Engine Management Basics
The management system of a car can be its most complex system, and also its most problematic system. The management systems job is to monitor many different engine conditions and report this information back to the PCM. The PCM will then use this information to make decisions based on these inputs and command the appropriate response from various actuators. For instance, the coolant temperature sensor may indicate that the engine is cold, so the PCM will command the fuel injectors to open longer until the engine is up to temperature. The PCM is programmed from the factory to deliver the best fuel economy and lowest emissions before considering power output. This is why many engines power outputs benefit so much from an after-market chip or flash.
In these systems, when the operator pushes the accelerator pedal, the pedal will pull on a cable attached to the throttle plate to open it. The throttle position sensor detects that the throttle has been opened and starts to add fuel to allow the engine to accelerate. This system has been slowly phasing out over the past decade. The management system has to react to the throttle plate opening, causing the engine to run slightly lean for a moment, which is not desirable for acceleration.
These systems have taken over in recent years. In these systems, the operator is not physically connected to the engine. The accelerator pedal has a sensor on it which monitors pedal movement. The accelerator becomes just another sensor or input to the PCM. When the PCM notices the pedal being pressed, it commands the throttle body open electronically and adds fuel at the same time. This avoids a lean mixture and allows for smoother engine operation.
Common Sensor Types
Potentiometers and Rheostats
Potentiometers and rheostats are both devices that are used to measure many different things on a car. (They can also be used as a contol device) A potentiometer will have 3 wires going to it, one will be power, one will be ground and the other will be signal. In a potentiometer, a wiper will move along a resistive element. 5V reference voltage is supplied to one end of the resistive element, ground is supplied to the other end of the resistive element and the signal wire to the PCM will be attached to the wiper. When the wiper is closer to the ground side of the resistive element, the signal wire sees very little voltage, but as the wiper moves towards the 5V wire side, signal voltage increases. It is important to know that a potentiometers output is a voltage signal. A rheostat, has only 2 wires going to it. It has a wiper and a resistive element just like a potentiometer but no ground wire. One is power and the other is signal to the PCM. 5V reference signal is still sent to the resistive element, and the signal wire is attached to the wiper, just like a potentiometer. The PCM will watch for current output on the signal wire. Rheostats are starting to be used more and more in automotive applications because they are cheaper to make and only need 2 wires.
PM generators use electromagnetic induction to supply an A/C voltage signal to the PCM. Remember, all we need is a magnetic field, a conductor and movement to induce a voltage into the conductor, even if the magnetic field is the piece that moves. In the pick-up coil, there is a permanent magnet, with a coil of wire wrapped around it. This pick-up coil is placed near a reluctor wheel that rotates with teeth on it. As a tooth aligns with the pick-up coil, the magnetic field is forced to wrap tightly around the coil. As the tooth passes the pick-up coil, the magnetic field is allowed to expand to fill the gap. This movement of the magnetic field back and forth causes a weak A/C voltage to be induced in the coil of wire. The PCM monitors the frequency of the signal to calculate rotational speed.
A hall-effect sensor is a 3 wire sensor, used to detect speed in an automotive application. One wire is 5V reference signal, one wire is ground and one wire is signal to the PCM. The hall-effect principle states that, if a conductor or semi-conductor is flowing current and is then exposed to a magnetic field, a voltage is produced at 90 degree angles to current flow. In a hall-effect sensor, current is sent through a semi-conductor which has a permanent magnet sitting across from it. The magnets lines of force will bisect the hall device at right angles to the current flow. A metal shutter disk spins between the hall device and the magnet. The disk has windows which allow the magnetic field to expand into the hall device and vanes which blocks the magnetic field. When the hall device is exposed to a magnetic field, a voltage is present in the signal wire and received by the PCM. The disk may have uneven spaces between windows and vanes, this indicates to the computer position as well as speed. This type of sensor produce a digital (on/off) signal so they are much more PCM friendly/usable.
