I have never worked on auto air conditioning systems before, so this was definitely a learning experience for me. I am writing about it here so I can use it as a reference in the future.
My wife took her car, 1996 Saturn SL2 to a Service Center last year July (2011) to get the Air Conditioner serviced. That service set us back $200 and the AC started to work, but only for that season. This year we had the same problem: when you switched on the AC and the fan, it would not cool down the interior of the car.
So I started to look into the problem. I knew that the AC pump needed to come on, but I could see that the magnetic clutch was never getting activated, and the pump was not getting switched on. The circuit diagram showed that the AC relay controlled the pump. When I closed the relay contacts with a piece of wire, the pump would start. I checked the relay on the bench, and there was nothing wrong with it. Also the AC fuse was fine, because one side of the relay coil was at +12 volts. The other side of the coil was controlled by the ECU (Engine Control Unit), pulling it to Ground to switch on the coil.
I didn’t know how the ECU decided to switch on the coil, I figured the AC switch and the fan switch was sensed by the ECU somehow. I checked both switches, and they were working fine. DO NOT DO THE FOLLOWING. What I did was I connected the one side of the AC switch to the coil so that when you switched on the AC inside the car, the coil would be activated. This worked fine, however still no cold air inside the car. I say don’t do this because ECU noticed that something was wrong and turned the Service light on every time the AC was turned on. I removed the wire.
In the meantime, my wife threatened me that unless the car AC is fixed in the next few days , she would take the car to the service center. This made sense to me, and I learned some more about how the car AC systems worked.
If we follow the path of the freon (or its replacement R-134) through the system starting from the pump, this is what we would see:
- The pump compresses the gas up to about 135 psi. The pressure could go as high as 200-250 psi. This is the high pressure side of the system (Hi – Side). They typically use small diameter metal pipes on this side. If you hold the pipe during operation, it will be warm to the touch.
- Follow the pipe and you will find the condenser located in parallel to the radiator. The same fan is used to cool both the radiator and the condenser. After the heat is removed by the fan, the refrigerant becomes liquid under high pressure.
- After the condenser, the pipe goes into refrigerant reservoir. Extra refrigerant is stored there.
- From there the pipe snakes into the passenger compartment, and goes into an evaporator. The refrigerant is released into an evaporator from a very small orifice. This is where the cooling occurs, and the fan pushes the hot air in the car through the evaporator.
- The hose that comes out of the evaporator is a larger diameter hose and the gas in it is at a lower pressure (Low side starts here and continues until the pump). The pressure in this pipe is 25 to 35 psi. While in operation this hose is cool to the touch.
For the AC to work properly, the pressure levels mentioned above need to be maintained. There is a pressure sensor in the high side labeled as ’3′ in the picture below. In some cars there may be a pressure sensor on the low side as well.
1996 Saturn Hi Side Sensor
In the picture below the connector to the same sensor is disconnected. To check if there is enough refrigerant, start the engine, turn on the fan and the AC, and connect a piece of wire into the socket with the wires. The ECU should activate the AC relay and turn on the magnetic clutch. If that happens, then the problem is that there is a refrigerant leak and the AC system need to be re-charged. It is also possible that over a long period of time some of the refrigerant is lost.
1996 Saturn Hi Side Sensor, Disconnected
From a local auto shop I bought a can of R-134 refrigerant with a built-in pressure gauge for $35. The gauge is re-usable with other cans without a pressure gauge ($15).
The picture below is a little fuzzy, but you can see the location of the high side pressure sensor and the AC pump in relation to the rest of the engine compartment.
In the picture below you can see the AC pump and the two service ports. The low side service port is labeled as ’1′, and the high side service port is labeled as ’2′.
AC Pump and Service Ports
The R-134 Can has instructions on it. Follow the safety instructions. The key thing here is to connect the refrigerant to the low side service port. But before then, you need to start the engine, and turn both the AC and the fan on. In my case, within 5 seconds the AC pump came on, and the AC started to work. I continued until almost all of the can was used up. So far AC is working great!