Experimenting with bypassing the AT 4th gear prohibition when cold - Toyota Yaris Forums

CTScott · 12-19-2011, 10:23 PM

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.

bronsin · 12-20-2011, 06:25 AM

IF you usually drive your car ~ five miles then shut it off (like going to work) having the car in 4th instead of third for five miles instead of 3 and a half could make a big difference. (would it really?)

Since manual transmissions can be shifted into fifth right away whatever the lockout feature is "helping" doesnt apply to them. Are manaul transmission cars suffering premature failures because of this. Seems unlikely.

You would think full time fourth gear would improve the Yaris city mpg features. Wouldnt Toyota want to trumpet that? Isnt it in their interest?

You would think so.

bronsin · 12-20-2011, 07:28 AM

Along with the shifting issue, is there anything else that would be "advanced" by the ECM seeing 168 degrees sooner than it actually is? Im thinking the carburation might be altered.

eTiMaGo · 12-20-2011, 08:04 AM

Quote:

Originally Posted by bronsin

Along with the shifting issue, is there anything else that would be "advanced" by the ECM seeing 168 degrees sooner than it actually is? Im thinking the carburation might be altered.

fuel injection and spark advance would be affected by the intake air temp, not coolant temps. Only other systems I can remember being linked to the coolant temp would be the radiator fan functions?

Klink10 · 12-20-2011, 08:42 AM

Very nice there CT. Will be following this one.

cali yaris · 12-20-2011, 11:22 AM

eTiMaGo · 12-20-2011, 12:12 PM

Quote:

Originally Posted by bronsin

Since manual transmissions can be shifted into fifth right away whatever the lockout feature is "helping" doesnt apply to them. Are manaul transmission cars suffering premature failures because of this. Seems unlikely.

Actually I am not sure if the AT cars do this to the same degree, but the ECU runs the car rather rich while it is in warmup mode, for the same reason of getting the cats up and running...

CTScott · 12-20-2011, 01:47 PM

Quote:

Originally Posted by eTiMaGo

Actually I am not sure if the AT cars do this to the same degree, but the ECU runs the car rather rich while it is in warmup mode, for the same reason of getting the cats up and running...

The Yaris sure does. Last night, after about a 5 mile run with the resistor in to fool the ECM into thinking that it was still in the "cold" mode, I pulled into the garage and noticed that the engine was revving. I looked up at my YarGauge and saw that it was at about 1600 RPM and the AFR was 13.7 (whereas after warming up it always sits dead on stoich when idling). I pressed and held my warm up button, and the RPMs and AFR immediately returned to their normal state.

bronsin · 12-20-2011, 03:50 PM

I have the ECHO 2001 Servic Manual and it shows the ECT switch as screwed into the left side of the cylinder head under the plastic valve cover.

Is it the same on the yaris?

The chart for the resistnace values seems different.

CTScott · 12-20-2011, 04:29 PM

Quote:

Originally Posted by bronsin

I have the ECHO 2001 Servic Manual and it shows the ECT switch as screwed into the left side of the cylinder head under the plastic valve cover.

Is it the same on the yaris?

The chart for the resistnace values seems different.

The Echo may not have the same sensor, but the placement is the same. It is on the left side of the cylinder head below the large wiring loom that sits between the engine and the battery. The picture below shows the sensor on Crashy (the non dirty one in the center of the pic). The connector has a pink and a blue wire.

ECT Sensor.jpg

The pink wire is a ground, so you can just connect a wire to the blue wire and run it through the firewall (see my cruise control DIY for instructions on going through the grommet above the ECM for the easiest way through).

Here is a close-up of the connector:

ECT Connector.jpg

Connect the wire that is connected to the blue wire to one leg of a normally open momentary switch. Connect the other leg of the switch to a 300 Ohm resistor (270 to 420 Ohms should work if you cant find a 300 Ohm one). Connect the other side of the resistor to chassis ground. Here is mine (prior to heat shrinking). I did not have a 300 Ohm resistor handy, so I connected two 150 Ohm resistors in series. The blue wire connects to the coolant temp. sensor's blue wire and the black wire connects to chassis ground under the dash:

ECT Bypass.jpg

bronsin · 12-20-2011, 04:40 PM

Ah thanks! I have a couple days off and nothing to do!

The chart for the resistance values for testing the switch is the same.

Does the cool engine light go out when you press the momentary switch?

CTScott · 12-20-2011, 05:28 PM

Quote:

Originally Posted by bronsin

Ah thanks! I have a couple days off and nothing to do!

The chart for the resistance values for testing the switch is the same.

Does the cool engine light go out when you press the momentary switch?

Yes. If you hold the switch for about two seconds the light will go out. About a second or two after you release it the cold light comes back on.

bronsin · 12-21-2011, 11:53 AM

Golly Gee how do you run a wire from the engine room to the interior?

CTScott · 12-21-2011, 11:57 AM

Quote:

Originally Posted by bronsin

Golly Gee how do you run a wire from the engine room to the interior?

Check out my cruise control DIY for explicit directions on getting through via the grommet above the ECM on the passenger side of the firewall. You can also go through the giant grommet on the driver's side (under the dash up by the steering column), but that one requires removing the battery to get to the engine side of where you come through.

http://www.yarisworld.com/forums/showthread.php?t=33750

bronsin · 12-21-2011, 08:39 PM

OK I get it. Have to wait til I go back to work Friday so I can solder up and shrink insulate the resistor to the switch.

