Tech Stuff

Sponsored by
HFO 1234yf Refrigerant and Coolant Mapping

A simple wax pellet thermostat.
<p>A simple wax pellet thermostat.</p>

Jeff Taylor boasts a 31-year career in the automotive industry with Eccles Auto Service in Dundas, Ontario, as a fully licensed professional lead technician. While continuing to be “on the bench” every day, Taylor also is heavily involved in government focus groups, serves as an accomplished technical writer and has competed in international diagnostic competitions as well as providing his expertise as an automotive technical instructor for a major aftermarket parts retailer.

When I look at the new vehicles that are rolling into the showrooms and being advertised on TV, the focus seems to be on customer comfort, convenience and fuel economy. This is definitely the focus of most manufacturers, and they continue to roll out products that achieve those goals. But the parts and pieces responsible for those improvements and the technology that most techs will sooner or later be required to diagnose or repair aren’t usually visible unless the hood is opened and even then it’s a stretch to see anything under all the noise insulating covers.

Two separate, but certainly not new, types of technology are starting to become more popular on many of the vehicles that techs will be seeing in the next few years, if they haven’t already seem them. Both these technologies have been popular in Europe for some time, and many techs who concentrate on European models will be familiar with them. But now the use of HFO 1234yf refrigerant and coolant mapping is showing up here in North America. The integration of both of these technologies has been somewhat seamless, but there are a few items that we need to understand when it comes to properly servicing and diagnosing these two innovative systems.

A mapped thermostat, in coolant housing.
<p>A mapped thermostat, in coolant housing.</p>

HFO 1234yf

There is still an enormous debate going on right now about the use of HFO 1234yf in the air conditioning systems of today’s vehicles, and that is not likely going to end soon. Some manufacturers used it, stopped using it and reverted back to R134a. This debate will continue as technology evolves, but for now HFO 1234yf is a reality. I am sure that most have heard the stories of flammability and the inherent risks of using it, but the fact remains: it’s being used. The advantage to using HFO 1234yf is that it has almost the same chemical characteristics as R134a (pressure temperature 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 efficiency aren’t readily noticeable. Changes that some may notice are: the predominance of thermostatic expansion valves (TXVs) over an orifice 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 efficiency).

The real adjustment that we as technicians are going to have to deal with is twofold: different equipment and HFO 1234yf is significantly 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 fittings are different, and the proper HFO 1234yf A/C machine will have a built-in refrigerant identifier to make sure that no contamination takes place in the recovery operation.

The heating element of a mapped thermostat and the wax pellet that opens the thermostat.
<p>The heating element of a mapped thermostat and the wax pellet that opens the thermostat.</p>

There are a number of SAE J Standards that HFO 1234yf machines, leak detectors and standalone refrigerant identifiers 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 expansion material that expands when heated, opening the thermostat’s valve and allowing coolant to flow 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 composition, varying opening points in the cooling system can be achieved. But the conventional 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 flows to the radiator.
  2. Thermostat opened. The coolant flows to the radiator to remove heat.
  3. Thermostatic control range. The designed opening temperature of the thermostat is maintained.

These three modes are fixed to the actual design opening temperature of the thermostat. This fixed temperature is going to be a compromise that the engine designers/engineers have to take into account when designing an engine as the performance, fuel economy and emissions output are very temperature dependent.

Here is a temperature/engine load/vehicle speed graph that illustrates how the engine’s coolant temperature is regulated due to operating conditions.
<p>Here is a temperature/engine load/vehicle speed graph that illustrates how the engine&rsquo;s coolant temperature is regulated due to operating conditions.</p>

For maximum performance the engine designer wants a lower opening temperature 185 degrees Fahrenheit (85 degrees Celsius) for maximum engine power. Cooler operating temperatures reduce engine knock and allow optimized ignition timing, especially under full load, but if the temperature is maintained this low then emissions increase and fuel economy decreases.

The fuel economy/emission engine designer wants a higher operating temperature of 230 degrees F (110 degrees C), as higher operating temperatures provide better fuel economy under part throttle, city driving, and partial load situations and drastically reduce emissions and increase fuel economy.

If these two characteristics could be combined you would have the best of both worlds, and that is exactly what a mapped thermostat achieves. The mapped thermostat still uses a wax pellet to open, and a spring to close, but it also incorporates a small heater inside the wax pellet that can be controlled by the PCM. Using a combination of inputs such as engine load, engine rpm, vehicle speed, intake air and coolant temperatures, the PCM can now control the characteristics of the thermostat.

If the opening temperature of the wax pellet is set at 230 degrees F (110 degrees C) without any electrical or PCM intervention the thermostat would open fully at 230 degrees F (110 degrees C), as this temperature level is conducive to good fuel economy, and better emissions, faster warm-up and better comfort for the operator inside the vehicle.

But if the PCM decides from operator input signals that a cooler engine temp of 185 degrees F (85 degrees C) is required for high speed or high load operation, a pulse width modulated heater circuit is actuated and the thermostat will open at a lower coolant temperature, allowing better performance.

As soon as the PCM sees that those conditions have been removed (back to part throttle, exited off highway, etc.), the heater circuit control slows current flow to the heater and the thermostat will go back to normal wax pellet operation and raise the actual set point back to 230 degrees F (110 degrees C), again improving the operating scenario.

