Making Sense of Sensors: Part 1

[john_trickle]

Last week I was working on a Nissan Maxima that was having an idle speed problem. After about an hour of testing and checking sensors I located the problem and had it fixed. No big deal, all in a days work. But for some strange reason it made me think about how far cars have come in the last 30 years.

I remember my first tune-up. It was on a yellow (yes, I even remember what color it was) 1966 Chevy Chevelle with a 307 engine. I changed the spark plugs, distributor cap, rotor and wires. Replaced the points and condenser, air and fuel filters, PCV valve and breather element. Then I took a dwell meter and adjusted the dwell angle and set the timing. Tweaked up the carburetor a little bit and I was done. No muss, no fuss, just a plain old tune-up.

Back then if a car had a problem it was fairly easy to find.
There were only a few things that could cause it and they were basically mechanical in nature. Maybe the timing was off or the points wore worn out or the carburetor needed adjusting. Once the symptoms were known, a good mechanic just about had the car diagnosed even before he brought the car into the shop.

Ahhh... the good old days, life was simple and good.

These days' things are a little different. Points are gone, timing is controlled electronically and carburetors are in a museum someplace. Today, a computer controls all these things. While computers have made engines more efficient, they have made things more difficult and locating a problem more complex. Years ago all a mechanic had in his toolbox was a dwell meter and a volt-ohm meter. Today he has a scanner, throttle switch simulator, breakout boxes and other specialized tools. All the tools needed to test electronic controls on modern cars. And somewhere along the line, he was no longer a mechanic he became a technician.

But more important than needing the right tools, he needs to understand how these systems work. Education is an ongoing battle. Technicians are constantly going to training schools to learn about new systems and how the parts work and interact with each other. Every year something new comes out so back to school he goes to learn about it.

Most of you who do your own work have been kind of lost in the last few years. You don't have the resource of continuing education to help you make sense of this new technology. Hopefully I can help you make sense of this mess and you can figure out what's wrong with your car.

In a modern fuel injection system there is an electronic control unit. Some car manufactures call it a PCM, ECU and other things. Generically we call it the brain, since it does the "thinking." Now comparing it to the human body will help you understand how it works. Your brain receives information from your senses; sight, touch, taste, smell and hearing. Using this information the brain tells your body what to do. For example your hand touches something hot. Your brain receives this information and decides it's too hot to touch and sends a signal to your hand to pull away. Simple input-processing-output.

Since a computer doesn't have hands, eyes, ears or noses they need to get information some how. That's where sensors come in. These sensors are the eyes and ears of the ECU. As the information comes in, the ECU processes it and determines what output to use to do something.

Here are some of these input sensors and how they work.

Mass Air Flow Sensor (MAF)
The mass airflow sensor is placed in the stream of intake air. It measures the intake flow rate by measuring a part of the entire intake flow. It consists of a hot wire that is supplied with electric current from the ECM. The temperature of the hot wire is controlled by the ECM a certain amount. The heat generated by the hot wire is reduced as the intake air flows around it. The more air, the greater the heat loss. Therefore, the ECM must supply more electric current to maintain the temperature of the hot wire as airflow increases. The ECM detects the airflow by means of this current change.

Intake Air Temperature sensor (IAT)
The intake air temperature sensor is mounted to the air duct housing. The sensor detects intake air temperature and transmits a signal to the ECM. The temperature-sensing unit uses a thermistor that is sensitive to the change in temperature. Electrical resistance of the thermistor decreases in response to the temperature rise.

Camshaft (or Crankshaft) Position Sensor (CPS)
The camshaft position sensor monitors engine speed and piston position. These input signals to the ECM are used to control fuel injection, ignition timing and other functions. The camshaft position sensor has a rotor plate and a wave-forming circuit. The rotor plate has 360 slits for a 1°(POS) signal and 6 slits for a 120°(REF) signal.

The wave-forming circuit consists of Light Emitting Diodes (LED) and photo diodes. The rotor plate is positioned between the LED and the photo diode. The LED transmits light to the photo diode. As the rotor plate turns, the slits cut the light to generate rough-shaped pulses. These pulses are converted into on-off signals by the wave-forming circuit and sent to the ECM.

