrigerants (6/23/94)

(question on the net) I was wondering what other z owner's are going to do about their airconditioning with the new laws?

Deja Vu all over again. I posted this to the BMW list just yesterady.

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(question on the net) I've got an A/C system for a 2002 sitting in my garage that will probably never get installed. The reason is that I don't want to deal with the R12 thing in the future. According to a guy hear at work that knows a great deal about refrigeration systems, the older style systems (up to the mid '70s I think) have an expansion valve setup that won't work with the new R134.

This not true. R-134a works a bit better if the superheat is adjusted via the hex screw inside the valve exit port but that isn't absolutely necessary. And if you do decide to change the valve, it is only about $20. BFD. Of vastly more concern for ANY system converted to 134a is the incompatability of the PAG oil with even traces of old refrigerant. Supposedly other types of oils that are compatable with traces of old refrigerant are under development but the only ones shown to date, the esters, are so sensitive to moisture that the moisture typically present in the dryer will cause it to break down. Plus it is a lousy lubricant. As of now, you MUST change everything that is porous and has been in contact with the old refrigerant. That includes the hoses, the dryer, all O-rings and seals and the evaporator if it has any elastomer seals. And as a practical matter, a compressor replacement is in order unless you want to take it completely apart to scrub the components and replace the gaskets.

Conversion is really a side issue because R-12 is going to be around for a long time. The Bush admin EPA was shilling for big industry as represented by MACS and SAE. The Clinton EPA is taking a much more pragmatic approach (strangely enough), particularly after the industry was caught lying about the ease of conversion and the reliability of 134a systems. They have already pushed back the deadline to stop production of R-12 by a year and the feeling is it will be pushed back further. Plus there are alternatives. R-406a, formerly known as GHG-12, developed by George Goble (ghg@ecn.purdue.edu) is an exact drop-in for R-12. Banned at industry request under the Bush admin, it was re-examined under the Clinton administration and given approval for everything except mobile use. The exception is strictly a political move in that the EPA doesn't want to admit all at once how bad it screwed up so the mobile exclusion will go away within the year. This blend has over 4 years' testing in thousands of vehicles and works fine. It even improves the cooling capacity of systems it is used in. Meanwhile, you can buy this blend from People's Welding in Indianapolis (try 800 info for the number - I don't have it handy) and once you own it, you can use it as you wish, including in your car.

Another alternative, OZ-12, was banned because MACS stirred up media hysteria because it is flammable (mix of propane and isobutane). A rational evaluation of the risks (a pound of this stuff right beside a hundred lbs of gasoline) was never considered. Reliable rumor has it that OZ-12 is about to re-appear with a fire supression additive that makes it non-flammable. Like 406a, OZ-12 slightly improves the performance of the system and is completely compatable with and can be mixed with R-12.

Finally, if all else fails, you can mix your own refrigerant using isobutane (NOT butane)