A photo-electric sensor uses one or more LED bulbs which are illuminated. Opposing the LED(s) are photo cell(s) which interprets the light from the LED(s) and sends a voltage signal to the PCM. Sitting between the LED(s) and the photo cell(s) is a slotted disk which can allow light through or block the light as it rotates. This type of sensor produces a (on/off) digital signal. These sensors work well but are sensitive to dirt and debris which can interfere with the light pulses.
A thermistor is a resistor which varies its resistance based on temperature. Most of the automotive thermistors are of an NTC (negative temperature coefficient) type, which means that as temperature goes up resistance goes down and vice versa. These sensors have 2 wires. The PCM will supply a 5V reference signal and watch the signal wire for a return voltage.
Mass Air Flow Sensor (MAF)
The mass air flow sensor measures the weight (mass) of the air entering the engine. A mass air flow sensor will have a “hot-wire” inside the sensor, exposed to the intake air stream. This wire is fed current from the PCM until it reaches a very high set temperature. When the wire gets to its set temperature, it cannot allow any more current through because as a wire gets hot, its resistance increases. When the throttle opens, more air passes through the MAF, the extra air cools the wire slightly and allows more current through the hot wire. The PCM monitors how much current flows through the hot wire, and uses this information to calculate how much air is entering the engine. MAF sensors take intake air temperature as a factor but some systems will have a separate intake air temperature (IAT) sensor built in or near by anyway.
Intake Air Temperature Sensor (IAT)
An IAT is a thermistor placed in the intake air stream which will detect the temperature of the air entering the engine.
Manifold Absolute Pressure Sensor (MAP)
A MAP sensor can be used on a naturally aspirated engine alone, or alongside a MAF sensor to detect how much air is entering the engine. The MAP sensor indicates engine load to the PCM. As the throttle plate opens, manifold vaccum drops or manifold pressure increases, the MAP sensor senses this and informs the PCM. It may also inform the PCM of boost levels on a turbocharged or supercharged engine.
Throttle Position Sensor (TPS)
The throttle position sensor is a potentiometer in the throttle body which detects how far open the throttle plate is. On a drive-by-wire system, this sensor is in place to confirm that the PCMs throttle command has been carried out properly as well as to fine tune throttle position. If the throttle position sensor detects that the throttle plate has not followed the PCMs command, the PCM will turn on the check engine light (CEL/MIL). In a drive-by-cable system, the TPS is the PCMs main input for the operators accelerator pedal position.
Accelerator Position Sensor
This sensor is commonly confused with the throttle position sensor (TPS) but is has a slightly different function. An accelerator position sensor is found on a drive-by-wire system and informs the PCM when the accelerator pedal is moved by the operator. In this system, the driver is not physically connected to the engine. When the pedal is pressed, this sensor which is normally integral with the accelerator pedal itself, requests that the PCM increase the amount of air and fuel to the engine. If this sensor fails it can cause undesirable acceleration, or no acceleration at all. To prevent this, these sensors will usually be 2 potentiometers or rheostats in one. One will increase resistance as the pedal is pressed, and the other will decrease resistance as the pedal is pressed and vice versa.
Coolant Temperature Sensor (CTS)
The CTS is a thermistor, placed in coolant flow to detect coolant temperature and inform the PCM. When the CTS reads colder the PCM will add extra fuel to the mixture until the engine is up to normal operating temperature or NOT for short.
Crankshaft Position Sensor (CKP)
The CKP or “crank sensor” can be a PM generator or a hall-effect sensor. The PM generator types can either run off the teeth on the flywheel/flexplate/torque converter, or they may have their own reluctor wheel on the crankshaft. They are responsible for informing the PCM where the crankshaft is at a particular moment, as well as if/how fast the crankshaft is spinning. The PCM typically uses this signal for spark timing because it indicates where the pistons are. This sensor also informs the PCM of a misfire condition. If a cylinder does not fire, the crankshaft will slow down until the next cylinder fires. If this happens a few times, the PCM will turn on the check engine light (CEL/MIL) and store a misfire fault code. In most applications if this sensor fails, the engine will not start.
