How to Troubleshoot the A/C in a Mercedes E320

1. Check to see that the AC controls are operating correctly. Start the engine and turn on the fan and air conditioner. If the controls are difficult to move or loose, have them replaced. If the fan does not switch on, a fuse might have blown or the fan motor could have blown. Replace the fan.
2. Inspect the fuse box for blown fuses. The fuse box in a Mercedes E320 is on the end of the driver's side dashboard. Open the door to access the panel. Consult the diagram on the inside of the fuse box cover to identify the fuse that corresponds to the AC system. If the fuse is damaged, replace it with another of the same amperage rating.
3. Ensure that the valve control is operating correctly by checking the two AC/heater hoses under the hood while the engine is running. The hoses protrude from the firewall. The firewall divides the engine and passenger compartments.The hoses are similar to the thickness of a garden hose. Place your hand on the two hoses. Both should be warm to the touch. However, one should be noticeably cooler than the other. If not, this could mean that the valve control needs to be replaced. Have a qualified mechanic investigate further.
4. Turn up the air conditioner to its highest setting with the engine running. Check under the hood to see whether the magnetic clutch connected to the AC compressor engages. If you do not see or hear the clutch turn over, the system might be low on refrigerant. Have a mechanic refill the system. An excessively noisy compressor could mean that it is close to failing.
5. Check all the AC hoses for leaks. If you notice a green, oily residue on them, they could be damaged and may need to be replaced.
6. Use the AC compression test gauge (with the engine off) to check for sufficient pressure in the AC system. The test gauge has red and blue hoses attached to red and blue monitors to calculate the pressure. Remove the black caps from the service ports on the E320's AC unit. Screw the blue hose to the low-side service port and the red hose to the high-side port. Check the pressure readings on the monitors. If the reading is not between 50 psi and 80 psi, have a qualified mechanic investigate further as this could mean there is a leak in the system.