There was a time when an overheating problem in an automatic transmission was blamed on a faulty cooler in the radiator, hot weather, or extreme service like towing. In all cases, something external to the transmission is addressed by extra cooling (air-cooled finned heat exchangers) or a trip to the radiator shop to have the transmission fluid cooler built into the bottom of the radiator cleaned and checked. The transmission mechanic didn’t have to be an expert on overheating problems; he had the radiator shop. To say things have changed is a bit of an understatement. The materials and supply chains for radiators went from copper and brass to aluminum alloy. Radiators we once repaired and serviced are now uniformly replaced. Heat removal is still the number one issue that modern vehicle automatic transmissions and new EV technologies must contend with. I’m not saying transmission mechanics will be doing cooling system maintenance, but the changing dynamics of vehicle repair are evolving as rapidly as everything else. This doesn’t mean the old glycol/water cooling systems will not still be there; they will be. It gets complicated quickly if you don’t know the terminology you will need to know when explaining to a customer why his transmission is overheating. This is why I wanted to start with a discussion of the phenomenon known as thermal transference.
Thermal transference is the heat flow in a body (automatic transmissions in this case) to or from external surroundings by conduction, convection, and radiation. What that means is the heat generated at frictional surfaces (planetary gear sets, clutch packs, bearings, vane pumps, sheaves, and belts, etc.) is transferred from a hightemperature area to a lower-temperature area (in the automatic transmission, the ATF performs this function). This transfer of heat changes the energy of both objects involved in the heat transfer. The built-up heat needs to be removed to protect the transmission and fluid from heat damage. This is the function of the auxiliary cooling system. In most applications, this is an external cooler. These systems can be configured using an external watercooled (bottom of engine radiator) or air-cooled (aluminum finned heat exchanger) system that cools the hot transmission fluid. Heat removed this way is called “convection” heat transfer by the motion of a fluid, where the heated fluid causes the heat to move away from the heat source, carrying the heat energy with it. In automatic transmissions, the fluid is pumped through the cooling system to the radiator (or heat exchanger), where the heat is transferred to the metal (aluminum in today’s cooling systems) wall of the heat exchanger (water or air type) and is released to the atmosphere (air flowing over the fins of the radiator or heat exchanger) by “conduction” heat transfer. If one end of an aluminum cooling fin is at a higher temperature, then heat energy will be transferred through the cooling fin toward the colder end because the high-speed heated particles will collide with the slower particles causing the transfer of heat energy to the slower ones. These fins then discharge the heat to the atmosphere by means of heat “radiation” (with radiation being defined as the emission of electromagnetic waves that carry the energy away from the emitting object). Simply stated, the heat is removed from the transmission by pumping the hot ATF through the heat exchanger in the liquid (or air) cooled heat exchanger, then returning the cooled fluid to the transmission. The goal is obviously to reduce operating temperatures and prevent overheating. Unfortunately, overheating can be caused by anything that decreases the ability of the cooling system to absorb, transport, and dissipate heat. Examples of this are transmission fluid level to high, transmission fluid level to low, low coolant level, restricted coolant lines, poor heat conductivity because of sludge or deposits, poor airflow through the radiator or heat exchanger, bad water pump or any of the many problems that affect the liquid-cooled cooling systems of most vehicles. In many cases today, radiators can barely cool the transmission when everything is working correctly.
Without the addition of external coolers and larger sumps, the ability of the vehicle owner to reduce the temperature in their automatic transmission becomes limited. Systems with dual roles and cool electrical components in EVs complicate the situation by generating another source of heat that must be removed.
For now, most EV OEMs call for non-conductive coolants composed of common base fluids like glycol with organic acid anti-corrosion technology designed for low conductivity. These corrosion inhibitors do not contribute to electrical conductivity, which makes them suitable. EV makers Tesla and Rivian recommend a G48 coolant (blue or blue-green color). Another point of interest is the cooling of the electrical motors for both EV OEMs, is done by indirect cooling. Indirect cooling systems don’t allow the coolant to touch the motor; they use heat exchangers to run coolant in a separate closed water jacket cooling system. This is very similar to the traditional radiator cooling system used on internal combustion engines (ICE). Because of the limitations of the indirect cooling methods, there has been a move to the direct cooling method, which has been used for many years by automatic transmissions. Direct cooling methods expose electric motors to oil-based coolants like ATF, which come into direct contact with the motor windings, rotor, and stator. This method of direct cooling the electric motors is currently used by the Ford Maverick Hybrid, which has two motors mounted as an integral part of the CVT transmission used by the ICE main drive engine.
In direct-cooled transmissions, heat is absorbed from the frictional surfaces through the transmission fluid and carried to the discharge area (radiator or heat exchanger). When these molecules are heated by moving internal components, the attraction of the atoms moves the heat outward, carrying it to the case or radiator/heat exchanger, where the heat energy is discharged. The same thing occurs when electric motors built into current automatic transmissions are cooled by the same fluid as the transmission. Reduced viscosity ATF is now used by OEMs as another strategy to improve heat removal by allowing quicker heat exchanger turnover rates, resulting in lower operating temperatures. Because direct cooling delivers improved cooling strategies for the electric motor equipped Hybrid EVs, these cooling systems are experiencing a surge in development and implementation. Quite a change from when we relied solely on ICE-style indirect cooling.
The path forward may require more specialized knowledge as we make things smaller with AI-driven design for improved thermal efficiency. Materials and pumps, lubricants, and coolants may change, but the phenomenon known as thermal transference will not change. The radiator shop may be gone, but explaining to an anxious customer how things work and why he may have cooling problems is more necessary than ever.
