The static pressure I measured was 52 psi, and both gauges were reading the same. But now the car was inside a heated shop, and the higher ambient temperature would raise these pressures as the car warmed up. Remember, refrigerant pressure will change with a change in temperature — that's one of the basic principles the system uses to do its job. Because that was going to also affect my test readings with the system running, I figured I'd wait a while for everything to stabilize before I continued.

When I returned to the Mountaineer, the static pressures had increased to 70 psi, and the air temperature (checked in front of the condenser) was 65° F. From experience, that told me that the system charge was high enough to test its operation and check for any leaks.

I like to stress the system as much as I can when testing to help flush out any problems that might not otherwise show up under more normal conditions. To do that, I open all the doors, set the controls to high blower, maximum A/C, maximum cold and command the recirculation door closed. Then I start it up and allow the system a few minutes of running time to stabilize.

My low side pressure reading was 24 psi, and the needle was "bouncing" a few psi either side of that. The high side was worse, reading 175 psi but moving from a low of 150 to a high of 200 — with no compressor clutch cycling! Air temperature measured at the center duct was 35° F, or a 30-degree difference between the outside temperature and the ducts, a difference that is generally considered "good." So while the front A/C was cooling well, the gauge readings were telling me something was wrong.

According to the service information, the pressure readings (or at least the averages) were in the normal range, so I didn't think the state of charge was a factor. Today's systems are much more sensitive to state of charge than ever, and I would certainly want to make sure that the proper amount of R134a was installed when I was done.

I also checked the temperature at the rear A/C outlets and got a reading of 52° F. That made some sense, considering the problem with the temperature door actuator. But my gut told me that wasn't all, so I crawled up under the car to measure the temperature at the rear evaporator core's inlet and outlet tubes.

One of the challenges in testing a dual A/C system is testing two different control systems based on one set of gauge readings. Like many dual systems, the front system uses an orifice tube to meter refrigerant to the evaporator, while the rear uses an expansion valve to do this job. That's where understanding how the A/C system works comes into play. The rear expansion valve is going to meter the flow of refrigerant into the evaporator, taking the high pressure from the inlet and reducing it before it enters. This pressure drop will also cause a temperature drop — and that I can measure.

Using my infrared temperature gun, I measured the inlet temperature at 82° F, and the outlet temperature at only 68° F. While there are no specifications for this measurement, I knew I should see a bigger difference than that. There were more problems in the rear than just a broken blend door actuator. I added these readings into my repair order notes and moved on to my next test.

I shut the engine down, and watched the pressure readings. Normally, they will equalize fairly quickly. But not this time — and I was becoming more convinced that there were restrictions in the system.