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Finally, I’ve found time to write the second part of this article about sensor mods.

In part one I showed you how to make 2 easy mods to control the signal from the water temperature and intake air temperature sensors. In this article I will show you how to control the Manifold Air Pressure (MAP) or Mass airflow (MAF) sensors, as well as a simple and cheap way to adjust the Oxygen sensors output.

If this is your first visit here please read the warnings in part one before doing any modifications to your sensor system.I offer this information only to help you understand the way a cars computer system works and how it can be manipulated, but will not be held responsible for any damage caused by your misuse of the information.

The MAP sensor measures the amount of vacuum in the intake manifold, more vacuum means less load and less fuel needed. It also measures the barometric pressure when you turn on the ignition to allow the computer to adjust the fuelling to compensate for altitude. Air density at sea level is greater than when you are at high altitude and the fuel map in the computer is adjusted to compensate for this difference. The MAP sensor is a variable resistance device that receives a fixed voltage signal of approx 5 volts from the computer and adjusts the voltage returned to the computer between 1 and 5 volts to tell the computer the amount of load on the engine and how much fuel is needed. A lower voltage means less fuel. Lower load also means the timing can be advanced without the engine pinging.

The MAF sensor sends the same type of signal, but in this device the amount of air entering the engine is used to set the return signal voltage. More air means more fuel and vice versa.

For both of these devices the method of signal modification is the same. these sensors are very sensitive so only a small amount of resistance is needed to modify the signal. A resistance in the 10 - 30 ohm range is sufficient to reduce the return voltage by the amount we need. I use a 50 ohm (ohms, NOT kohms) variable potentiometer in the 5 volt VREF wire as this is sufficient to reduce the return voltage by the small amount required. A 25 ohm pot would probably be sufficient and far less sensitive in adjustment but I have yet to find a pot of this size.

To make the modification, first identify the wires going into the sensor. There a normally 3 wires, a 5 volt VREF, Ground, and Output Signal. Cut the 5 volt VREF wire near the sensor and solder extension wires to each of the cut ends. Run these 2 extension wires to wherever you have mounted your 50 ohm potentiometer and solder 1 of the wires to the center terminal of the pot. Solder the other wire to either of the outer terminals. Which outer terminal is only important for the direction of rotation, so select the one that suits you. Always solder and shrink wrap connections made in signal wires as the minute currents are extremely sensitive to poorly made connections. Make the connection near the sensor rather than the computer as the VREF wire at the computer end may be shared with other components and we don’t want to affect their operation.

The connection could also be made in the Output Signal wire, however, using the VREF wire makes it far easier to measure the voltage change on a consistent basis. On my Chevy Cavalier the original VREF voltage was 5.06 volts. Reducing the voltage down to 4.8 volts is enough to stall out the engine, 4.95 volts allows the engine to run well without problems. That’s how sensitive these devices are!

Set the potentiometer to the lowest resistance side ( turn to the limit in the direction of the outer terminal you connected to) and start the engine. It should run completely normal if the connections are made correctly. Turn up the resistance slowly until you feel the engine start to hesitate. You have now reduced fuel to such a low level that the engine will barely run, and will overheat and do damage if used for any length of time. Turn the pot back down to a safer level for your motor!

By using this modification in combination with the air intake temperature (AIT) sensor, we can now adjust the timing by balancing the return signals from these 2 sensors. The AIT signal retards the timing as well as reduces fuel as the temperature rises. The MAP or MAF sensor advances the timing and reduces fuel as the load it senses decreases. So, by carefully juggling the 2 controls we can reduce fuelling and keep the timing where we want it, even on a distributerless ignition system.

The final sensor that we can modify is the Oxygen (O2) sensor in the exhaust system. This device is the most important of all the sensors as it over-rides all the other sensors by giving a consistent feedback of conditions inside the hot engine.

