Closed Loop Monitor

Here I will document the progress made on the closed loop monitor. Everything will be listed in reverse order so the newest news is always at the top.


     Alright, I've been driving with the closed loop monitor for a few weeks now. It really works great as far as telling you if your in open or closed loop mode. The few weeks of driving have been very informative in learning how the car is thinking and in what conditions it uses both closed and open loop mode. From this time I have learned that closed loop is used much less often than I originally thought. While cruising down a level road at a constant speed the Tercel appears to use closed loop mode up until about 40-45 mph. Above that it goes into open loop mode for cruising. This was surprising to me. Most of my everyday driving is far above 40 mph. This means that probably 90% of the time I'm driving I'm in open loop mode. In any case I'll describe the other situations in which you stay in closed loop mode. Accelerating to about 30-35 you can actually stay in closed loop mode if you accelerate very slowly. If you press that pedal down much at all your going to kick it right into open loop though. Of course nearly every time you lift off the pedal completely you are kicked into closed loop mode. This appears to be the case in all situations except when you are traveling at 70-75 mph in which case the ECU will hesitate for a few seconds before putting it into closed loop.

     So then, what have we learned? Well I personally now know closed loop is much less important than previously thought. I figured while I'm driving down the highway at 70 mph I'd be in closed loop effeciently reaping the benefits of a 14.7:1 air fuel ratio. This is not so! In order to gather more information I would have to install a wideband oxygen sensor, and I'm probably not going to go that far... at the moment.

     With that I'm wrapping up this project. It has been great and informative as well as cheap! If you have any questions or comments feel free to email me at


     I got the monitor installed on the car last night. The installation was very easy. As you can see from the pictures below the oxygen sensor signal was very easy to find and tap into. After tapping into the O2 signal I just found and tapped into another ground near the steering column and also found my 12v ignition source there as well. Here are pictures of the installation.

     It seems to work pretty well,but I still have to dial it in. Unfortunately, I didn't get a chance to go for a ride with it last night. I'll be taking a trip this weekend and should be able to get it tuned real well.
     Ah yes, that brings up the tuning part. Tuning is extremely easy. All you have to do is make sure you car is running and up to operating temperatures. At this point the LED will either be on or off. All you do is turn the potentiometer until the light starts pulsing on and off. The adjustment isn't the finest in the world so it'll be a little tricky finding the best spot. But, its really not that hard at all. It took me maybe 30 seconds to get it roughly tuned. After that I have a feeling you'll need to drive with it a bit to get it right where you want it. All in all, its very easy.
     Other than that I did take a video of the monitor in operation. You'll notice how I step on the gas and the light goes out. The air/fuel is rich at this point. Then I release the gas pedal and it gets bright as the mixture goes lean. After a few seconds it goes dim. Its pulsing at this point, but its hard to see in the video. Now the engine is back in closed loop mode.


     Well, that really about wraps this project up. I'll probably update once more to voice my final opnions on the monitor after I have used it for a while. I must say I'm excited to see how it reacts in normal driving.


     This has been a device I have been wanting to try for quite some time now. As a subscriber to I read about this device a long time ago. I found myself going back and reading this article a few times. Its just now that I finally went and ordered the components for it and made it.
     By now the bright fellows out there will have already concluded what this device does, but I'll explain for the rest of us. The closed loop monitor does exactly what the name suggests. It monitors when you are in closed loop mode (when your engine is using the oxygen sensor to control the fuel injectors) versus when you are in open loop mode (using the map sensor mainly). Very simply the light will blink on and off when you are in closed loop mode. When you are in open loop it will turn off, and when you let off the gas it will turn on. Nothing fancy really.
     So I gathered up the components and ordered most of them off The other parts I just picked up at the local RadioShack. The parts arrived a few days later and I went to work on it the first night I got it. I'll tell you, it takes a few solders to get used to working on this small stuff. In the end it turned out alright. I'd like to put it all in a case of some sort, but for the prototype I'm not going to worry about it. So, this is what we have so far.

     So as you can see the circuit is really quite simple. Theres one chip, a resistor, a potentiometer, and an LED. Make a few connections and presto, a closed loop monitor. All I have to do now is hook it up to my car. This is also very simple. Theres three wires to hook up. One goes to a 12v ignition source. The next goes to the oxygen sensor signal. The last one goes to a ground. It doesn't get much easier than this my friends.
     I'll update again once I get it installed and tuned. Thats brings up another point. Yes, you do have to tune it once it is installed. Don't worry, its also very simple. But, I'll explain that once I get it installed onto the car.