Experimenting with bypassing the AT 4th gear prohibition when cold

[CTScott]

There have been a few discussions on here over time about the possibility of bypassing the 4th gear prohibition on Yaris with automatic transmissions when the engine is cold. Until the "cold" indicator goes off on the cluster, the Yaris will not shift from 3rd to 4th gear. Typically this just takes a couple of minutes, since the indicator goes out when the engine coolant temperature rises above 140 F, but it can be annoying when starting up and quickly pulling onto a major road or highway.

I speculate that this "feature" is primarily emissions related. Catalytic converters must reach an operating temperature in order to be effective, so forcing higher engine RPMs by restricting the highest gear should hasten reaching the cat's operating temperature. This was pure speculation, as the service manual doesn't explain the "whys", but rather just the "hows".

According to the service manual, the 4th gear prohibition is on until the engine coolant reaches 140 F and the AT fluid reaches 50 F. Both sensors are thermistors, so fooling the ECM into thinking that the coolant or AT fluid is warmer than it actually is just requires adding an appropriate value resistor in parallel with sensor. My test fixture consisted of a 300 ohm resistor and a momentary switch. Pressing the switch connected the resistor in parallel with the sensor, making the ECM read the coolant temp. at 168 F. The momentary switch would allow me to determine if passing the threshold needed to be maintained or if just hitting the threshold for a moment would be sufficient to bypass the prohibition.

I started my testing in 40+ degree F temperatures, so bypassing the AT fluid sensor was not necessary, as the AT fluid temp would reach 50 F within a few seconds of starting the engine. At that temperature the coolant would reach 140 F about a mile and a half from my house, which was also right about where I had enough road to hit 4th gear, so it was difficult to tell if the resistor and switch worked or if the coolant had simply reached the threshold.

So, I next disconnected the coolant temp. sensor and replaced it with a 1K Ohm resistor, which would make the ECM see a constant 109 F coolant temperature. With resistor in place I could drive for as long as I wanted while maintaining the prohibition.

Pressing the switch to bump the coolant temp. up to 168 F resulted in the transmission immediately up shifting from 3rd to 4th, and releasing the switch, dropping the coolant temp. back to 109 F, did not drop it out of 4th, nor did it prevent the transmission from up shifting back into 4th.

So, the 4th gear prohibition can easily be bypassed, on demand, using a 300 Ohm resistor and a momentary switch. I will post up a complete DIY within the next couple of days.