The "brains" of a computerized engine control system is the Powertrain Control Module (PCM), which is also called the Electronic Control Module (ECM), Electronic Control Unit (ECU), microprocessor, control module or brain box. Regardless of the name, the unit is really nothing more than a sophisticated calculator. There's nothing magic about its capabilities, though some can do some pretty amazing things. Even so, the PCM's basic function is to crunch numbers. It takes in raw data from its various sensors, translates the sensor data into numbers, then processes the numbers to determine what needs to be done next.

The number crunching is controlled by a program. A program is nothing more than a set of step-by-step instructions that tells the computer how to manipulate the data that's fed into it. It's like a road map with many possible routes and paths -- and all based on a system of logic. "If this, then that" is the decision making process that's used to determine the sequence of events that take place as the numbers are crunched.

On GM computers, the program is contained in an "integrated circuit" chip, a small rectangular black chip with numerous legs. It's a special kind of chip called a "Program Read Only Memory" chip. Changing the PROM chip recalibrates the fuel and ignition curves as well as other functions the computer controls. GM devised the replaceable PROM chip so the same computer could be recalibrated for use in different applications. Ford, Chrysler and the imports do not have replaceable PROM chips.

One of the nice things about having a replaceable PROM chip is that it allows the computer to be "retuned" for increased engine performance. Aftermarket performance PROMS have become a popular item in recent years, a fact that has not escaped the attention of the Environmental Protection Agency. The only type of aftermarket PROMS that are emissions legal are ones that do not increase emissions.

Back to the computer. The end product of all the number crunching is a control output -- which is usually the switching on or off of an electronic circuit. The computer does this by grounding the circuit electronically. This completes the circuit allowing current to flow through a solenoid or relay (sometimes referred to as an "actuator" device).

COMPUTER OUTPUTS
When the incoming signal from the coolant sensor tells the computer that the engine is getting hot, the output command that follows is to turn on the cooling fan. The computer does this by grounding the relay circuit that controls the electric cooling fan. When the relay clicks on, the electric cooling fan starts to spin and cools the radiator.

Another COMPUTER control function is locking up the torque converter on automatic equipped vehicles. When input from the vehicle speed sensor tells the computer the car is going fast enough to lockup the converter (for better fuel economy), the computer grounds the lockup solenoid circuit which causes the converter lockup clutch to engage.

To help the catalytic converter burn the pollutants in the exhaust, the computer controls the routing of air from the air pump. When the coolant sensor tells the computer the engine is cold, air is routed to the exhaust manifolds to help reduce carbon monoxide (CO) and unburned hydrocarbons (HC) in the exhaust. But as the engine warms up, vehicles with "three-way plus oxygen" converters need the air rerouted into a middle chamber in the converter so it can reduce oxides of nitrogen (NOX).

Some other functions controlled by the engine computer include opening the purge valve on the charcoal canister so accumulated fuel vapors can be siphoned into the engine and burned, controlling the exhaust gas recirculation (EGR) valve, turning the electric fuel pump on and off, and regulating the alternator's output voltage.

FUEL LOOPS
One of the most important control functions that the computer handles is regulating the fuel mixture. It does this by using what's called a "feedback loop." The primary sensor input here comes from an oxygen (O2) sensor (called a "Lambda" sensor in import applications) located in the exhaust manifold. The O2 sensor's voltage signal changes in response to the concentration of oxygen in the exhaust. A high concentration of oxygen is interpreted as a lean fuel mixture, while a low concentration means a rich mixture.

The computer monitors the constantly fluctuating input from the O2 sensor, and orders the fuel mixture to do exactly the opposite of what the O2 sensor reads to compensate. In other words, a lean O2 sensor reading makes the fuel mixture go rich, and a rich O2 sensor reading makes the fuel mixture go lean. The constant readjusting and flip flopping the fuel mixture allows the computer to maintain a relatively balanced fuel mixture is maintained for maximum fuel economy and low emissions.

But all this doesn't happen the instant the engine starts. It takes a few minutes for most oxygen sensors to heat up and start producing a signal. So until there's an O2 sensor signal, the computer stays in what's called an "open loop" mode of operation. In this mode, the fuel mixture is slightly rich and does not change. This is necessary to maintain a good idle while the engine is warming up. Once the computer starts to receive a signal from the O2 sensor and/or the coolant sensor tells it the engine has warmed up enough to start leaning out the fuel mixture, the computer shifts into the "closed loop" mode. In closed loop, it uses the signal from the O2 sensor to constantly fine tune the fuel mixture.

So how does the computer actually change the fuel mixture? On engines with carburetors, it does it by changing the "duty cycle" (on time versus off time) of a mixture control (M/C) solenoid in the carburetor's fuel metering circuit. The M/C solenoid is a little valve that cycles open and shut very rapidly (you can actually hear it buzz while it's running). Increasing the on time allows more fuel to flow through the carburetor's jets, which richens the fuel mixture. Decreasing the on time restricts the flow of fuel to lean the fuel mix. Mechanics can read the duty cycle of the solenoid by hooking up a dwell meter to its test lead, or by using a scan tool to monitor it through the computer itself.

