Auto Service Professional

APR 2016

Magazine for the auto service professional

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32 | April 2016 The Chassis relationships, boiling point) and little had to be redesigned to make it work with the current A/C architecture. Yes, the refrigerant oil is specialized and the O-ring and hose material composition changed slightly, but the physical layout remains the same. The compressor and condenser designs are virtually identical to R134a components, and tweaks in the evaporator for better thermal effciency aren't readily noticeable. Changes that some may notice are: the predominance of thermostatic expansion valves (TXVs) over an orifce tube and variable displacement compressors, and the suction (low side) and liquid (high side) lines leading to and from the evaporator may be a pipe-inside-a-pipe design to pre-cool the liquid refrigerant as it enters the TXV for expansion (for better thermal effciency). The real adjustment that we as techni- cians are going to have to deal with is twofold: different equipment and HFO 1234yf is signifcantly more expensive. A dedicated A/C machine and leak detector are mandatory to perform service on any vehicle that has HFO 1234yf, similar to the conversion from R12 to R134a wherein the machines aren't going to be compatible. Vehicle connection fttings are different, and the proper HFO 1234yf A/C machine will have a built-in refrigerant identifer to make sure that no contamination takes place in the recovery operation. There are a number of SAE J Standards that HFO 1234yf machines, leak detec- tors and standalone refrigerant identifers must meet, so pay attention when looking at purchasing equipment. The Mobile Air Conditioner Society (MACS) has a full list of all the SAE J Standards on its website along with lots of other very useful information. The real good news about HFO 1234yf is the fact that it works so similar to R134a and doesn't involve any of the high pressures that CO2 systems require to work properly. Coolant mapping The basic thermostat that we are all familiar with uses a concoction of wax and aluminum (wax pellet) as an expan- sion material that expands when heated, opening the thermostat's valve and allow- ing coolant to fow into the radiator. When the waxy mix cools and contracts a spring forces the thermostat's valve to close. This setup has proved very reliable for many years, and by adjusting the mixture of the wax pellet expansion material composi- tion, varying opening points in the cooling system can be achieved. But the conven- tional wax pellet-designed thermostat is limited to three basic operating modes. 1. Thermostat closed. The coolant stays in the engine, and using the bypass circuit no coolant fows to the radiator. 2. Thermostat opened. The coolant fows to the radiator to remove heat. 3. Thermostatic control range. The designed Art courtesy of Mahle Inc. The heating element of a mapped thermostat and the wax pellet that opens the thermostat.

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