Jim Smart
April 9, 2015

We remember classic Mustangs most for hot summer drives, perspiration, and sticky vinyl. However, your nostalgic summer cruising does not have to be miserable if your climate-control strategy is up to par. It is April; time to get your air conditioning working before the heat and humidity arrive. And if you don’t have air conditioning in your Mustang, it is time to think about getting it before the Mercury rises.

Classic Auto Air has a number of solutions for both original factory systems and aftermarket in-dash and underdash systems for classic Mustangs, and we’re here to tell you all about them. Ray Sanchez of Mustangs Etc. in Van Nuys, California, is going to walk us through the fundamentals of classic Mustang air conditioning and what you need to know to keep cool all summer long.

Automotive air conditioning systems work on the principle of heat transfer via the continuous cycle of a low boiling point liquid to a gas to a liquid. Press the nozzle on a can of carburetor cleaner or engine starting fluid and feel the cold spray. The rapid expansion of gas and solvent from the spray nozzle feels cold doesn’t it? It feels cold because it is rapidly going from a liquid to a gas at a low boiling point of -21 degrees F. Refrigeration is not the process of putting cold in, but instead taking heat out. Heat always travels to a cooler medium—heat transfer—which serves as the basis for refrigeration.

How Does Air Conditioning Work?

Nearly any liquid can be used as a refrigerant, depending on how cool you want to be. However, we want the greatest efficiency from a refrigerant to be cool. Efficiency comes from how much heat a refrigerant absorbs in its liquid state and as it vaporizes, which means it must have a low boiling point when it becomes a gas. R-12, as one example, is the perfect refrigerant because it boils at -21.7 degrees F. That makes it plenty cold when it flows into your Mustang’s air conditioning evaporator coils inside the interior. R-12 absorbs a lot of heat as it makes its way through the cabin evaporator contributing to your comfort on a hot day. Cold evaporator coils cause moisture to condense on the coils, which drains through the evaporator drain onto the pavement beneath. If you live in a dry climate, rarely will you see condensation on the ground. However, in the Southern regions where there is high humidity, expect to see plenty of moisture from the evaporator drain.

Your Mustang’s air-conditioning compressor, which is an engine-driven pump, pressurizes the system. Refrigerant responds to the amount of pressure placed upon it by the compressor. The compressor pressurizes refrigerant as a gas (liquid cannot be compressed) in the receiver/dryer tank attached to the condenser in front of the radiator. As gaseous refrigerant flows through the condenser, it becomes a liquid under pressure. Hot refrigerant in liquid form flows to the expansion valve at the firewall where flow is regulated into the evaporator inside the air conditioner. As it is released into the evaporator it becomes very cold. As cold refrigerant leaves the evaporator, it becomes a gas, and flows back to the compressor. The expansion valve regulates the flow of liquid refrigerant based on temperature and pressure in the liquid line heading into the evaporator.

As refrigerant flows through the evaporator, it carries latent heat from the passenger cabin back to the compressor. You may wonder how something so cold can carry heat away. It has everything to do with system pressure and how refrigerant behaves under pressure. When refrigerant leaves the evaporator, it is actually boiling at -21.7 degrees F much like water boils at 212 degrees F.

Refrigerant has a much lower boiling point than water at -21.7 degrees F. Liquid nitrogen and liquid oxygen boil off into a gas at extremely cold temperatures, with nitrogen at -320.42 degrees F and oxygen at -297.31 degrees F. Ammonia, also used as a refrigerant in commercial applications, boils at -28.01 degrees F, which is very similar to R-12 and R-22. Our point here is simple: The cold temperatures in which these elements boil helps explain why refrigerant gets so cold in a liquid state.

Because your Mustang’s interior is considerably warmer than the refrigerant inside the evaporator coils, heat transfer to that cold refrigerant makes perfect sense. Ambient heat from the interior is carried out via the cold refrigerant where it is compressed and enters the receiver/dryer and condenser and is transferred to the air flowing across the condenser.

Why Won’t This Thing Cool?

