I know a lot of us on here understand the sensors on vehicles, but some of the people on here are just learning, so I thought this may help them understand the functions of certain sensors and the failure symptoms. Not all these sensors are on the 3rd gen, but most are.
All vehicles from 1980 on (except for some big trucks) have some sort of computerized engine controls. The fuel/air mixture and often the ignition timing are electronically adjusted according to engine load, speed, temperature, even the altitude above sea level!
The newer cars also use either the engine control computer or a separate computer to shift the transmission.
All manufacturers call their computer components by different names, but all the systems are similar in function. Here is a list of the most common sensors and what they do. Most all sensors provide a variable resistance to the computer to let it know what's happening. 5 volts are fed to the sensor, and it returns 0 to 5 volts back to the computer. Although later systems can do without certain sensors and still run, many systems will run poorly or not at all because of one defective sensor.
When a sensor voltage falls out of specifications, the computer will illuminate a "CHECK ENGINE, SERVICE ENGINE SOON, OR MAINTENANCE REQUIRED" light on the dash. The new technical name for this light is the "MIL" light. This stands for Malfunction Indicator Light. If this light is on in you car, it means the computer has received a bad reading from one of the sensors.
Most computers have a set of trouble codes which can be recalled even after the engine is turned off. These codes tell which sensor gave an "out of specs" reading. If the light comes on then goes off, the problem is intermittent. A code has been stored, however, (on all domestic, most newer imports) , and this code can be retrieved to help find out what sensor went out of specs.
If the light is on all the time, the sensor is out of range all the time. Your car may go into a "limp home mode" where it assumes a set of values for the bad sensor(s). The car will get bad gas mileage when in limp home mode: it can run really badly!
MAP SENSOR: This stands for manifold Absolute Pressure Sensor. It's nothing but an electronic vacuum gage. As the manifold vacuum changes, the MAP sensor supplies a variable voltage to the computer.
FAILURE SYMPTOMS: Poor running, stalling, light on dash
NOTE: A MAP sensor can be inaccurate, sending an incorrect voltage to the computer. If this voltage is still within the range of voltage the computer expects, a light or trouble code for the MAP sensor may not be set. Fords are particularly susceptible to this. problem. Thus, if the MAP is old, it might be a good maintenance item.
TEMPERATURE SENSOR: Often a car will have several of these. All of them have a coolant/engine block temperature sensor. Some also have a manifold temp sensor and an intake air temperature sensor.
FAILURE SYMPTOMS: Rich or lean mixture, black smoke (rich), light on dash
THROTTLE POSITION SENSOR: This lets the computer know how far you press the accelerator pedal down. It often has a wide open throttle and a closed throttle switch either as part of it or as separate components.
FAILURE SYMPTOMS: Hesitation on quick acceleration (passing, "floorboarding it") , sometimes bad idle. May not illuminate light or set codes.
MASS AIRFLOW SENSOR: The latest thing: this actually measures how much air enters the engine and adjusts the fuel/air mixture accordingly.
FAILURE SYMPTOMS: Poor running, stalling: all conditions. May not illuminate light or set codes.
OXYGEN SENSOR: This measures the exhaust oxygen content. The computer fine tunes the mixture using data from this sensor.
FAILURE SYMPTOMS: Poor fuel economy, driveability, light on dash.
THE HEGO SENSOR (Heated Oxygen Sensor): First seen on Fords, this is just an Oxygen sensor, except it's HEATED! Heated Exhaust Gas Oxygen sensor. It is electrically heated so it works immediately on engine startup: the other type must be heated by the exhaust before it starts to work. Computers using these place a lot more emphasis on the data from the Oxygen sensor, and use this data sooner while the engine is still cold.
CRANKSHAFT/DISTRIBUTOR/CAMSHAFT POSITION SENSOR: Used on all engines with computerized timing advance, this tells the computer which cylinder should be firing. Some engines use a sensor on the front pulley of the engine (Harmonic balancer or vibration dampener), another sensor in the engine block on a special toothed wheel on the crankshaft inside the engine block, and a third sensor on the camshaft gear. Earlier systems have a pickup inside the distributor as the only input as to crank position.
FAILURE SYMPTOMS: No start, hesitation, misfire
BAROMETRIC PRESSURE SENSOR: This adjusts the mixture according to altitude: the same settings won't work both in Denver and Death Valley!
FAILURE SYMPTOMS: Poor fuel economy, light on dash
In addition, the computer often monitors things like vehicle speed, brake pedal on or off, power steering pressure, A/C on or off, and transmission shifter position (tells it what gear you're in).
FAILURE SYMPTOMS: Poor fuel economy, light on dash, stalling while parking or at traffic lights
All vehicles do not have all of these sensors.
THE CRITICAL SENSORS:
All sensors must function properly to obtain maximum fuel economy. However, only a few of them are crucial for proper engine operation. For instance, on many vehicles there's a vehicle speed sensor. If the speedometer cable breaks, the "CHECK ENGINE" light will come on, but the car will appear to run normally. (A vehicle speed sensor CAN make your transmission not shift on a computer controlled transmission!) On the other hand, a bad MAP sensor can make some cars (especially Fords) barely run!
For most cars the "essential" or "major" sensors are: MAP sensor, Mass Airflow Sensor (if so equipped), Engine Block Temperature Sensor, and the Crankshaft/Distributor Position Sensor. These sensors can make an engine barely run or not run at all. The other sensors make very fine adjustments to the fuel/air mix and timing and will not cause a gross poor running condition.
This is important, because often a "major" sensor will cause a "minor" sensor to read out of specs. If the car barely runs, or doesn't run at all, suspect one of these "major" sensors (and other non-computer stuff, like wiring, coil, or fuel supply) before trying to trace down a code generated by a "minor" sensor.
FOR EXAMPLE : BE CAREFUL WITH COMPUTER CODES: THEY CAN MISLEAD YOU !
Your car smokes black, smells like gas, and barely runs. The computer gives a "rich oxygen sensor" trouble code. The Oxygen sensor is replaced. The car still smokes black, still has a "rich oxygen sensor" trouble code. The codes said the Oxygen Sensor reads rich.
This just confirms what any good mechanic could tell by smelling the exhaust: it's running rich!!
The car really needs a temperature, MAP, or Mass Airflow sensor , or possibly a carburetor rebuild or fuel injection service.I've seen Chryslers with bad Throttle Body Injectors smoke, give rich Oxy sensor readings at idle, but setting no other trouble codes.
LIMP HOME MODE: When the computer decides enough sensor inputs are out of proper range, it will go to a "limp home mode". This mode has effects from almost unnoticeable to fairly radical, depending on what car you have. The mild case is a MIL light and bad gas mileage. Some cars (with computer shifted transmissions) will stay in second gear when in limp home mode!
In limp home mode, the computer uses assumed values to control fuel and spark rather than measured (and constantly adjusted) values. This results in poor fuel economy and bad driveability.
OBD 1 AND OBD 2 (OBD I AND OBD II)
In an effort to standardize computer control systems on vehicles, the government mandated manufacturers put a standardized data collection system on newer vehicles. This system is called OBD (On Board Data) one and two. The cars may have other proprietary data interfaces, but they MUST have the OBD interface if they are sold in the U.S.
The new interfaces have literally hundreds of trouble codes which can help a lot in diagnosing your vehicle. code scanners to read these codes have gotten much cheaper recently. Several parts stores will read these codes for free.