Automatic Transmission Cooler Testing and Development Part 1

We wanted to see how well the factory transmission cooler circuit performs under various conditions in the real world.  So we set about splicing in pressure and temperature sensors to the factory transmission cooler circuit.  On the FR-S there is an external heat exchanger that transfers heat from the transmission fluid to the coolant.  The coolant that circulates through the heat exchanger is also part of the heater core circuit and is therefore at a lofty 200 degrees Fahrenheit.  Fortunately, the FR-S uses Toyota WS transmission fluid which is a synthetic fluid and is capable of withstanding much higher temperatures than traditional mineral based fluids that start breaking down at 180 degrees.

Steady state testing at idle showed the transmission fluid to stabilize at 189 degrees.  Once on the highway, steady state cruising yielded 194 degree transmission fluid temperatures.  City driving with its stop and go pattern had elevated temperatures, but even with moderate acceleration, they were by no means alarming at 205 degrees.  Overall, the factory system works as intended for daily driving duties.

Driving up a mountain pass enthusiastically, however, reveals that the factory heat exchanger is unable to shed heat fast enough to maintain reasonable temperatures.  Within 2 minutes of spirited driving, the temperatures shot up to 235 degrees.  While the synthetic transmission fluid can withstand high temperatures, the other components in the transmission cannot.  Generally, seals will start to harden and crack over 260 degrees and the clutches will start to slip at 295 degrees.  Therefore, we really don’t want temperatures to exceed 240 – 250 degrees for longevity.

Track testing with the factory setup naturally showed how woefully inadequate the system is for that kind of stress.  While the car is advertised as being able to carry a set of track wheels and tires in the trunk with the rear seat folded down, it is not truly designed for track duty.  In 8 minutes of track driving, the transmission temperatures climbed up to 244 degrees.  In short, an automatic equipped car needs to have additional cooling if it is to be driven on a track.  See Part 2 for our Transmission Cooler Development and testing.