But then I'll be good to go.

I suggest installing the resistor(s) someplace in the middle of the wire going to the blue wire of the temperature sensor. The resistors wire is solid and soldered directly to the switch would not be as flexible as a stranded wire. Less chance of the wire fatigueing and breaking that way.

bronsin · 12-25-2011, 11:13 AM

I had a problem getting the grommet near the computer out. I have a torn muscle in my lower back and a blown disk in my neck. So every move I make has to be thought out ahead of time.

I cant bend over or reach far enough to touch the grommet. Unfortunately I pushed the grommet in towards the passenger compartment with a long screwdriver. Now I cant get it out and I cant push anything through it.

I cant figger out where the grommoet is in relation to the inside of the car. Is it near the pink thing in the picture? Behind the big white thing?

Thanks!

CTScott · 12-25-2011, 11:57 AM

It's around that area, but under the carpet.

bronsin · 12-31-2011, 11:21 AM

I found out where the grommet hole above the ECM comes out in the passenger compartment. Its to the right of the big white thing in the picture. About half an inch from it. Way up by the top of the big white thing.

What with my back and left arm and neck troubles its taking a while. Still have to figger out a way to reach in there far enough to run the wire...

12-19-2011, 10:23 PM	#1
CTScott ULTIMATE View User's Garage Drives: 09 5dr LB, 2x 08 3dr LB Join Date: Oct 2008 Location: USA, CT Posts: 13,460	Experimenting with bypassing the AT 4th gear prohibition when cold 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. __________________

12-20-2011, 11:22 AM	#6
cali yaris ULTIMATE View User's Garage Drives: 07 Yaris Turbo Join Date: May 2007 Location: Canoga Park, CA Posts: 14,859	__________________ Micro Image forums, online store and shop are now closed. It was a great eight year run, but it was time to focus on other things. I'm still selling parts on eBay under micro*image seller ID and customers can still make requests for anything specific.

12-21-2011, 08:39 PM	#15
bronsin Drives: 2009 Base Hatch 2 Dr Auto Join Date: Dec 2008 Location: nj Posts: 4,790	OK I get it. Have to wait til I go back to work Friday so I can solder up and shrink insulate the resistor to the switch. But then I'll be good to go. I suggest installing the resistor(s) someplace in the middle of the wire going to the blue wire of the temperature sensor. The resistors wire is solid and soldered directly to the switch would not be as flexible as a stranded wire. Less chance of the wire fatigueing and breaking that way. Last edited by bronsin; 12-22-2011 at 08:02 AM.

12-25-2011, 11:57 AM	#17
CTScott ULTIMATE View User's Garage Drives: 09 5dr LB, 2x 08 3dr LB Join Date: Oct 2008 Location: USA, CT Posts: 13,460	It's around that area, but under the carpet. __________________

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12-20-2011, 06:25 AM	#2
bronsin Drives: 2009 Base Hatch 2 Dr Auto Join Date: Dec 2008 Location: nj Posts: 4,790	IF you usually drive your car ~ five miles then shut it off (like going to work) having the car in 4th instead of third for five miles instead of 3 and a half could make a big difference. (would it really?) Since manual transmissions can be shifted into fifth right away whatever the lockout feature is "helping" doesnt apply to them. Are manaul transmission cars suffering premature failures because of this. Seems unlikely. You would think full time fourth gear would improve the Yaris city mpg features. Wouldnt Toyota want to trumpet that? Isnt it in their interest? You would think so.

12-20-2011, 07:28 AM	#3
bronsin Drives: 2009 Base Hatch 2 Dr Auto Join Date: Dec 2008 Location: nj Posts: 4,790	Along with the shifting issue, is there anything else that would be "advanced" by the ECM seeing 168 degrees sooner than it actually is? Im thinking the carburation might be altered.

12-20-2011, 08:42 AM	#5
Klink10 View User's Garage Drives: 2007 Yaris Liftback Join Date: Sep 2007 Location: 910 Posts: 1,229	Very nice there CT. Will be following this one.

12-20-2011, 03:50 PM	#9
bronsin Drives: 2009 Base Hatch 2 Dr Auto Join Date: Dec 2008 Location: nj Posts: 4,790	I have the ECHO 2001 Servic Manual and it shows the ECT switch as screwed into the left side of the cylinder head under the plastic valve cover. Is it the same on the yaris? The chart for the resistnace values seems different.

12-20-2011, 04:40 PM	#11
bronsin Drives: 2009 Base Hatch 2 Dr Auto Join Date: Dec 2008 Location: nj Posts: 4,790	Ah thanks! I have a couple days off and nothing to do! The chart for the resistance values for testing the switch is the same. Does the cool engine light go out when you press the momentary switch?

12-21-2011, 11:53 AM	#13
bronsin Drives: 2009 Base Hatch 2 Dr Auto Join Date: Dec 2008 Location: nj Posts: 4,790	Golly Gee how do you run a wire from the engine room to the interior?

12-31-2011, 11:21 AM	#18
bronsin Drives: 2009 Base Hatch 2 Dr Auto Join Date: Dec 2008 Location: nj Posts: 4,790	I found out where the grommet hole above the ECM comes out in the passenger compartment. Its to the right of the big white thing in the picture. About half an inch from it. Way up by the top of the big white thing. What with my back and left arm and neck troubles its taking a while. Still have to figger out a way to reach in there far enough to run the wire...