Air conditioning stops cooling when all of the dots are not connected—and they must all connect. It isn’t always the loss of refrigerant that causes cooling woes. It takes the proper sequence of electrical switching for cooling to happen at all. At least three switches must close in succession to complete circuits before you will get compressor clutch engagement. If any segment of this circuit is open, you will not get compressor clutch engagement and it will not get cool. Electrically, here’s what must happen to get compressor clutch engagement:

- Fan switch
- Thermostatic switch
- Vacuum motor switch

If any of these switches are open, you will not have cold air. If there is a break or short to ground anywhere in the system, you will not have cold air. If you have an aftermarket in-dash climate-control system, safety pressure switches must also be closed. There is a low-pressure switch and a high-pressure switch, often incorporated into one. If there’s no refrigerant in the system, the low-pressure switch will be open. By the same token, if there’s too much system pressure (high head pressure), the high-pressure switch will be open. Cooling can also terminate if the expansion valve malfunctions due to a mechanical bind, ruptured diaphragm, or depleted capillary tube. The same can be said for the thermostatic control switch. If it fails to function properly, the compressor clutch will not engage.

Without refrigerant, air conditioning will not cool. Refrigeration mandates a completely dry environment under pressure void of any element aside from refrigerant. To achieve cooling, the refrigeration system must to be pulled down into a vacuum (20-30 inches of vacuum) for at least 30 minutes before refrigerant can be installed. If you have to discharge existing refrigerant, do so responsibly and capture it safely. Do not discharge refrigerant into the atmosphere.

Servicing Questions & Answers

Automotive refrigerant selection used to be simple. We had one, perhaps two refrigerant types: R-12 and sometimes R-22, the latter of which is more for commercial and residential air conditioning. R-12 was deemed environmentally unfriendly by the feds some 20 years ago and was replaced by R-134a, which is a different animal entirely.

R-12 is Dichlorodifluoromethane, an odorless, colorless gas also known as Freon 12. Freon is a name trademarked by DuPont. Because R-12 consists of chlorofluorocarbon halomethane (CFC), it was outlawed by the federal government and the Montreal Protocol in 1996 due to its risk to the ozone layer, meaning it could neither be manufactured nor serviced in the United States. R-12 has also been used as a propellant in aerosol cans. The use of R-12 in new cars ended in 1994.

Where the R-12 issue gets involved with classic Mustangs is whether to stick with R-12, which is expensive, or convert to R-134a, which isn’t as expensive. There are as many reasons to stick with R-12 as there are reasons to convert to R-134a. R-12 is a more efficient refrigerant in terms of cooling capacity. It cools better than R-134a and isn’t corrosive. However, R-134a is easier to find and use.

Converting your A/C system to R-134a requires the abilities of a licensed professional who understands how to do it properly. R-134a behaves differently than R-12, is corrosive, and will leak through conventional refrigeration hoses. Conversion must include R-134a compatible hoses. If you have an R-12 system that’s working properly and is void of leaks, there’s no reason to change over to R-134a. Even if your system has lost some R-12 refrigerant, recharge and press on. The best time to retrofit to R-134a is if you’re replacing a major component, such as the compressor, condenser, or evaporator. Hose, dryer, or expansion valve replacement isn’t sufficient grounds for conversion to R-134a.


01. Refrigeration is a simple process of circulating R-12 or R-134a through your Mustang’s air-conditioning system in a contaminant-free environment under pressure to transfer cabin heat to the outside air. The engine-driven compressor squeezes and moves gaseous refrigerant to the receiver/dryer and through the condenser ahead of the radiator. Gaseous refrigerant condenses into a liquid and moves toward the expansion valve (also known as a throttle/suction valve) where it is controlled and released into the evaporator inside the car as a cold liquid based on refrigerant temperature. As it leaves the evaporator, it turns back into a gas as it heads for the compressor, taking interior heat with it.

02-03. We know from examining high- and low-side pressure with the engine running and compressor engaged what the condition of the air conditioning system is. The high side (red) is compressor and condenser, where refrigerant is a gas and under high pressure. The low side (blue) is the suction side, where refrigerant turns into a liquid. Ray Sanchez is servicing an R-134a system here with the engine idling and compressor engaged. He is looking for normal high- and low-side pressures for the ambient temperature.

04. Gauge/manifold lines are connected via these quick-connects as shown on an R-134a system with a Sanden compressor. R-134a employs different quick-connects than R-12 does, so there’s no chance of mixing them up. An R-12 system will not tolerate R-134a without modifications and a complete system flush.

05. Ford shows us the proper way to evacuate and service a classic Mustang’s air conditioning system. There are two system service ports known as pressure (red) and suction (blue), also known as high and low sides. Refrigerant is serviced through the low side only—never the high side. System evacuation (vacuum) is performed through the low side only for 20-30 minutes before adding refrigerant. Ray Sanchez is servicing an R-134a system here with the engine idling and compressor engaged. He is looking for normal high- and low-side pressures for the ambient temperature.

