Engine Coolant Temperature Sensor (ECT) Mod
1/27/2003

Galen (Dr. Zhivago on the forums) shows you how he installed a device to modify the output of the Engine Coolant Temperature sensor.

Hey All,

First let me start out by explaining what the ECT is: The ECT (Engine Coolant Temperature) Sensor on our engines is a Negative Temperature Coefficient (NTC) Thermistor. What this means is that as the temperature of the ECT Sensor rises, the resistance of it drops.

Here's a chart detailing the characteristics of the ECT Sensor in our engines. This is from a Toyota training document. The ECT Sensor used is the same in all Toyotas.



As you can see by the chart, the Vertical scale on the right shows Resistance and the Horizontal scale shows Temperature. You can see the relationship between the two variables.

Why is this important to know? The ECT tells the ECU what the internal temperature of the engine is which determines a few things. One thing that it affects is how much fuel is dumped into the cylinders. When an engine is cold, the ECU delivers more fuel and runs more timing advance because cold fuel does not atomize as well as warm fuel.

By modifying the ECT Signal, we can fool the ECU into delivering more fuel into the engine and running a little more timing advance, thereby enhancing power and throttle response. The ECT isn't the ultimate authority when it comes to telling the ECU what to do, but it has a heavily weighted opinion.

The mod involves installing a Potentiometer (Variable Resistor) in series between the ECT and the ECU. By adding resistance to the ECTs circuit, the ECU believes the engine is colder and makes the above mentioned changes almost instantaneously. It's a pretty neat trick.

Since we are using a Potentiometer, we can "Dial In" the appropriate amount of added resistance and we can also turn it off with the quick turn of a knob. Once the Pot is wired into the circuit, the mod is initially adjusted like so: Start the engine with the Pot "Zeroed Out" or turned fully counter-clockwise. Then once the engine has reached normal operating temperature, slowly turn the pot until the idle RPM's have increased 250-350 RPM's. Take it for a spin to feel the results!

On my truck, I currently have the ECT modded idle set to 1050 RPM's. My normal idle is 700. On my truck, this seems to give the biggest kick and it does not "bog" between shifts. Too much ECT modification can cause bogging between shifts.

By the way, this mod has been around for over 10 years. I didn't originate it. I ran it on my 87 Supra Turbo with great results. I hadn't heard of or seen anyone running it on their 4Runner, so I thought I'd dust it off and install it on my truck. As usual, it worked like a charm. I've been running it for about 6 weeks now. In that time, it has not caused any check engine lights to come on, nor has it appeared to negatively affect fuel economy.


Midiwall was kind enough to put together a page detailing the installation of the ECT Mod on his 96 4Runner Limited. Mark was/is my Beta Tester for this mod. This article is not meant to be an "End-All, Be-All" how to for this mod, but merely a guide to get you started. We will refine it in the near future.

Here's a link to the article. I know you will all have questions. So, please feel free to ask away.

I hope you all found this is useful and interesting.

Dr. Z

PS Thanks Mark!