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Making Sense of Sensors: Part 1

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Making Sense of Sensors: Part 1

   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.

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