06. This is R-134a being serviced into an R-134a system. As you can see, the canister has been turned upside down in order to deliver liquid R-134a quickly with the engine running and compressor engaged. Right side up yields a slower delivery because you are adding gaseous refrigerant. The red service valve must be closed.

07. This is R-12 being added to an R-12 system via a small 14-ounce can. R-12 is legally for sale only to licensed refrigeration professionals, though it can be found online through a number of sources including eBay.

08. Although we’re always promoting doing things yourself at Mustang Monthly, refrigeration has become a more complex science best left to a trusted professional who can service your Mustang’s air conditioning properly. The SERCON 9000 is a refrigerant recycling and recovery machine for use in professionally licensed shops. And this is the type of equipment you want to see for R-12 or R-134a servicing.

09. This is an adjustable thermostatic control switch, which cycles the compressor clutch based on evaporator temperature. When evaporator temperature reaches a given value set by the thermostatic switch, the switch opens and the compressor clutch disengages. This switch uses a sealed capillary tube and bulb filled with refrigerant to “sense” evaporator temperature. As evaporator temperature drops, pressure inside the capillary tube drops, exerting less pressure on the diaphragm and opening the contacts. As the evaporator warms, pressure within the capillary tube increases, closing the switch contacts and engaging the compressor.

10. Refrigerant loss happens when lines chafe like this one against a radiator support. Lines must penetrate all bulkheads without chafing. Refrigerant can leak at joints, in condenser and evaporator tubes, at the compressor, and at the receiver/dryer. Before your Mustang’s air conditioning is serviced in the wake of refrigerant loss, perform a leak check.

11. Here’s a York aluminum-piston compressor with an R-134a conversion as witnessed by these service fittings. You can run R-134a with your Mustang’s classic York or Tecumseh compressor. However, system modifications and flushing must happen first. R-134a runs hotter with higher head pressures. However, a healthy York or Tecumseh compressor should stand up to the difference in pressure. An aftermarket Sanden compressor offers greater efficiency.

12. The sight glass is used to observe refrigerant flow as gaseous refrigerant becoming a liquid on its way to the evaporator.

13. When you’re servicing a classic Mustang air conditioning system with R-12, this is how it is performed. Service valves at the compressor enable you to connect the high and low side manifold gauges to observe pressures, pull the system down into a vacuum, and service with refrigerant. Again, low side (suction) only for servicing.

14. This is a Bacharach H-10G refrigerant leak detector. Most automotive air conditioning shops should have a leak detector (sniffer) like this, which makes it easier to search out and correct leaks and eliminate loss of refrigerant.

15. Leaks can occur almost anywhere in an air-conditioning system. The best places to check are around the compressor (shaft seal and case joints), fittings and unions, condenser and receiver, evaporator, and expansion valve. If your system has O-ring seals and hasn’t been serviced in a long time, suspect O-ring seals, which can dry rot and crack.

16. Box temperature should be around 40 degrees F if your system is working properly. Keep in mind that when your Mustang is sitting still there isn’t sufficient airflow across the condenser in front, which will adversely affect box temperature.

17. Another issue that bites us is belt tension. Make sure the compressor belt is tight, meaning no more than a 1⁄2-inch of play either way. You don’t want too much belt tension, which can cause premature bearing wear in the compressor. Belt slippage can occur without it making a sound, so always keep it checked.

18. This is a Tecumseh cast-iron compressor on a ’68 Mustang. York compressors are cast aluminum, yet similar in appearance to the Tecumseh. Ford used a variety of both Tecumseh and York compressors in classic Mustangs through 1978. The Ford Master Parts Catalog will tell you which compressor your Mustang should have. The York is a lighter compressor thanks to its aluminum construction.

19. This is the Tecumseh cast-iron compressor. The Tecumseh is heavier than the York, yet more rugged. Which compressor is debatable. Both are proven compressors.

20. This '71-'73 Mustang vacuum diaphragm is about what you can expect to find on any '67-'78 Mustang air conditioning system because they're all the same, though controls tend to differ. The engine's intake manifold vacuum is vectored to a vacuum reservoir or canister through a check calve, which maintains vacuum in the canister. A vacuum line from the canister to the multi-position control valve gets the job done, depending where you move the control.

21. This is the vacuum-controlled hot water valve for ’67-’70 Mustangs. When you move the heat selector, vacuum is routed to this valve, allowing hot engine coolant to move through this valve to your heater core.


