Timely Topics -- Making Sense of Sensors (Part 1)

www.acuravigorclub.com	Timely Topics Archive
*A Monthly Article for Vigor Enthusiasts (2/04)*

If you have a copy of the Acura Vigor Service Manual, you've probably seen the test procedures associated with the Vig's many sensors. This month, we're gonna put some of that stuff into perspective—"context," if you will—and then blow right past it with some test procedures that are both simpler and better than those described in the manual.

First, some "perspective" on sensors. We'll start with... what are sensors? The service manual talks about this sensor and that sensor, but... what the heck is a "sensor?"

PGM-FI, the Vigor's Engine Management System, is the software (actually, firmware) that controls the operation of the engine. You can think of it as an operating system for a computer. The hardware for this computer is the Vig's ECU.

Your home or office computer has two types of devices connected to it: Input devices—keyboard, mouse, scanner, etc., and output devices—a monitor, printer, etc. Some things can function as both input and output devices—floppy and hard disc drives, CD-RW drives, etc. When your computer gets information from these devices, they act as inputs. When your computer puts information onto these devices, they function as outputs.

Automotive computers use input devices called sensors. The output devices are called actuators. Sensors provide the ECU/PGM-FI with information about the operation of the engine, such as its temperature, speed, and load. Based on this information, the ECU/PGM-FI can make adjustments to the fuel injectors (whose solenoids are actuators) and ignition timing. The ABS Control Unit uses wheel sensors—inputs—to tell it how fast the wheels are turning. It also uses solenoids (actuators)—outputs—to apply and releases the brakes. Sensors are inputs. They tell the computer "what's goin' on." Actuators are outputs. They do things based on "what's goin' on."

Although there are many sensors on a Vigor, for our purposes, there are only three types of sensors: resistive sensors, voltage generating sensors, and switch sensors. Every sensor on a Vig will fall into one of these three categories.

Resistive Sensors—react to changes in a mechanical condition by changing their resistance. The computer or control unit supplies a regulated voltage (called a reference voltage or Vref) to the sensor, and measures the voltage drop across the sensor to determine the position of the throttle plate, the temperature of the coolant, the air pressure in the manifold, or the state of some other "thing." (For a review of voltage drops, see the Dec 03 and Jan 04 articles in the Timely Topics Archive.)

Voltage Generating Sensors—do just that—they produce (generate) a voltage according to the state of the device they're monitoring. Knock sensors and oxygen sensors are two examples of voltage generating sensors.

Switch Sensors—"toggle" a voltage from a "low" level to a "high" level—from an "off" state to an "on" state. A simple switch sensor is a brake switch, which indicates whether or not the brake is applied. A more complicated switch sensor is a phototransistor speed sensor, whose rate of switching between on and off tells the computer how fast something is rotating.

Potentiometer

A potentiometer is a variable resistor frequently used a sensor. Potentiometers have three terminals (see illustration). One is connected to the voltage supply. One is connected to ground. The third provides a variable output voltage to the control unit.

A potentiometer is a mechanical device. That third terminal is connected to a moveable contact which slides across the resistor. As it slides, the resistance of the potentiometer varies at that terminal. As a result, the voltage at that terminal will vary.

You can think of a potentiometer as a voltage divider. If the slider is close to the supply side of the resistor, the voltage at the slider will be "close to" the supply voltage. See the illustration below, where we have a 12V supply..

If the slider is close to the ground side of the resistor (illustration at far right), the voltage will be "close to" zero (ground). If the slider is in the exact center of the resistor (center illustration), the voltage at the slider's output terminal will be half the supply voltage. It will be as though there two resistors, one "above" and one "below" the slider.

Naturally, the slider can be anywhere along the length of the resistor, not just top, bottom, and center.

Manual potentiometers (with a knob that rotates the slider across the resistor) are used on a Vigor to dim the dash lights, and to control the volume of the stereo. But when used as a sensor, a potentiometer's slider is controlled remotely by another mechanical device. Let's see how a potentiometer is used as a sensor on the Vigor. We'll use the Throttle Angle Sensor (usually called a Throttle Position Sensor or TPS) as our example.

In the illustrations below, we can see the a schematic view of the throttle body (gray area) and the sensor. The TPS is mounted to the throttle body in such a way that the moveable contact of the sensor's potentiometer will rotate on the same axis as the throttle plate. The "top" of the potentiometer (the rightmost terminal in the illustrations) is connected to Vref, 5V. The "bottom" (the leftmost terminal) is connected to a 0V reference. These are supplied by the ECU. The moveable contact is connected to the center terminal, as shown in the first illustration. This is the TPS voltage signal that is sent to the ECU.

At closed throttle, the moveable contact (called a "wiper arm") will be all the way at the bottom of the potentiometer, so the potentiometer will drop all the Vref voltage supplied, and return a TPS signal close to 0V.


Closed Throttle	Part Throttle	Wide-open Throttle

At full, wide-open throttle, the wiper arm is at the "top" of the potentiometer, so very little voltage will be dropped. As a result, the TPS signal to ECU will be very close to Vref.

If the throttle plate is partly open, the wiper arm will be somewhere between Vref and 0V, and its voltage will be relative to the exact position of the throttle plate. (See the voltage "curve" shown at right.) In this way, the ECU can look at the TPS voltage and know exactly where the throttle plate is.

Okay... but how can we check the Throttle Position Sensor? How can we tell a good one from a bad one? Well, it's a two-step process.

The first step is to disconnect the TPS connector, and with key ON-engine OFF (KOEO) measure the Vref between the YEL/WHT (+) and GRN/WHT ( - ) wires on the harness connector. There should be approximately 5V. By the way, if you do have approximately 5V here, you can safely assume that the ECU has good power and ground! If not, you're probably looking at an ungrounded or a bad ECU.

Checking the TPS Vref

According to the Acura Vigor Service Manual, the next step is to connect an expensive special test harness to the ECU and measure the TPS signal there. The logic is, if there is a good Vref at the TPS connector, and the TPS signal is around 0.5V (closed throttle) to 4.5V (WOT, or wide open throttle), then we can assume the TPS is working. Problem is, most of us don't own this test harness.

Does that mean we give up? Of course not! Turn the page (er... click "Continue") for the solution.

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