Camshaft Position Sensor (CMP)
The CMP can be a PM generator or a hall-effect sensor. They are responsible for informing the PCM of the camshafts position, as well as the engines position, at a particular moment. When piston 1 is at TDC, that could mean that it is about to fire or that it is between exhaust and intake stroke (overlap). The camshaft determines which one. The PCM uses this information to determine when an intake valve is open, so it can activate that injector. If a CMP sensor fails the PCM will monitor the CKP to know where the pistons are and inject half the amount of fuel needed any time that piston moves down. In 4+ cylinder applications, at least 2 cylinders will be doing this at the same time. When the intake valve does open, the correct amount of fuel will be in the intake runner but some of it may have pooled in the runner. This is called “group fire” mode and is not as efficient but it will allow the engine to run.
Knock Sensors (KS)
Knock sensors are a piezoelectric sensor which produce a voltage with engine vibrations. This informs the PCM when detonation has occurred in the cylinder so the PCM or ICM can retard ignition timing. A typical engine may have one knock sensor for every 2 cylinders. An actual legitimate test for theses sensors, is to watch ignition timing on the scan tool and tap the engine block with a hammer and watch for the ignition timing to retard for a moment. However, if you are not a professional, please do not do this. Modern OBD2 systems will be able to pick up problems with the knock sensors and set a fault code.
Oxygen Sensors (O2S or HO2S)
All the other sensors are used to calculate how much air/fuel should be added. The O2 sensors sit in the exhaust and measure the amount of oxygen left over after combustion to determine if the engine is running rich or lean. They essentially measure the air/fuel ratio, instead of the PCM using the other sensors to calculate the approximate air/fuel ratio. These sensors will only work when they reach a temperature of approximately 316°C (600°F) so modern O2 sensors have a heating element to get them up to temperature very quickly. When the O2 sensors are not hot enough, the PCM will not listen to them, it has to rely on the other sensors to determine how much air/fuel to add. This is called open loop operation. When the O2 sensors get up to temperature, the PCM will start listening to the O2 sensors as well as the other sensors, this is called closed loop operation. Engines will have at least one O2 sensor for every bank of cylinders before the catalytic converter and one after the catalytic converter to determine if the cat is doing its job. There should be less oxygen in the exhaust after the cat than before the cat. Most O2 sensors use zirconium dioxide which will generate a voltage, proportional to the amount of oxygen in the exhaust. The sensor will range from 0.1V to 0.9V. When there is a lot of left over oxygen in the exhaust (lean) the O2 will produce a low voltage signal of about 0.1V-0.4V. When there is little oxygen left over in the exhaust (rich) the O2 will produce a high voltage signal of about 0.6V-0.9V. So the O2s produce more voltage with LACK of oxygen. A stoichiometric air/fuel ratio, with proper combustion will produce about 0.5V. O2 sensors are constantly switching from rich to lean conditions as exhaust flows past the sensors. The PCM must determine the average overall signal to find the true running condition of the engine. Other types of O2 sensors will vary resistance instead of producing a voltage signal.
Power Steering Pressure Sensor
When the operator turns the steering wheel, extra load is placed on the engine because the power steering pump becomes harder to turn. If the engine is idling, it may not be producing enough torque to turn the pump and maintain a stable idle. The power steering pressure sensor detects this so the PCM can increase air and/or fuel slightly to compensate for this.
Other PCM Inputs
A/C Compressor Engagement
The A/C compressor is a load on the engine. The PCM will need to compensate for this extra load to maintain a proper idle.
The PRNDL switch is the electronic switch usually located under the shifter lever on vehicles equipped with an automatic transmission. It informs the PCM and TCM which range the operator has selected. This is important to the PCM because when a gear is engaged, extra load is placed on the engine because the turbine in the torque converter will not be able to turn until the wheels are released by the brakes.
The PCM receives all these different inputs and makes decisions on how to react. It will take in information from all sensors and modules, compare this information to its programmed specifications and use this information to decide what to do about it. It will then command the appropriate actuator to carry out an action. Basic PCM operation involves a microprocessor and several forms of memory which the microprocessor can read, write and erase depending on the memory type. PCMs have replaced many different mechanical and vacuum operated systems over the past years and will continue to be an engines primary control device for many years to come.