Major component replacement calls for a good system flush to remove contaminants. At that point you might as well switch over. If you’ve experienced a blown compressor, you’re going to have to replace the condenser, receiver/dryer, hoses, expansion valve, and the evaporator. When compressors fail, they can distribute contaminants throughout the refrigeration system. You can flush the system and hope those contaminants go away. However, it is a gamble because flushing doesn’t always remove all contaminants. This is why complete system replacement removes all doubt.

To convert to R-134a, here’s what you have to do:

- Flush all mineral oil (compressor lubrication) from the system
- Replace the receiver/dryer, which cannot be purged of mineral oil
- Replace all O-rings, if so equipped
- Install a high-pressure shutoff switch and wire it between the compressor and compressor clutch power source
- Change the orifice tube and expansion valve for compatibility with R-134a pressures and temperatures
- Service the system with PAG refrigeration oil
- Some systems will need a larger condenser because R-134a isn’t as efficient as R-12. R-134a is 80-90 percent less efficient than R-12
- And while you’re at it, plan on replacing all hoses and lines with pieces compatible with R-134a

The law requires the installation of R-134a service fittings to prevent accidentally servicing the system with R-12

Although you might be tempted to do an R-134a conversion on the cheap at a shop or in your driveway, don’t get suckered into this approach. Because the chemical makeup and operating pressures of R-134a are different than R-12, there’s no cheap way to do this conversion properly. The above steps must be followed. Try to mix R-134a with R-12 and mineral oil and you will experience compressor failure and unwanted expense. Don’t fall for it. The air conditioning aftermarket is fond of drop-in replacements for R-12 systems, including propane and a host of other risky substitutes.

Understanding Climate-Control Function

If you have a factory in-dash heating and air-conditioning system, you may be stumped as to how the vacuum-operated control system works. When you slide the function lever to a given mode, such as MAX, FRESH, VENT, HEAT or DEFROST, you are using engine intake manifold vacuum to bring about each function. The approach to function varies from ‘67-’78, but basic principles are the same from year to year.

The control slider is tied to a vacuum selector valve (all years), which channels vacuum to a given vacuum motor or actuator. The motor/actuators move a series of air doors to get heat, defrost, vent, or air conditioning. A vacuum reservoir located in the engine compartment or right wheelwell stores enough vacuum (negative pressure) to allow abundant vacuum function in all kinds of driving. A vacuum check valve between the engine and vacuum reservoir enables the vacuum system to hold pressure even with the engine shut off or in wide-open throttle conditions. A key to proper function is to have a strong vacuum signal and all hoses connected to the right destinations. Each hose is color-coded, making it easier to get them connected to the right spot. Each vacuum hose kit from Classic Auto Air is color-coded for easy installation.

Operating hand in hand with the vacuum selector is the heat control cable, which controls cabin heat level and hot water valve function. When you slide the lever for heat, vacuum is directed to the hot water valve, which allows hot coolant to flow through the heater core. As you slide the control for more heat, you’re moving the temperature blend door to allow a balance of hot and cool air to keep temperature consistent.

There are four vacuum motors—A/C-heat door for feet or dash, restrictor air door, outside air/recirculation door, and heat/defrost door. These motors can fail to function, which is often the motor diaphragm that tears and there’s a vacuum leak. Doors can bind too. You can spray a light amount of silicone on the door seals to free them up if they stick.

The fan switch controls both blower and compressor clutch function. If the switch is in the off position, you will not get compressor clutch engagement. Fan switch function isn’t one of variable resistance. It is a three-position switch that routes current flow from a variable resistor at the heating/air conditioning plenum to ground to complete the circuit, making blower operation possible. If you lose a fan speed, it is either the fan switch or the variable resistor at the plenum. It is an easy fix.

Just One More Thing …

Auto air conditioning is only as effective as airflow across the condenser. This means your Mustang must have an efficient radiator cooling fan and shroud designed to haul healthy amounts of air through the condenser for successful heat transfer. Although we see all kinds of cooling fans used in air-conditioned Mustangs, the best type to use is the humble thermostatic clutch fan with as much blade as you can cram in there.

Thermostatic clutch fans operate only when they are needed. They are quiet and very efficient. And they move huge quantities of air through a radiator and condenser, and when you’re at freeway speeds, a clutch fan is along for the ride because it isn’t needed. If you have a noisy and inefficient six-blade steel fan or flex fan, you’re not giving your Mustang’s air conditioning the cooling capacity it deserves. Determine proper fan size and use a shroud. Fan penetration into the shroud should be one-half of the fan’s width.

Electric cooling fans are also a good idea because they come on only when they are needed. You just have to determine the best fan for your application. Flex-A-lite’s website is the best place to find fan-sizing information.