HOW DOES THE MAP SENSOR WORK?
The Manifold Absolute Pressure (MAP) sensor signal is electrically used in a similar way to the use of Mass Air Flow (MAF) sensor signal (although internally it is built differently).
It takes a 5 volt signal from the computer, and returns a lower direct current signal in accordance with the vacuum in the engine. A higher output voltage means lower engine vacuum, which is then calculated as “more fuel is needed”. Lower output signal indicates higher engine vacuum, which requires less fuel. It's not just fuel control though. The MAP senor signal gives the computer a dynamic indication of engine load. The computer then uses this data to control not only fuel injection, but also gear shift and cylinder ignition timing. In some cases it is even used to calculate changes in barometric pressure, to automatically adjust for different altitudes.
HOW DOES THE MAF SENSOR WORK?
The Mass Air Flow (MAF) sensor helps the computer to calculate the flow and mass of the air entering the engine. It does that by measuring the cooling effect of air flow over a heated wire element. The electronic circuit inside the sensor attempts to keep the sensor at a fixed temp.
When it is cooled more by an increased air flow, more current is needed to maintain a constant temperature. The increase in current is converted into a signal and that signal goes to the computer. In most cars this signal would be a high frequency signal. Not as high as a radio wave, but much faster changing than the (relatively) slow frequency of the Oxygen sensor.
During low air flow rates, such as at engine idle, the MAF sensor produces a lower frequency signal. During high air flow rates, such as at wide open throttle-road load, the MAF sensor increases the frequency. The control module then converts these frequencies into their corresponding Grams-Per-Second values.
Yet again, some MAF sensors may work on a straight DC signal 0-5Volts such as the typical MAP sensor. This is the case in some older MAP Sensor designs that have a trap door with a potentiometer connected to its shaft.
The invention we're talking about here is a simple play with resistors. A resistor is a little piece of carbon that somewhat blocks electrical current. Higher value means it resists more. The potentiometer (“pot” for short) is a resistor, a variable resistor, which varies its value by turning the knob. But it is still only a resistor. There is another resistor, a fixed value resistor, in series to the pot as shown in the diagram below.
The MAP or Manifold Absolute Pressure Sensor is a little though expensive device installed in your intake manifold, or installed on the firewall and connected to the manifold with a thin hose. It has 5 Volts or 12 Volts coming in, and it simply senses the vacuum in the manifold and attenuates (reduces, weakens) this incoming voltage by a certain factor. In other words it reduces the supply voltage to a direct-current voltage in the range of 15% to 60% of the supply voltage (depending on the car's design these numbers will vary), and this varying (but non-pulsing) signal is then sent back to the computer.
The arrangement of resistors in the MAP Sensor Enhancer simply takes this already attenuated (reduced, weakened) signal – and attenuates it further. Too much attenuation kills the engine, it will simply shut off. Yet if you control it correctly you can lean down the mixture from the stoichiometric (a big word that simply means “balance of ingredients”) which is factory set at 14.7:1 (14.7 parts of air to 1 part gasoline) – down to 20:1, maybe even 50:1 or 100:1.
This device is totally passive and will work just the same if the signal coming in is 12 volts, 5 volts, or whatever comes on the line. The diagram in the book is the SIMPLEST way of doing this. The line from the sensor to the ECU is cut, and you place a pot on the line as shown in the photo contained in the book. Further below you will see the improved enhancer based on the same principle.