THE OBD II ENGINE MANAGEMENT SYSTEMThe performance and emissions that today's engines deliver would be impossible without the electronics that manage everything from ignition and fuel delivery to every aspect of emissions control. Electronics makes possible V8 engines that delivery excellent performance, good fuel economy and produce almost no pollution. But there's a price to be paid for today's technology, and that price is complexity.
Many powertrain control modules (PCMs) today have 16-bit and even 32-bit processors. Though not as powerful as the latest desktop personal computers, PCMs can still crunch a lot of information. It's been said that today's automotive PCMs have more computing power than the Space Shuttle's main processors. Kind of scary to think about, isn't it? So does it take a rocket scientist to troubleshoot and repair drivability problems in today's cars? No, but it does take a lot of knowledge, experience and sophisticated diagnostic equipment. Fortunately, you don't have to be an expert to replace sensors and other components. But you do have to be familiar with the basics of computerized engine control, know what many of the sensors do, and understand how difficult it is to accurately diagnose and repair many drivability problems today.
BASIC SENSORS
From the outside, most PCMs look similar: just a metal box with some connectors on it. The PCM's job is to manage the powertrain. This includes the engine's ignition system, fuel injection system and emission controls. The PCM receives inputs from a wide variety of sensors and switches. Some of the more important ones are:
* The Oxygen Sensor provides information about the fuel mixture. The PCM uses this to constantly readjust and fine tune the air/fuel ratio. This keeps emissions and fuel consumption to a minimum. A bad O2 sensor will typically make an engine run rich, use more fuel and pollute. O2 sensors tend to deteriorate with age and may be contaminated if the engine burns oil or develops a coolant leak. On 1996 and newer vehicles, there is also an additional O2 sensor behind the catalytic converter to monitor converter efficiency. Though most O2 sensors have no recommended replacement interval (replace as "needed" only), sluggish O2 sensors can be replaced to restore like-new performance. Unheated 1 or 2 wire wire O2 sensors on 1976 through early 1990s applications can be replaced every 30,000 to 50,000 miles. Heated 3 and 4-wire O2 sensors on mid-1980s through mid-1990s applications can be changed every 60,000 miles. And on OBD II equipped vehicles, the sensor could be replaced once it has seen 100,000 miles.
* The Coolant Sensor monitors engine temperature. The PCM uses this information to regulate a wide variety of ignition, fuel and emission control functions. When the engine is cold, for example, the fuel mixture needs to be richer to improve drivability. Once the engine reaches a certain temperature, the PCM starts using the signal from the O2 sensor to vary the fuel mixture. This is called "closed loop" operation, and it is necessary to keep emissions to a minimum.
* The Throttle Position Sensor (TPS) keeps the PCM informed about throttle position. The PCM uses this input to change spark timing and the fuel mixture as engine load changes. A problem here can cause a flat spot during acceleration (like a bad accelerator pump in a carburetor) as well as other drivability complaints.
* The Airflow Sensor (of which there are several types), tells the PCM how much air the engine is drawing in as it runs. The PCM uses this to further vary the fuel mixture as needed. There are several types of airflow sensors including hot wire mass airflow sensors and the older flap-style vane airflow sensors. All are very expensive to replace. Some engines do not have an airflow sensor and only estimate how much air the engine is actually taking in monitoring engine rpm and using inputs from the throttle position sensor, a manifold absolute pressure sensor (MAP) and manifold air temperature (MAT) sensor. Problems with the airflow sensor can upset the fuel mixture and various drivability problems (hard starting, hesitation, stalling, rough idle, etc.).
* The Crankshaft Position Sensor serves the same function as the pickup assembly in an engine with a distributor. It does two things: it monitors engine rpm and helps the computer determine relative position of the crankshaft so the PCM can control spark timing and fuel delivery in the proper sequence. The PCM also uses the crank sensor's input to regulate idle speed, which it does by sending a signal to an idle speed control motor or idle air bypass motor. On some engines, an additional camshaft position sensor is used to provide additional input to the PCM about valve timing.