The microprocessor is the electronic device which makes all of the decisions for the engine. It is the brain of the engine. Microprocessors are programmed from the factory and usually cannot be reprogrammed without being physically removed from the circuit board and replaced with one with a different program.
Read Only Memory (ROM)
The ROM cannot be erased or modified, the microprocessor can only “read” the information stored in this type of memory. The microprocessor will look at information from the engines sensors and compare it to the ROM data to make a decision on what to do next.
Programmable Read Only Memory (PROM)
PROMs are ROMs that can be removed and replaced from the PCM. Engineers would often need to get cars onto the assembly line before they could work out all of the programming. This was a way that the engineers could get the vehicle into production, work out the programming and then offer a free “upgrade” to people who purchased the vehicle later. This also offered a chance for after-market programmers to upgrade the factory programming to an after-market program without replacing the entire PCM. This was of coarse replaced by EEPROM.
Electrically Erasable Programmable Read Only Memory (EEPROM)
This type of ROM can be erased and reprogrammed, usually from the OBD (On Board Diagnostic) port. This is called a “flash” or “re-flash” and just like PROM, engineers use this to get vehicles into production as fast as possible and work on fine tuning the programming later. This is what really allowed after-market tuners to easily change the programming to allow for more power, but usually at the expense of fuel economy (at WOT) and increased emissions.
Random Access Memory (RAM)
The microprocessor can read, write and erase RAM memory in any order it needs to. It stores temporary data that the microprocessor can refer to, like sensor data, which can change suddenly. It can also store values and calculations that the microprocessor writes to is so it can refer to it later. When the engine is turned off, RAM data is erased because it will not be of any use the next time the engine is started.
Keep Alive Memory (KAM)
KAM memory is like RAM except it does not get erased when the engine is turned of. However, if the module loses power, KAM is lost. A simple example of this (not in the PCM) is your radio preset stations, they are not lost when you turn the engine off but if the battery goes completely dead or the radio is unplugged, all the presets are lost.
Non-Volatile Random Access Memory (NVRAM)
NVRAM is like KAM except it does not get erased if the power source is disconnected. A simple example of this (not in the PCM) is the odometer.
Throttle Actuator (Drive-By-Wire Only)
In a drive-by-wire system, the throttle plate is controlled by a stepper motor which is controlled by the PCM. A stepper motor is an electric motor which can rotate to a specific degree of rotation and hold there. Typical throttle actuators have about 6-10 positions they can be set to based on driving conditions and operator input. These actuators may also need to be adapted to the PCM if the battery is disconnected or the throttle body is unplugged.
Fuel Injector Pulse Width
Fuel injector pulse width is the time (in ms) the fuel injector stays open, spraying fuel. The injectors are fed constant power (when the ignition is turned to run or the fuel pump is running) and the PCM will give a ground signal, when it wants an injector to spray fuel. For more on fuel injectors and fuel sytems, check out our fuel injection page.
Ignition Coils (if coils are PCM controlled)
In most modern systems the PCM is responsible for spark timing as well. With the help of independent coils (one coil per cylinder) the PCM can control spark timing based on the knock sensors (as well as other sensors) inputs to control engine knock. PCM controlled coils also allow different ignition timing from cylinder to cylinder.
The Dreaded Check Engine Light (CEL/MIL)
When the PCM detects that something is wrong with the engine, engine components or the management system, the PCM will illuminate the CEL/MIL and store a fault code. The PCM will need to be scanned to retrieve the fault code and begin diagnosis of the problem. The CEL/MIL can come on for many different reasons ranging from the fuel cap not being tight to a several thousand dollar module needing to be replaced. Many people see the light come on and simply ignore it. “The car drives fine.” or “Its been on for since I bought the car.” are the famous lines. Several months later they are shocked when the vehicle doesn’t start or suddenly stops, leaving them stranded and costing them much more money (including towing) to get their vehicle back on the road. At this point people usually say “What a piece of $#!^.” But the car did warn you! It may (or may not be) the vehicles fault that it had a problem, but it is the owners fault for letting it go until the vehicle became immobile. If your engine light does come on, at least get it scanned so you know what you are dealing with. This can save you time, money and a headache down the road.