In effect an O2 sensor is a small chemical battery that emits a variable voltage depending on the amount of oxygen present in the exhaust gasses. The voltage created is very small, between 0 and 1 volt on most sensors, although there are newer types of sensors that work in a different way by sending back a varying current and operate at up to 3 volts.

You will need to identify the type of sensor as this modification will only work on the 0-1 volt sensors. These come in several versions, depending on whether they have a heater element built in. The simplest type is the 1 wire sensor which only has a signal wire. next come the 2 and 3 wire sensors, the additional wires being a 12v feed for the heater and a ground wire for the heater on the 3 wire version. 4 and 5 wire sensors are also known as wideband or AFR sensors and need a different device to control them.

For those of us who like to tinker and have the older type sensor, here is a simple to build unit that does the job and is ideal for experimenting.

The biggest problem we face with the O2 sensor is the fact that it is like a self contained battery so we cannot use the cars 12 volt system directly to add voltage. Adding voltage to the signal is what we need to do to reduce fuel as the computer adjusts its fuel map based on the signal voltage it receives and a low voltage signal means the exhaust gas has a hign level of oxygen so more fuel is needed and vice versa.

This simple device works by adding in a tiny amount of voltage to the signal using power from an AA battery. The amount of current is tiny so a battery lasts a long time. I use this on my Cavalier and the Duracell battery is still going strong after a years use, although I wasn’t expecting it to last so long.

the computer is expecting a signal of between 0 and 1 volt from the sensor, and adjusts fuelling to try to keep the average signal around 0.5 volts. The computer also expects the signal to vary a certain number of times per second and not go outside of preset parameters. If the sensors output stays outside of the preset parameters for any length of time the computer will think that the sensor has failed and ignore it, using safe settings which use excessive fuel to make sure the engine stays cool, but will overheat your wallet.

Unless you have added a modification like a Hydroxy generator, the amount of voltage added should be kept to a minimum or your engine will be running weak on fuel with potential overheating damage. Hydroxy generators provide a fuel catalyst that makes the gasoline burn far more effectively inside the engine, but unfortunately the computer sees the cleaner exhaust as a lean condition and adds fuel to compensate, not the result we want. This is where the EFIE or this device comes into play. We can add back sufficient voltage to the signal that the computer sees so that it will not counteract the fuel gains that the Hydroxy generator provides.

Here’s the wiring diagram for the “poor man’s” battery powered device. It takes the signal generated by the O2 sensor and adds a variable amount of millivolts to the signal going to the computer.

This means the computer still sees a varying signal, just one thats a bit higher than the sensor is sending. Depending on how tightly the computers parameters are set, the added voltage can be as much as 450 millivolts, although this can trigger a check engine light on many cars.

As an example, the computer on my Cavalier needs to see a low level voltage passing below 285 millivolts from time to time to believe the sensor is functioning correctly. That means the most voltage I can add is approx 280 millivolts. Unfortunately that is not sufficient to compensate for the amount of Hydroxy my generator adds so I have the manual task of moving the control knob from time to time to drop the voltage added and keep the computer happy. It means I can keep the level close to 400mv most of the time though, which is the level needed to compensate for the amount of added Hydroxy and give the best fuel consumption. Someday I will get round to building a device to do this for me automatically, but not easy ATM as the only workshop I have is the trunk of the car. The alternative would be to reduce the output from the Hydroxy generator and get less fuel savings.

I hope these 2 articles have helped you to understand the way a cars computer control system functions and have also shown that tuning a system with a computer is not the ‘black art’ that many garages would have you believe so that they can charge excessive rates for fixing things. A decent scanning system that allows you to read the cars computer on your PC is a definate advantage though as it takes away the trial and error from the modifications. I use the ‘Allinone” unit to connect my laptop to any OBD2 equipped cars computer, and PCMscan software to monitor the sensor signals. These 2 items provide all that you need to check the operation of a cars computer system. The best 200 bucks I ever spent.