Most people never see an air-conditioner evaporator core. Designers often tuck the evaporator core away, deep inside the dash. Like most auto parts, evaporator cores sometimes fail. When an evaporator core fails, the result is a loss of refrigerant and a lack of cooling.
What is an evaporator core?
The evaporator core is a small heat exchanger, much like a radiator. Refrigerant enters the core as a low pressure liquid. Heat in the vehicle passenger compartment causes the liquid refrigerant to boil. Boiling converts the liquid to a gas and the change of state absorbs energy. Refrigerant transports the heat from the passenger compartment. When we remove the heat, the air in the vehicle becomes cool.
Humidity in the air collects on the cold evaporator core. This is much like the water beads that form on a glass containing ice. This moisture drips into a tray and drains under the vehicle. By removing humidity, the evaporator core makes the passenger compartment more comfortable.
Why evaporator cores fail
Evaporator cores fail when a leak develops. Several things may cause this to happen. Failure in new vehicles is normally a result of defects in the manufacturing process. They make evaporator cores from aluminum and weld or solder them together. If a weld does not hold, a leak develops. This is normally the case on new vehicles that have a leaking evaporator core.
Normally evaporator cores fail after years of use. Many things may contribute to this. Some vehicles have less robust design and fail more often than others. Chrysler products seem to have far more evaporator failures than General Motors products.
In use, the evaporator core gets very cold. The temperature is also uneven between the top and bottom. When we turn the system off, the evaporator core returns to ambient temperature. This fluctuation in temperature causes expansion and contraction of the aluminum core. Cracking may occur due to the thermal changes and leaks are the result.
A blower motor pushes air through the evaporator core. Air may contain dust and other debris. The cold surface of the evaporator core is wet, because of humidity. Debris sticks to the wet surface and begins to build up. These areas hold moisture and encourage corrosion.
To make air conditioners more efficient, the aluminum in the evaporator core is very thin. A small amount of corrosion quickly eats through the thin aluminum. This is considered outside-in corrosion.
When we allow leaves to collect on the cowl vent, the problem is worse. Suction at the vents, draws this debris in. Organic material breaks down and may form acids that accelerate corrosion of the evaporator core. Parking in an area free of trees will help. If this is not possible, using a car cover may prevent the problem.
Internal air conditioner corrosion
Many technicians suspect acids in the air conditioner system cause inside-out corrosion. This might occur from off-specification refrigerant or from moisture entering the system.
Engineers place driers in the system to help remove moisture. The desiccant in these components may fail in time. This leaves the system unprotected and moisture in the system may form acid. This is much worse if they improperly service the system.
When an air conditioner is opened, we should normally replace the drier. We then remove all air by vacuuming the system. A failure to follow procedures may result in a much larger problem, such as a leaking evaporator. The improper addition of refrigerant can also cause problems.
Finding leaks in an evaporator core
Because we cannot see the evaporator core, knowing if it is leaking can be difficult. We find most leaks with an electronic leak detector. With the system running, we insert the detector’s probe through the evaporator drain. Other times we must turn the system off and continue to check for several minutes to find the leak.
Smaller leaks require adding dye to the system. Special florescent dye leaks out with the refrigerant and shows at the evaporator drain. This may take several weeks to show up. Often, a technician adds dye and has the client return after driving for a few weeks.
Allowing an evaporator to leak
We have to remove the entire dash on many vehicles to replace the evaporator. Because of the high costs, people sometimes simply add more refrigerant. This temporarily solves the problem.
Oil circulates with the refrigerant and lubricates the compressor. When the system is not in use, oil collects in the evaporator. Because of its weight, oil settles to the bottom of the tubes. Refrigerant pressurizes these tubes and pushes oil toward the leak. When refrigerant leaks, it forces the oil out through the hole in the evaporator tube. This occurs whether the system is in use or not. The leaking refrigerant forces the oil from the evaporator core 24-hours a day. Soon the system runs low and the compressor burns up. This adds considerable expense to an already costly job. Replacing the evaporator core is better than damaging the system.
A leaking evaporator is expensive and we cannot normally prevent the problem. A proper repair helps prevent collateral damage and an even larger expense.