Mark Houlahan
Tech Editor, Mustang Monthly
April 1, 2000

Step By Step

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The diagnosis was typical for Fox Mustangs. The evaporator was leaking and so were several hoses and seals. A full overhaul will be required.
There are several “checks” that you can make before you take your Mustang to an A/C repair facility. The A/C pressure switch is mounted at the top of the dryer assembly and prevents failure of the compressor when the refrigerant charge drops below a certain level. If the A/C suddenly stops working, this switch can be at fault.
Disconnect the pressure switch and, with the engine running and the A/C turned to “Max,” jumper the two connector terminals together. Once the connector terminals have been jumped together, the A/C compressor clutch should come on. If it does, either the pressure switch is bad or you have a low-refrigerant charge. If the clutch doesn’t engage, then you have no power going to the pressure switch, or there is a problem between the pressure switch and the compressor clutch—more on that in a minute.
Check the compressor clutch for any damage. During years of use, the rubber damper in the clutch disc will break away, thereby preventing the clutch from properly engaging. Replacing the clutch, pulley, and field coil—which pulls the clutch in when energized—is a simple repair that is accomplished with a set of snap-ring pliers and a few metric tools.
Using an ohmmeter, you can determine if there is a short or open in the wiring from the pressure switch to the compressor clutch. Double-check this diode found in the clutch harness behind the compressor to ensure that the diode is still functioning by only allowing power to feed one way. The clutch connector should be inspected as well since it can be broken from mishandling, and cause intermittent operation of the clutch.
The A/C control panel is another place to check for power to the pressure switch. After removing the pop-off trim cover, remove the four screws retaining the controls, and check for power to and from the control-knob connector. You also can check for power to the blower motor and for any damage to vacuum lines here.
If your A/C seems to be working but isn’t blowing cold enough, or starts out cold and warms up the longer the car is running, then you need to check your manual temperature door cable and adjust it. The problem is the temperature-blend door is not fully sealing out the heater-core hot air. Push the door lever to close it, and adjust the cable to match.
If your A/C blows cold but doesn’t come out the right vent, or moves from the dash to the defroster on acceleration, then check out this vacuum check valve on the firewall. The line from the vacuum tree enters the side, and the lines going into the bottom are for the vacuum reservoir (left line going in to the right inner fender) and to the main vacuum controls (right line going into the firewall). Several problems could be here, including a leaking reservoir (possibly from an accident or age), a bad check valve, or weak vacuum from the engine.
Other vacuum problems could be under the dash as well. Check the vacuum connectors to ensure all the lines are firmly seated, and check the vacuum motors for leaks or bad diaphragms. You can also see the blower-resistor block mounted into the HVAC case, too. If your fan works only on “high,” then check the resistors for failure.
We find it hard to believe how many people don’t use their air conditioning efficiently. On a hot day when all your windows have been rolled up, you should start your A/C on “Norm A/C,” with the temperature maxed and the fan on high. Then crack the windows a few inches, and let the cold air push out the hot air. Using “Norm A/C” puts less burden on the system by cooling outside air instead of cooling the extra hot inside air. After a few minutes, simply turn the control knob to “Max A/C,” adjust the fan to your comfort level, and roll up the windows.
These core support decals let you or the technician know the proper serpentine belt routing...
...and the proper refrigerant charge for your A/C system. If yours are missing, they can be purchased from most late-model parts dealers.
Next to the evaporator, the most common leak point on the late-model Mustang A/C systems are the sealing O-rings found at various connection points. The worst of those have to be where the condenser connects to the rest of the system by the air filter housing. These connections see a lot of vibration and twisting with the engine moving around, thus they tend to leak earlier and more rapidly than other O-rings in the system.
Another area not quite as common but that should still be checked by you or the technician is the end crimp of any rubber hose. These rubber hoses are crimped to the metal tubing via a press, and the ends can deteriorate with age, heat, ozone, and so on, and cause a small leak that will drain a system of its refrigerant over the course of the year.
After all our diagnostics led to the agreement that we had a low refrigerant charge, we visited McGee Goodyear to have one of its technicians partially charge the system and check for leaks. Using an electronic refrigerant detector, the condenser was checked first. No problems arose there.
Next up were the condenser O-rings. A slight leakage was detected but nothing off the scale.
The hoses and compressor areas were tested as well, and the detector located a major leak at the compressor manifold. The manifold is sealed with more O-rings, so right now we are looking at nothing more than O-rings and a new dryer, but the evaporator has yet to be tested.
Lastly, the technician went inside and cranked the blower down to low and turned the key on. The shrill pitch of the detector tattled on the evaporator. We will have to “crack” the dash to pull out the evaporator. So, the final score is the evaporator, dryer, liquid line (with fixed orifice inside), and an O-ring kit. If you get started on yours now, and get all the parts rounded up, you can join us in an issue or two when we actually dive in and make these repairs. Stay tuned.

Late-model Mustangs—actually, all Fox-platform vehicles—seem to have a common problem with their air conditioning systems having catastrophic failures of their subsystems. The reason an air conditioning system blows warm air is a lack of refrigerant. A leaking O-ring or O-rings sometimes cause this lack of refrigerant, but more often than not, the real culprit is a major part, such as the evaporator or condenser. Replacing these parts can be expensive, but you can save yourself some money by completing your own repairs. No special tools are needed for any of the repairs, except for inexpensive spring-lock release tools, so repairs can be made in your garage with your own handtools. Most parts can be bought in aftermarket parts circles, or you can go straight to Ford for all your parts needs.

The only real problem for the do-it-yourselfer is detecting the problem in the first place. While you can purchase a refrigerant detector to test for leaks, the cost of the tool is prohibitive for one-time use. A better suggestion is to take your Mustang to a reputable repair shop or A/C service specialist and have them diagnose the system. Ask for a detailed list of items that need replacing, and don’t be afraid to ask them to point out where some of these items are—though our underhood detail photos should answer all your questions.

We opted to visit a local tire store that does most of our tire mounting, balancing, and alignments for the various company magazines based in Lakeland, Florida. The store, McGee Goodyear, was quite convenient for us, and we knew we could trust the folks there. Ask around for local repair facilities that you can use. Once we have our diagnosis written down and our parts ordered, we will tackle the repair in a subsequent issue.