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.

[bronsin]

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]

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]

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]

Very nice there CT. Will be following this one.

[cali yaris]

http://t2.gstatic.com/images?q=tbn:ANd9GcThbX7OZbyTXv_i8JNhUL6Oq5l2eyVZS HhP2KRqdlNnQy0lk_TcjD8Pjtro

[eTiMaGo]

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]

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]

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]

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.

44904

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:

44905

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:

44906

[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?

[CTScott]

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]

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

[CTScott]

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

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]

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]

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]

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

[bronsin]

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...

[bronsin]

I got it hooked up. I started up the engine and pressed the momentary switch. After about 2 seconds the CEL goes out. Then it came back on.

I managed to misread the directions and hooked it up backwards.

To aid dyslexics like myself run a wire straight from the blue wire of the temperature sensor to one side of the momentary switch.

The other side of the momentary switch gets the resistor hooked up to it. Then connect a wire to the resistor and run it to ground.

No time for any further testing.

[swede]

would there be any issues if you used a 2 position maintained switch instead of a momentary? This would allow me to leave the switch turned on instead of having to hold it down while driving. But at the same time it could open the possibility of forgetting to turn it off and running the car like that for months, any thoughts if this could damage anything?

[bronsin]

would there be any issues if you used a 2 position maintained switch instead of a momentary? This would allow me to leave the switch turned on instead of having to hold it down while driving. But at the same time it could open the possibility of forgetting to turn it off and running the car like that for months, any thoughts if this could damage anything?


No you really really dont want to do that.

If the computer sees 300 ohms from the temperature sensor, it thinks the coolant has warmed up. It allows the transmission to shift into high right after that.

But you dont want the engine running like it thinks its warmed up when it isnt. When you push the momentary button, the green Cool Engine Light goes out for a couple of seconds.

Then it comes back on after you let go the button.

What we are hoping is happening is the transmission is fooled into thinking it can go into high BUT the engine is still being carburated like its not warmed up. So you want a momentary impulse. Not a continuous one.

Also, using a block heater as I am experimenting with, the CEL goes out after .4-.6 mile (as opposed to to ~ 1 mile without it) But Im not out of the neighborhood yet so I dont need high gear.

Coming home from work in another story. There Im on the freeway after less than .5 mile.

Thats what the mod is for.

[swede]

No you really really dont want to do that.

If the computer sees 300 ohms from the temperature sensor, it thinks the coolant has warmed up. It allows the transmission to shift into high right after that.

But you dont want the engine running like it thinks its warmed up when it isnt. When you push the momentary button, the green Cool Engine Light goes out for a couple of seconds.

Then it comes back on after you let go the button.

What we are hoping is happening is the transmission is fooled into thinking it can go into high BUT the engine is still being carburated like its not warmed up. So you want a momentary impulse. Not a continuous one.

Also, using a block heater as I am experimenting with, the CEL goes out after .4-.6 mile (as opposed to to ~ 1 mile without it) But Im not out of the neighborhood yet so I dont need high gear.

Coming home from work in another story. There Im on the freeway after less than .5 mile.

Thats what the mod is for.


ahhhh ok I understand now, I had to re-read the first post again. I thought that the transmission would go back to third gear once the light came back on. I must have skipped that paragraph in CTScott's explanation.

I did order a block heater from ebay last week so i am just waiting for that to come in the mail now. Driving in third gear for so long is really annoying to me since I have only owned MT cars for the last ~10 years.

I will see how the block heater works and decide if I still want to do this mod. Like you were saying I can't plug my car in at work so I may end up doing this anyways

thanks!!

[bronsin]

I tried out the mod today and it worked great.

I was at work, .5 mile from the freeway, and it was ~ 30 degrees. Pretty cold for an afternoon high in winter near Philly. Normal winter high ~ 40 degrees.

I started the engine and pushed the button until the Cold Engine Light went out. About 5 seconds. It came back on again after about another five seconds.

I was able to get up to ~ 35 mpg .2 miles before the freeway and it shifted right into 4th with the CEL still on. The CEL went out after .8 mile. So I had .5 mile of 4th gear where I previously would have been in 3rd. This out of a total ~ 6.6 mile trip.

Ive learned to leave my heat OFF when starting out because that dramatically increases the time it takes for the CEL to go out. And note the CEL goes out at 140 degrees of coolant temp which is NOT normal operating temp. (more like 200 degrees)

Also I have the AC fuse pulled so I dont have to worry about running the compressor inadventantly.

Running the defroster on 4 and the heat on max red at 40 degrees gives you a CEL out at 1.3 miles. Thats with my block heater on for 2 hours prior to startup.

Normal miles to CEL out with the block heater @ 40 degrees is .4 mile.

[xorrbit]

Oh wow, this is amazing info. I seriously thought something was wrong with my Yaris after observing this 'feature'. It didn't click to me that it only shifted to 4th once the cool light went off but it definitely makes sense now. I'm adding to my list of 'todo' mods.

Thanks for the info and the DIY instructions!

[Focus_Sh1ft]

Wow this is great - just finally saw this today. The warm up phase is so tricky to tune in open loop without dumping fuel. Also don't have cats anymore so emissions...? :laugh:

Will definitely be trying this out sometime. Thanks, once again, CTScott.

[Jcp123]

Genius! I'd love to try this. My route home from work puts me right on a 55mph PSL state highway and waiting for 4th and lockup to engage is hurting my fuel economy significantly. Love to try this.

[bronsin]

UPDATE

A winter of activating the 4th gear cold engine enabler revealed that no detectable in mpg resulted. Fortunately for me its rare for it to be so cold that Im not in fourth when on a freeway after starting.

IF you had to start the engine and go right away to 50 mph that would suck!

[JudgeM]

My 98 Corona does this but its way more apparent than the Vios and both these cars are in a tropical climate. On a cool night, 25c ambient temp, it will not shift to 4th for a good 5-6 km. If that car were in cold climate, I bet it'd never change to 4th.

I found some old article about toyota ECT on the net and it says this for protection of the transmission.

[bronsin]

My 98 Corona does this but its way more apparent than the Vios and both these cars are in a tropical climate. On a cool night, 25c ambient temp, it will not shift to 4th for a good 5-6 km. If that car were in cold climate, I bet it'd never change to 4th.

I found some old article about toyota ECT on the net and it says this for protection of the transmission.

Makes sense I guess. How good it is for the engine I don't know!