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Evaporators, out of sight but not out of mind

By Paul Weissler, MACS Senior Technical Correspondent
The evaporator is out of sight, but with A/C  technicians it can’t be out of mind. When you can’t find a leak anywhere else, it’s the part that becomes suspect. By now, experienced technicians know they can’t rely on a vacuum-hold test to indicate anything more than a gross leak in the system. In the “good old days,” checking the evaporator for a small leak came down to slipping a leak detector probe up the evaporator case drain and listening for it to alarm.
There are these noteworthy problems with that technique:
1) the foam rubber seals and adhesives used in the evaporator
cases would cause most leak detectors to false-trigger. The new,
infrared leak detectors that meet SAE J2791 might be much less
likely, but then: (2) reaching, even finding the evaporator case
drains on most cars today is often impossible.
Figure 1
The other popular method: trace dye in the condensate. The technician puts a paper towel on the floor under the evaporator drains area, runs the A/C  and checks the conden-
sate-wetted towel with an ultraviolet light. This is reasonably effective if there’s a large enough leak and no buildup of airborne fibrous debris on the evaporator core face. Otherwise the dye will be absorbed by the debris. The time for enough oiled dye
to wick through the debris and mix with the condensate that drains to the ground could be a lot longer than most motorists are willing to tolerate in a hot spell.
Or another: remove an access cover for an evaporator fin sensor, and probe the evaporator core face with an electronic leak detector or a borescope (such as one with an external display which provides a relatively large viewing area). If there’s no suitable access cover, some technicians have had success carefully drilling a large enough access hole for a leak detector probe, then sealing it with a plastic cap when they’re done. In some cases, it may be possible to work the detector probe through the A/C register duct to get close enough to the evaporator core. But that is unlikely to find a small leak.
If the evaporator has seeped some oil, or there’s oily dye on the core face, that may be visible with the borescope, particularly if it has a UV light and the system contains dye.
Because of the difficulty of replacing an evaporator on most cars, extra effort to confirm an evaporator leak is invariably worthwhile. If the evaporator core is loaded with debris, using a disinfecting cleaner to remove it could help with diagnosis. However, it does add significantly to initial cost, and that makes it a hard sell as a purely diagnostic aid.
So if there’s no visible dye in the condensate, but the evaporator still is suspect, working the leak detector probe around the core face is the alternative.
And if you find there is no evaporator leak, you’ll be a lot hap- pier as a result. We know a lot of technicians who decided the problem “had to be the evaporator,” spent hours on an R&R, and then had to go back to Square One.
One of the often-missed locations is where the expansion valve mounts to the firewall. It’s not particularly visible, so a dye trace on the underside will be just about impossible to see. However, you should be able to work the electronic leak detector probe into the area and listen.
However, even testing for the possibility of leakage at the evaporator valve joint may be out of sight or access. On an increasing percentage of systems with expansion valves, the valve is built on the evaporator inlet, inside the evaporator case.
The expansion valve itself, and its connection to the evaporator are also potential leakers. So if you’re probing inside the evaporator case, you should try to get the probe as close as possible to where the expansion valve is located. Those systems also have refrigerant line joints right at the firewall, so don’t forget them.
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