Air conditioning can be a cool thing—pun intended. When your vintage Mustang is so equipped, especially in the deep south, where summertime temps can reach the mid- 90s and higher, it can make the difference between "let’s go out" and "never mind." Of course, if you ask southerners about the summer, they’ll tell you it isn’t the heat; it’s the humidity. The Select-Aire air conditioner of yore did a great job of cooling the cab and removing the humidity—when it functioned properly.
This story is about diagnosing a system with all its components, all in seemingly good working order: belts in place, compressor in place, and all lines attached. Obviously, if the car has a "bald spot" where the compressor is supposed to be, much of what we say won’t apply—but some of it will.
The car we are using is a ’71 Mach 1 with a 429 CJ and C6 automatic. This car’s air was working when it was bought but recently quit—no compressor function. So we contacted Al Sedita at Classic Auto Air and asked him to go over the car with us and recommend some checks the owner can make on his or her own—possibly saving a buck or two in the process.
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Musing about what to do first are Dana "The Cheater" Chetham (white T-shirt and legs), Bob "The Big Kahuna" Myhrer (next to Dana), then left to right, Curtis Clark, Doug Greshian, and Mike Fleshman.
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Our 429 CJ is complete and has all the components for the A/C to work-but it doesn’t. We suspect an electrical problem.
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Al recommends a good set of gauges for the type of refrigerant you have. If you have 134A, you can get the couplers and use your R12 gauges.
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The right tool for the job. The guys at Classic Auto Air use a specialized wrench to undo the valves. This shot is an example. You should have your gauges hooked up before you do this. The tool prevents the flats on the valve from being damaged. If you’ve used the wrong tool and chewed up the threads on the high-pressure hose, Classic Auto Air has the correct valves to replace them. Just be sure that you evacuate the system before replacement.
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By law, if your system has been converted to run 134A, a sticker should be on the compressor or displayed prominently in the engine bay. Classic Auto Air uses this machine to detect the type of refrigerant in the system. Ours was pure R12. They have seen cars with grab bags of refrigerants. One car had Propane, R12, and 134. This situation could be bad news for a shop during recovery since any of those three will contaminate a tank of refrigerant. With R12 running at $29 for a 12-ounce can, the results could be hazardous.
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First Doug checks for a good connection at the fuse block. A test light is essential for this. You might be surprised by how many cars are OK aside from a blown fuse. Doug recommends checking the service manual for the correct size fuse. The ’71-’73 Mustang uses a 30 amp fuse. Our fuse link was fine, so we moved on.
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From the underdash fuse panel, we turned the ignition to the on position, activated the A/C, and moved to the compressor-switch link. Trace the black wire from the compressor back to the link, and separate. Use the test light to see if there is power. Our car showed none.
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Doug also checked the link at the firewall. On the ’71-’73 Mustangs, the male and female plugs are both green. The plug had been rat food at one point but showed no frayed wires. The female side showed no power.
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Doug then removed the glovebox inside the Mach’s cab to check the thermostat switch on top of the case. You can see two wires plugged into a silver block; the block is the A/C thermostat or de-icing switch—no power here either. At this point, both Doug and Mike Fleshman suspected the clutch switch on the control panel.
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After partially removing the panel, the clutch engaged. Still, Doug felt we needed to replace the switch, since it was an intermittent problem. Doug suggested bending the female clips to tighten the fit and improve the connection. We did this, but the operation was still intermittent.
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We dumped the old switch for a new one from Classic Auto Air. This solved one problem but led us to the other checks you can do at home.
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We’d not yet checked for a condenser with separated coils. Since the fins cool the system, if they are separated, they can affect the efficiency of the A/C. One row won’t hurt you so bad, but multiple rows will.
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Al also told us that the fittings could show tell-tale signs of problems. Because there is oil in the pressurized system and the pressure can push oil out around damaged O-rings or fittings, there may be a dark, sooty film on the fitting. This film is composed of debris from the road and engine bay. This leak may be slow or quick, but eventually it will rob you of freon.
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The valves can also leak. Again, they’ll have oil on them, but the debris may be less if the caps are still on the system. These are available from Classic Auto Air.
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Check the hoses for pinching like we see here on our test car. The high-pressure hose has been pinched and crushed by an aftermarket hose. Though this one doesn’t seem to leak, hoses like this should be replaced. Check the hoses for damage at the connections and along the passenger side as well as over the exhaust manifolds. Classic Auto Air has new hoses that look and feel like the originals.
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One thing missing from this system is a sight glass, an effective tool for checking an A/C system with R12 for correct function. On correct systems, you should be able to see the refrigerant and air bubbles moving past the sight glass.
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Check the heater hoses and heater valve for correct function. Sometimes the valve inside the heater hose can be damaged, allowing too much coolant (and thereby heat) to enter the heater core, cutting cooling efficiency.
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While in the region, look at the pulleys. Al recommends that you take the belts off and turn each A/C pulley. If the pulley drags or feels like it has sand in it, it should be replaced.
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Check all the blend doors for correct function, and consult the service manual for the function of the doors.
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Our Mach 1’s vacuum motor on the restrictor door was bound up and causing problems. Doug freed the malfunctioning door, causing it to function correctly again.
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The Mustangs’ restrictor doors get rusty due to cowl-vent leaks and bad heater cores. The only real fix is to disassemble the A/C box, bead-blast the door and rod, and paint the assembly.
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Al recommends checking the vacuum reservoir for its ability to hold a vacuum (one of its functions). This is especially important if you have a Hi-Lift cam with a low vacuum reading. The can keeps a vacuum when operating properly. You can check this by moving the selector switch after the car has sat with the engine off for a half hour or so. You should be able to hear a hiss as the A/C selector switch is moved back and forth. If you don’t, consider replacing the can.
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Here are the corrected A/C lines and a new bracket from Classic Auto Air. Doug put new O-rings on all the fittings, set all the hoses in place, and put tension on them once they were in the correct position.
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After we installed the correct hoses, Doug evacuated the system and put the new R12 freon into the car. Note the fluctuation on the gauge above. This is a sign of a bad set of reed valves. Though not serious enough to cause major problems, the compressor could stand a rebuild. If the compressor were new, the reading would be solid.
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Mike leak-tested the system and found it to be sound. Sometimes the test can net a bad evaporator, especially if it is an original aluminum one. This would require the system to be drained and the evaporator core replaced. Fortunately, ours was in good condition, and we got 40 degrees at the vent from our new hoses and original system.
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Late in the day, we did the final test. Dana "The Cheater" looked under the car to make sure the system was draining. It was, and all was right with the A/C world.
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