Coolant Temperature Sensor (CTS)
The engine coolant temperature sensor is used to detect the engine coolant temperature. The sensor modifies a voltage signal from the ECM. The modified signal returns to the ECM as the engine coolant temperature input. The sensor uses a thermistor that is sensitive to the change in temperature. The electrical resistance of the thermistor decreases as temperature increases.

Knock Sensor
The knock sensor is attached to the cylinder block. It senses engine knocking using a piezoelectric element. A knocking vibration from the cylinder block is sensed as vibrational pressure. This pressure is converted into a voltage signal and sent to the ECM.

Heated Oxygen Sensor (HO2S)
The HO2S is placed into the exhaust manifold. It detects the amount of oxygen in the exhaust gas compared to the outside air. The sensor has a closed-end tube made of ceramic zirconia. The zirconia generates voltage from approximately 1V in richer conditions to 0V in leaner conditions. The sensor signal is sent to the ECM. The ECM adjusts the injection pulse duration to achieve the ideal air-fuel ratio. The ideal air-fuel ratio occurs near the radical change from 1V to 0V.

Throttle Position Sensor (TPS)
The throttle position sensor responds to the accelerator pedal movement. This sensor is a kind of potentiometer that transforms the throttle position into output voltage, and emits the voltage signal to the ECM. In addition, the sensor detects the opening and closing speed of the throttle valve and feeds the voltage signal to the ECM. The ECM receiving the signal from the throttle position sensor determines idle position of the throttle valve. This sensor controls engine operation such as fuel cut. On the other hand, the "Wide open and closed throttle position switch", which is built into the throttle position sensor unit, is not used for engine control.

Vehicle Speed Sensor (VSS)The vehicle speed sensor is installed in the transaxle. It contains a pulse generator that provides a vehicle speed signal to the speedometer. The speedometer then sends a signal to the ECM.

Power Steering Pressure Switch (PSPS)
The power steering oil pressure switch is attached to the power steering high-pressure tube and detects a power steering load. When a power steering load is detected, it signals the ECM. The ECM adjusts the idle speed for the increased load.

Manifold Absolute Pressure (MAP)
The Manifold Absolute Pressure sensor measures changes in the intake manifold pressure resulting from engine load and speed changes. The computer sends a 5-volt reference signal to the MAP sensor. As pressure changes in the intake manifold occur, the electrical resistance of the MAP sensor also changes. By monitoring the sensor output voltage, the computer can determine the manifold absolute pressure. The higher the MAP voltage output the lower the engine vacuum, which requires more fuel. The lower the MAP voltage output the higher the engine vacuum, which requires less fuel. Under certain conditions, the MAP sensor is also used to measure barometric pressure. This allows the computer to automatically adjust for different altitudes. The computer uses the MAP sensor to control fuel delivery and ignition timing.

Cranking Signal
The control module uses this signal to tell when the vehicle is in the STARTING mode. This information is used to allow enrichment and cancel diagnostics while engine is cranking.

Now not all engines have all of these sensors. And there are other inputs other then the ones I've described here. The ECU gets input signals from the A/T Control unit, Ignition Switch and Air Conditioning Switch.

The ones I have listed here are the most directly responsible for proper fuel management. If you have a drivability problem, rough idle or starting problem, these are the most likely to be the cause. Understanding them will go a long way in helping you figure out what's wrong with your car.

While these sensors can, and do, go bad it's important to always check the basics before condemning a sensor. You have to look at the connectors and make sure they are clean and tight. A loose or dirty connector can cause the same symptoms as a bad sensor.

Check the wiring as well. Make sure there are no broken or chafed wires. Make sure any vacuum lines that may be connected to a sensor are in good shape. In short, check everything else before you check the sensors. About 75% of the time it is not a bad sensor, just a bad connection.

Well, that just about covers the inputs. Next time we'll discuss what happens when the ECU digests this information and what outputs it controls.