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Making Sense of Sensors—Part 1 (cont.) Piezo Resistive Sensors are resistors constructed on a thin chip of silicon wafer. One of the properties of silicon is that its resistance changes whenever the wafer is twisted, warped, or distorted. It's usually used as a pressure sensing device, but it can also be used to detect flex forces, such as deceleration for an SRS air-bag circuit. Manifold Absolute Pressure (MAP) Sensor. Probably the most important piezo resistive sensor in a Vigor is the Manifold Absolute Pressure (MAP) sensor. It monitors intake air volume for the ECU. The ECU uses this information to determine the fuel injection duration and ignition timing. In a Vigor's PGM-FI system, the principal measurements used to determine fuel metering are manifold pressure and engine speed (rpm). This kind of fuel metering system is frequently called a speed-density system because it uses engine speed and air pressure (density) in the manifold. Air pressure is calculated as manifold absolute pressure (MAP), which is the difference between atmospheric pressure (29.9 in/Hg at sea level) and the low pressure in the manifold we call vacuum. If the ECU knows the speed of the engine and the pressure in the manifold, it can calculate the weight (mass) of the air the engine is pumping and it can meter the fuel accordingly. High manifold pressure (low intake vacuum) indicates a high load, requiring a rich mixture and an advance in ignition timing. Low manifold pressure (high vacuum) indicates very little load, requiring a leaner mixture and a less advanced (retarded) ignition timing. Inside the MAP sensor is a silicon chip and a small vacuum chamber. One side of the chip is exposed to intake manifold pressure, and the other side is exposed to the "perfect vacuum" in the chamber. A change in the intake manifold pressure will cause the silicon chip to flex, changing its resistance. An integrated circuit converts the fluctuations in resistance to a voltage that is sent to ECU. In the illustration below, manifold vacuum is applied at a hose connection on the bottom of the sensor.
The sensor has three terminals, although only one is shown in the illustration above: One for power, one for ground, and (shown) one for the voltage signal to the ECU.
Verifying a MAP sensor according to the Acura Vigor Service Manual consists of (1) measuring the Vref at the harness side of the MAP sensor connector and (2) using that expensive test harness to measure the MAP voltage with KOEO. The first part is simple enough. Disconnect the connector and (KOEO) measure the voltage between the YEL/WHT (+) and GRN/WHT ( - ). The Vref should be approximately 5V. But without the special test harness, how can a home mechanic take it any further? Ha! We know what to do! Backprobe the voltage. The WHT/BLU (+) wire is the MAP signal, relative to GRN/WHT ( - ). According to our chart above, with KOEO (no vacuum), there should be approximately 3V. With KOER, the voltage should drop to around 1.5V at idle. As you open the throttle plate, vacuum should drop and the MAP voltage should rise. If it does, you're good to go. If it doesn't, then check the Vref. If it's good, then the MAP sensor is bad. If Vref isn't there, then the ECU or the wiring from the ECU is at fault. By the way, have you noticed that all these sensors use a GRN/WHT wire for the 0V reference from the ECU? In fact, all those GRN/WHT wires are connected together! Summary Hopefully, this article will make two things clear.
Next month we'll continue with other types of sensors and ways of checking them.
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