This in effect gives the PCM a number of operating modes that sets it apart from the standard three mode operation of the traditional wax pellet thermostat.

This photo shows the mapped thermostat connection and housing in a BMW.
<p>This photo shows the mapped thermostat connection and housing in a BMW.</p>

This temperature regulation happens unnoticed by the operator and can happen many times during a normal drive, but in the event that the thermostat goes bad, the wax pellet will still function as a fail safe and trouble codes will be set to alert the driver of the situation. One major advantage to the mapped thermostat design is that it doesn’t take much to incorporate it into current engine strategies or architecture, and doesn’t really require any major systems to be redesigned. But it will require the use of a scan tool to diagnose the system, since a mapped cooling system can set a number of codes and can be bidirectional controlled in some cases to make diagnostics easier.

The use of an electrically controlled thermostat is not the only way that the cooling system can be mapped or controlled for better overall efficiency. Ford uses an arrangement of control valves to control coolant flow on its 1.6L EcoBoost engines. The idea is to warm up the engine as fast as possible, reduce the internal engine operating friction and in turn lower emissions, and aid in heating the passenger compartment faster. This engine still uses a conventional wax pellet-designed thermostat and thermo-syphon cooling to cool the turbo when the engine is shut off, but fine-tuning of the cooling system incorporates a coolant shutoff solenoid valve, and a coolant bypass solenoid valve. These valves are controlled by the PCM using a low side driver and allow the PCM to calibrate and tailor the coolant flow in four distinct operating modes, or “phases,” as Ford calls them.

Phase 1: With an ambient temperature of 60 to 75 degrees F (16 to 24 degrees C) or above (this temperature range can be customized by the engineers) both the coolant shutoff and the coolant bypass solenoid valves are closed. The coolant does not circulate in the engine or through any other cooling circuits. This lack of flow significantly reduces the warm-up time of the engine, and reduces start-up emissions and lowers fuel economy during warm-up.

Phase 2: With an ambient temperature of 60 to 75 degrees F (16 to 24 degrees C) or below (again, this temperature range can be customized by the engineers), the coolant shutoff valve is opened; this allows coolant to circulate through the engine into the heater core, engine oil cooler, transmission oil cooler and around the bottom of the thermostat housing to warm the thermostat.

Phase 3: When the coolant reaches 158 degrees F (70 degrees C) and the engine load is greater than 70% or the engine is revving greater than 4,000 rpm, the coolant bypass solenoid will open. This allows more coolant to flow from the engine to the thermostat housing, increasing the coolant flow through the engine, reducing cooling system pressure and better regulating temperature variations in the engine block.

Phase 4: When the coolant reaches 180 degrees F (82 degrees C) the thermostat will open and allow coolant to flow to the radiator. The PCM can vary the temperature of the coolant coming in contact with the thermostat by using the coolant bypass solenoid. This results in a variable operating temperature zone of between 180 degrees F (82 degrees C) and 198 degrees F (92 degrees C). This allows higher operating temperature during part throttle resulting in increased fuel economy.

Both of these solenoids can set trouble codes for circuit diagnostics and electrical faults and the standard thermostat codes P0128 and P0125 can result from a bad wax pellet thermostat, and low coolant like before.

And yes, there was a recall on some of these engines for a failure of the bypass valve that allowed the engine to overheat.

This shows the mapped thermostat connection and housing on a GM Malibu. The connection is behind the metal cover.
<p>This shows the mapped thermostat connection and housing on a GM Malibu. The connection is behind the metal cover.</p>

The mapped cooling system has been very popular on European vehicles for a number of years now, but it is now being used here by some domestic manufacturers. GM has been using a mapped thermostat in a few models, such as the Chevy Cruze Eco version, and it is starting to incorporate it onto the 3.6L V6 engine used in many of its models. Other technology is being used to enhance the cooling system. There are now split systems in hybrids, parallel, cross and reverse flow coolant patterns in the engine; all in the drive to make the cooling system more efficient.

GM has developed a patented technology called “targeted cooling” that is designed to provide deliberate cooling of the hotter parts of the engine, while still providing faster warm-ups in order to lower emissions.

Toyota uses specially designed plastic sleeves to direct coolant flow in the cylinder block to increase cooling efficiency. Some manufacturers have even integrated the exhaust manifold into the cylinder head assembly to speed up coolant heating and reduce the emissions the engine creates.

What the future holds

Changes in the automotive industry are the norm. Being forced to buy new tools to repair and diagnose systems is not new by any means, but the changes that we are seeing are making it more difficult to handle repairs properly without having the appropriate equipment. Vehicle manufacturers are under intense pressure to produce vehicles that produce fewer emissions and do less harm to the environment, so progress isn’t going to stop.

This will only mean more electronics, finer control of existing systems and different techniques arriving at the same goals, and ultimately requiring the techs and shop owners to purchase new tools and equipment.   ●

Post a comment

Comments (3)


Post a Comment

Subscribe Today

Subscribe to Auto Service Professional

Sign up for a FREE subscription to Auto Service Professional magazine