With cruising season upon us and our Shoestring C5 back in action after an extended hiatus, we decided to devote this month's installment to addressing a couple of irksome driveability issues. The first had to do with the air-conditioning system, which seemed to have lost much of its chilling power since we bought the Vette in the summer of 2006.
Before we get to the repair, let's take a look at the system itself. As a newer vehicle, our C5 uses what is known as a V7-style compressor. A V7 uses a variable-displacement design that matches air-conditioning demand by changing the compressor stroke, rather than cycling the compressor clutch as in a conventional system.
In a V7 compressor, a control valve located in the rear head of the unit senses the low-side pressure, causing the compressor mechanism to perform this stroke. The V7 compressor is therefore constantly running, and the system does not cycle. Consequently, the diagnostic procedures differ from those used on the older, fixed-displacement systems.
Because of the complexity of the diagnosis and repair procedures, we once again relied upon the Corvette experts at Bill Buck Chevrolet's Corvette Connection, in Venice, Florida. Lead Corvette Tech Jamie Bogdas began by running some preliminary visual checks of the A/C fuse and electrical connections, and also tested the operation of the cooling fans. He looked for any debris that might have been blocking air flow to the condenser, then tested the operation of the A/C blower.
With everything working within factory specifications, Bogdas moved on to perform a system performance test. This procedure involves idling the engine with the A/C control head in the "Upper" mode, the blower speed set to high, and the temp at full cold. Operating the system at these settings allowed him to ascertain whether the compressor clutch was engaging. It was, so Bogdas turned off the ignition and connected a pressure gauge to test for low refrigerant. The idea was to see if both the low- and high-side pressures were equal and within thefactory-specified values (30 to 50 psi). They weren't, leading Bogdas to diagnose a system leak.
After evacuating and reclaiming the refrigerant in the system, it was time to run a vacuum test. By sucking the system vacuum down to 30 pounds and watching how long it held pressure, Bogdas was able to determine the approximate severity of the leak. Ours held for over 15 minutes, indicating a slow leak and implicating something other than the evaporator or compressor (the two areas where A/C leaks usually form).
When an A/C system is not operated for a long period of time, the seals in the system can dry out and shrink, allowing refrigerant to escape. Suspecting this was our problem, Bogdas recharged the system with the factory-specified 1.63 lbs of R134a refrigerant, adding an infrared-visible dye to help him trace the source of the leak.
Next, he inserted a thermometer into the center vent and idled the car for approximately 10 minutes with the A/C on. Turning the system on allowed its high pressure to increase to between 200 and 250 psi, forcing out any refrigerant that might have been leaking at a weak point. When the engine was shut down, and the high and low pressures started to equalize, the dye-laden refrigerant should have begun to seep out.
With no dye evident, Bogdas concluded that our system's seals had simply dried out, and that the fresh charge of R134a and lubricant in the system would restore them to good operational health. Considering that the car's A/C has performed flawlessly since then, and that a follow-up dye scan revealed no previously overlooked leaks, we're confident his diagnosis was correct.