A powertrain control module (PCM) is a small computer that manages the operations of your engine and transmission. It receives inputs from a network of sensors and activates other electronic components to adjust the way your vehicle performs.
PCMs are more common on older vehicles before modern OBD II standards. These electronics boxes were some of the first attempts by auto manufacturers to automate common drivability issues. Newer cars separate the engine control unit and the transmission control unit.
At Importapart, we know that engine electronics are often confusing. Fortunately, most vehicles use similar designs and features. Here’s what you need to know about control units.
The Powertrain Control Module Is the Brain of the Engine
Think about your human brain for a moment. It is more than just a sloshy grey ballast wobbling about in your head. It provides some essential functions, and your life depends on it.
You take in sensory inputs from your eyes, nose, ears, etc. Then you use that information to send commands to your hands, legs, and whatever else.
A control module works similarly. It takes inputs from sensors placed around the vehicle. Then it uses that information to make decisions and send commands to actuators.
Sensors typically include:
- Cam Position Sensor – Indicates when the engine is at the top of the firing order.
- Crank Position Sensor – Tells the PCM how fast the engine is spinning.
- Oxygen Sensor – A feedback signal measures the exhaust gasses.
- Intake Sensor – Measures the volume of air coming into the engine.
- Throttle Position Sensor – Estimates the amount of throttle input from the driver.
- Speed Sensor – Counts the rotation of the wheels and estimates vehicle speed.
Every manufacture has a different strategy and might include or exclude various inputs. Some other conventional sensors measure engine temperature, oil pressure, and shift lever position.
Actuators the electronic module controls include:
- Ignition Coils – Fires spark plugs in the sequence of the timing order.
- Fuel Injectors – Sprays fuel into the cylinders before ignition.
- Fuel Pump – Maintains steady pressure on the fuel rail.
- Shift Solenoids – Selects the gears and hydraulic pressures in the transmission.
- Torque Convert Clutch – Locks the torque converter to the engine at freeway speed.
In more sophisticated vehicles, the control module might also control the throttle valve, variable valve timing gears, and emissions control equipment. Let’s take a look at the decisions your module makes and get a little insight into when things go wrong.
Decision One – Startup
When you turn the key to the on position, the computer powers up and cycles the fuel pump briefly to prime the lines. Then it waits for a signal from the crank position sensor. Once you cycle the key to start, the engine cranks away. When the module sees a message from the crank sensor, it reactivates the pump, fuel injectors, and ignition coil to start the engine.
If the module itself has gone bad, your car will not start. It might fail to see the signal or be unable to activate the fuel pump.
Decision Two – Normal Operating Mode
Your control module can run your engine in three different modes:
- Open Loop – On cold start during the warmup cycle.
- Closed Loop – Running at normal operating temperature.
- Fail-Safe – A default setting used when something is wrong.
A cold engine runs a little harder. The control module allows more variation in the air-to-fuel mixture, timing, and shift patterns at first. Once the engine reaches operating temperature, the program slips into closed loop and tunes the engine’s performance as you drive.
When the module detects a severe problem, it activates fail-safe mode. Typically, this locks the transmission in a single gear and limits speed. If you cannot accelerate normally or exceed 35 miles per hour, it is likely your module is in fail-safe mode.
Sometimes a faulty control module will not leave open loop mode or remains stuck in fail-safe mode. Reduced fuel economy or bad performance with no other explanation might be due to a defective control module.
Decision Three – When to Shift
A PCM manages the transmission control as well as the motor. The transmission requires good stable power through the torque converter and reliable information about the engine and wheel speed.
The module selects the gear and engages the clutch packs. When the module fails, you may experience delayed shifting or harsh engagement. Sometimes low fuel economy at freeway speeds is a sign that the torque converter clutch is not activating as it should.
Decision Four – How to Adapt
Your control module learns how you drive and adjusts the operations of your powertrain to suit your habits. The computer can trim the fuel, ignition, and shift profile within certain limits, but it takes some time for the module to learn.
Unhooking the battery, resetting, or replacing the module forces it to relearn the adaptations. Most drivers don’t even notice these subtle changes, but you may experience slight lags or lower fuel economy for a time.
When Your Engine Control Module Fails
A module is usually reliable and should be the last thing suspected. Modules do not have moving parts that wear down over time or fluids that leak. Most drivability problems occur when sensors or actuators fail. Onboard diagnostics provide fault codes when the computer detects a problem.
An engine warning lamp lets the driver know that the computer thinks something is wrong. Technicians use diagnostics computers to communicate with the vehicle’s module and create troubleshooting plans based on the stored codes and operating data.
When a module does fail, it is typically because of an electrical fault or some environmental factor. Common electrical faults include shorted circuits or stray secondary voltage. Environmental factors like excessive heat or vibration can cause the circuit boards to crack.
Modules are also susceptible to damage from water or corrosion, so it is essential to keep them dry. Most manufacturers seal computers in separate boxes to protect them from condensation or leaks. When seals fail, these components are vulnerable.
Order the Correct Replacement Part
If you are confident that the module is bad, remanufactured units are affordable options, but replacement modules are tricky. They all look similar. Differences in programing or pin assignments make a considerable impact on how your vehicle operates. Specific manufacturer part numbers and engineering updates complicate the issue.
Importapart can help you locate the correct computer for your vehicle. Contact us for help identifying the component you need.