Auto Service World
Feature   July 1, 2007   by Jim Anderton, Technical Editor

Listening to the receiver/drier

Fragged and contaminated A/C systems send important messages through accumulators and receiver/driers

Automotive A/C technology distils down to two basic types: orifice tube and thermal expansion valve, or TVX. Both are common, with OT types seen more often on traditional “domestic” models and TVX types on imports. Both also feature that ubiquitous tin can under the hood, the accumulator on OT systems and the receiver drier on TVX vehicles. Both have two main functions. Accumulators store excess liquid refrigerant flowing out of the evaporator, preventing the compressor from inhaling a “slug” of incompressible fluid refrigerant.

In TVX systems, the receiver dryer, located just downstream of the condenser on the high pressure side, acts as a high-pressure liquid/gas separator, ensuring that the thermal expansion valve gets a steady stream of high-pressure liquid refrigerant. They’re both important functionally, but the other common purpose is key from a service perspective: they absorb moisture and trap debris.

Moisture is a slow killer of A/C systems, negatively affecting both the lubricity of the system oil and forming corrosive acids. As this toxic waste works through the system, it can cause rapid compressor wear, adding metallic particles to the mix and eventually clogging orifice tubes and thermal expansion valves. The result is “Black Death.” Debris trapping is a secondary consideration; if there’s metal in the system, major surgery is in order. Many orifice tube systems are designed with screens and can be easily accessed for replacement. In addition, compressor inlet port debris screens are available for many systems. Add-on aftermarket filters are also available, and all add a measure of protection.

The first line of defence in a repaired system however is much simpler: replace the accumulator or receiver/drier. It’s generally a given for compressor replacement jobs, but relative to the cost of even a simple leak test and recharge job (especially with the need to treat ODP substances like radioactive waste) the cost of replacing these simple components is minimal. If the system needs flushing, accumulator/receiver replacement is a no-brainer and a good rule is to assume that any system that has leaked down to zero gauge pressure needs a fresh “can.”

“Swap out” of accumulators and receiver driers can be monkey-easy or a bloody-knuckles PITA, depending on what you’re working on. Accumulator systems are generally easier to service, especially with the can on the firewall, but the time saved might by spent on the orifice tube if it’s necessary to cut out a “permanent” type.

TVX systems, however, can put the receiver/drier almost anywhere between the compressor outlet and evaporator, with common placements being under or behind the condenser and rad cradle structure. Both can suffer from serious cathodic corrosion where steel line fittings thread into aluminium. It’s possible to break off a fitting, adding line replacement or a fussy bench job chasing the fitting threads. Don’t be tempted to use anti-seize on these threads unless you can be sure that the petroleum-based compound can be kept away from the sealing washer. And naturally, use new washers, treated as recommended by the vehicle manufacturer, normally with a drop or two of system lube. As always, component replacement will pull oil from the system, so check your manual for replacement volumes.

It’s important to remember that aftermarket replacements often look quite different from the OEM units. That’s O.K. if the unit is specified for your application and it fits, both in its bracket and to the lines. It shouldn’t be necessary to re-bend or stretch a line to make the connection. If you’re repairing one of the few remaining retrofitted R-12 systems, remember that the new unit may not have a sight glass. That’s O.K. since a little oil froth is common in R-134a systems, so the visual method isn’t a reliable way to recharge anyway.