On fuel injected engines, the computer switches the injectors on and off in little pulses (once every revolution of the crankshaft or just before the intake valve opens depending on the type of system). The longer the injector is on, the more fuel it squirts into the engine. Thus the computer can make the fuel mixture richer or leaner by increasing or decreasing the duration of the injector pulses.

On many engines, the computer also regulates idle speed. It does this on carbureted engines by driving a little electric motor (the idle speed control motor) in and out to change the throttle opening. This keeps the idle speed within the programmed rpm range. On fuel injected engines, idle speed is regulated by opening or closing a small passageway that allows air to bypass the throttle.

CONTROLLING IGNITION TIMING
Another vital control function handled by the computer is ignition timing. To alter the amount of spark advance, the computer monitors input from two important sensors: the throttle position sensor (TPS) and manifold absolute pressure (MAP) sensor. The TPS tells the computer how far open the throttle is and how quickly it is being opened. The MAP sensor reads intake vacuum, which the computer uses to determine engine load (reduced vacuum indicates increased load).

When the engine is accelerating or under load, the computer backs off the amount of timing advance to prevent the engine from pinging (too much spark advance causes detonation). The computer does this by retarding (delaying) the timing signal that goes from the ignition module to the ignition coil. The computer can also alter ignition timing to compensate for changes in altitude by using input from a barometric pressure (BARO or BP) sensor.

WHEN THINGS GO AMISS
Most computers have what's called a "limp in" or "failsafe" mode that takes over when a vital sensor signal is lost. When this happens, the computer substitutes an approximate value for the missing signal. The engine will still run, but not very well because the computer doesn't have the right information to keep things in proper balance.

The computer also has a certain amount of built-in self-diagnostic capability. But this ability is currently limited to detecting faults within the computer system itself, not in components that are "outside" the system. For example, the computer can recognize there's a problem when it loses a sensor signal or a sensor signal doesn't make sense (engine idling but the TPS says the throttle is wide open). But it can't detect a fouled spark plug, leaky exhaust valve, worn timing chain or vacuum leak. This limitation, however, has been overcome with the arrival of OBD II on 1996 vehicles. Ignition, fuel and compression related misfires can now be detected. For more information, return to the main menu and go to the screen on OBD II.

Because of its complexity, the computer ends up being blamed for many unrelated driveability problems. Consequently, a lot of computers are needlessly replaced -- which leads to needless comebacks because the new computer failed to cure the problem. According to Delco Electronics, a very high percentage (well in excess of 50%) of the supposedly defective computers that are returned under warranty have nothing wrong with them.

So what does that mean? It means a lot of computers are replaced unnecessarily because technicians jump to conclusions and blame that which they understand least. So make sure you've done your diagnostic homework before you replace anything: be it the computer or a sensor.

REMAN ECMS
The engine computer, sensors and other emission controls on 1994 and older cars are all covered by a federal 5 year/50,000 mile emissions warranty -- which changed to 8 years and 80,000 miles in 1995 on the computer and catalytic converter only, and 2 years/24,000 miles on the sensors and other emission components. So if the computer or any other emission control component fails while under warranty, the vehicle can be returned to the new car dealer for free repairs. Once the vehicle is out of warranty, however, defective parts can be replaced with aftermarket parts.

Remanufactured engine computers are more like a reconditioned TV or VCR than a rebuilt starter or alternator. The computer's various input and output circuits are exercised, and then repairs are made as needed. Some electronic components with a high failure rate may be replaced whether they test good or bad to improve reliability. The computer may also be subjected to vibration and thermal tests to make sure it doesn't have any intermittent problems.

Computers don't wear out in the same respect that mechanical components do. Either the circuits perform properly or they don't. There seems to be no correlation between the number of miles driven and the frequency of repairs for onboard computers. Failures are often sudden and unpredictable. But many computers are damaged by voltage overloads and shorts.

HANDLE WITH CARE
Computers are expensive, so handle with care! Fragile connectors and delicate electronics can be easily damaged by careless handling. Static electricity can damage a computer, so leave it in its packaging until it is ready to be installed. The vehicle's battery should be disconnected before installing the unit, and a grounding strap should be worn on your wrist to protect against static discharges. On GM computers, the PROM from the old computer has to be swapped to the replacement computer. Care must be taken not to damage the PROM (use a PROM removal tool and don't touch the PROM's pins).

The replacement computer will take awhile to adjust itself to its new situation. The time this takes will vary from one vehicle to another, but generally takes about four or five miles of driving. Accelerating the car from 35 to 55 mph using progressively more throttle each time can speed up the process.