* The Manifold Absolute Pressure (MAP) sensor measures intake vacuum, which the PCM also uses to determine engine load. The MAP sensor's input affects ignition timing primarily, but also fuel delivery. * Knock Sensors are used to detect vibrations produced by detonation. When the PCM receives a signal from the knock sensor, it momentarily retards timing while the engine is used load to protect the engine against spark knock.
* The EGR Position Sensor tells the PCM when the exhaust gas recirculation (EGR) valve opens (and how much). This allows the PCM to detect problems with the EGR system that would increase pollution.
* The Vehicle Speed Sensor (VSS) keeps the PCM informed about how fast the vehicle is traveling. This is needed to control other functions such as torque converter lockup. The VSS signal is also used by other control modules, too, including the Antilock Brake System (ABS).
A couple of things to keep in mind when selling replacement sensors. Parts that are physically interchangeable may not be calibrated the same and won't work properly if installed in the wrong application. To make sure your customer gets the correct replacement part, it may be necessary to refer to the vehicle VIN as well as OEM numbers on the original part. Also, some aftermarket parts may not look exactly the same as the original. A "universal" O2 sensor, for example, may fit a large number of applications but usually requires cutting and splicing wires to install.
OTHER THINGS THE PCM DOES
On many vehicles the PCM also controls the transmission. But on some vehicles, a separate Transmission Control Module (TCM) is used to oversee gear changes and the torque converter. But even if there's a separate module for the transmission, the PCM and TCM talk to each other and share data so each knows what the other is doing. On many newer vehicles, the PCM also regulates charging system voltage, cycles the cooling fan on and off, interacts with the antilock brake system (ABS) module to reduce power if the vehicle has traction control, and may even interact with the automatic temperature control (ATC) module to operate the cycling of the air conditioning compressor clutch. The PCM may also be assigned vehicle security tasks.
One of the PCM's most important jobs is to make sure all the engine's sensors are working properly and that the engine isn't polluting. Since the earliest days of the onboard computer, a certain amount of self-diagnostic capability has always been required to detect problems that might upset the smooth operation of the system. On older vehicles, the diagnostics were relatively crude. If a sensor circuit went open (no signal) or shorted, the gross failure would set a trouble code and turn on the Check Engine light. But many conditions that didn't cause a total failure could also upset engine performance and drivability. What's more, the earlier systems had no way of monitoring many conditions that could increase pollution. So the Environmental Protection Agency required every city and state that didn't meet Federal clean air standards to institute some type of vehicle emissions inspection program.
EMISSIONS & OBD II
Emissions testing has certainly helped boost the sales of aftermarket PCMs, sensors and emission control parts. But more importantly, it has made a significant improvement in the air quality of most large metropolitan areas. Even so, many motorists will only seek repairs if forced to do so because their vehicle failed an emissions test. Many put off repairs until their vehicle is barely drivable or dies and leaves them stranded.
With computerized engine control systems, it doesn't take much of a sensor input problem to adversely affect drivability and emissions. A sluggish O2 sensor, a defective coolant sensor that always stays cold, a throttle position sensor that has a dead spot, an airflow sensor that isn't reading accurately, etc., can all hurt performance, fuel economy and emissions. Unlike earlier onboard diagnostic systems that set a diagnostic trouble code only when a sensor failed or read out of range, OBD II monitors most engine functions while the vehicle is being driven and is designed to detect almost any problem that can cause emissions to exceed the federal limit by 1.5 times.
OBD II is extremely sensitive. Some say it is overly sensitive because the vehicle manufacturers have been overly cautious in setting trigger points below the 1.5 threshold to reduce the risk of expensive emission recalls. As a result, some vehicles may not actually have a emissions problem when the Check Engine light is on. Nevertheless, the problem should always be investigated to determine the cause.