The Chrysler Engine Control Unit

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[ ECU Types | Component Descriptions ]

These pages are designed to explain the engine control computer for Chrysler's 2.2L and 2.5L, fuel injected 4 cylinder engines.  Much of this information also applies to the Mitsubishi 3.0L V6 engines as well.  Most of the technical details are specific to the 1984 through 1987 computers, but many of the design aspects remained the same after 1987.  This type of computer system is sometimes referred to as the ECC (engine control computer), ECU (engine control unit), ECM (engine control module), EEC (electronic engine controller), etc.  Here the computer and controller system will be referred to as the ECU, to make explanations more clear.


For vehicles built from 1984 through 1987, the ECU is actually divided into two parts.  This was done to isolate the electrical noise and interference generated by the power module from the passenger compartment.  The Logic Module is located inside the passenger compartment, usually behind the front passenger-side kick panel which is below the dashboard and glove compartment.  The Power Module is located in the engine compartment in the left, front fender next to the battery.  The intake to the air box is connected to the power module so that the engine pulls cool air through it from behind the front bumper.  This keeps the module cool (it has several transistor drivers and regulators) and also supplies cool air to the engine intake.  It is important not to change this since the power module could potentially overheat when the car is running, but not moving.

For 1988 and 1989 vehicles, the logic and power modules were combined into one module and located in the same spot as the power module was on previous models.  This module is known as the SMEC (single module engine controller), but the logic and power module circuit boards actually remain separate and distinct inside the SMEC case (they are even potted separately).  This greatly reduced the amount of wiring needed between the passenger compartment and the engine bay, but the signals between the two boards inside still pass through the wiring locally.  The 60-way connector leads to the logic board while the 14-way connector leads to the power board.

In 1990, the SMEC was redesigned into a single circuit board and is known as the SBEC (single board engine controller).  This new design incorporated many changes to the microprocessor and control circuitry, but conceptually there are still processor components (logic module) and a controller components (power module) in the designs of the SMEC and SBEC.  Therefore, these pages will continue to refer to the two modules, even though they may not physically exist.  For more information on a specific module, use on of the following links:

The 1984 TBI Logic Module
The 1984 Turbo I Logic Module
The 1984 TBI Power Module
The 1984 Turbo Power Module

The 1985 TBI Logic Module
The 1985 Turbo I Logic Module
The 1986 TBI Logic Module
The 1986 Turbo I Logic Module
The L-body Turbo II Logic Module
The 1987 TBI Logic Module
The 1987 Turbo I Logic Module
The 1987 Turbo II Logic Module

The 1985 - 1987 TBI Power Module
The 1985 - 1987 Turbo Power Module

The 1988 - 1989 TBI SMEC
The 1988 - 1989 Turbo I SMEC
The 1988 - 1989 Turbo II SMEC

The 1990 - 1994 TBI SBEC
The 1990 - 1993 Turbo I SBEC
The 1990 Turbo II SBEC
The 1991 - 1993 Turbo III SBEC
The 1989 - 1990 Turbo IV VNT SBEC

You can identify some of the modules simply by looking at them.  All 1986 or earlier logic modules and the 1987 L-body logic module will have the MAP sensor mounted directly to the module.  Other 1987 logic modules have the sensor mounted externally with a cover over the area where the MAP sensor would be, or no cover at all.  A power module can be identified from the two large-pin connectors along the side.  One is a 10-way and the other is a 12-way connector.  A SMEC can be identified by a large pin 14-way connector and a small pin 60-way connector with a bolt in the center.  A SBEC is usually a completely different shape from the SMEC (a simple box) and only has a small pin 60-way connector.  If you ever need to replace a module, be sure to get the part number and serial number from the unit you have so that you can get the correct replacement.  There were some mid-year changes to a few of the modules, so the correct replacement is important.

The logic module is responsible for processing the signals from all of the sensors and responding to them accordingly.  The power module is responsible for producing ignition the fuel injector pulses and regulating the alternator field coil to maintain proper voltage levels.  It also produces a regulated 8V power supply for the logic module, which the logic module uses for internal power and to generate 5V for some of its sensors.  The power module sends and receives commands to and from the logic module for ignition timing, injector timing and pulse width, etc via a set of control lines (also true for the SMEC, but not true for the SBEC).  Some aspects of engine control are accomplished directly by the logic module through the use of motors, solenoids, and relays.

Fault Codes, Switch Tests, and Limp-In Mode

The logic module performs many diagnostics on various sensors and components at various times.  The details of these diagnostics can be found in the sections below.  For any component that is tested and fails, one or more fault codes are stored in the logic module's memory.  The codes can be retrieved without any special tools.  See the Fault Code Description Pages for information on how to do this and how to translate the codes.  Some logic module inputs can be tested using switch tests, after the fault codes have been displayed.  These are performed by simply activating the input (brake pedal, A/C switch, etc).  If the input is working properly, the power loss light will come on.  See the section on logic module inputs for more details.  Certain sensors will also cause the logic module to enter a limp-in mode for that sensor.  This allows the logic module to ignore the bad sensor and rely on other sensors for similar data.  Under certain conditions, the logic module will have to make guesses for certain readings and it may reduce engine performance to prevent engine damage.  Some sensors do not have fault codes because there is no way for the logic module to test them.  In these cases, proper sensor operation must be verified through the use of a diagnostic readout box connected to the diagnostic connector.  Certain limp-in modes will cause the "power loss" (or "limited" or "check engine") light to turn on while the engine is running.  The 1984 through 1987 logic modules will stay in limp-in mode until the engine is shut off.  The 1988 and later modules (SMECs and SBECs) will continue testing the failed sensor.  If it becomes operational again, the module will actually recover from limp-in mode and continue to operate normally.  The record of the failure is still kept in the form of a fault code.  If the check engine light was activated by the limp-in mode, in some cases the light will stay on and in other cases it will go out.

Component Descriptions

What follows are sets of links to detailed explanations of the components listed.  I have compiled all the information I have from all the sources I could find.  Some of these include the service manuals, a technical training guide, the Chrysler FAQ, and many small sources on the web.  Most of this information has been verified to be correct by me.  Explanations of the fault codes are present for all applicable components as well as appropriate links on how to repair them.  See the Fault Code Description Pages for detailed information on fault codes.  Many of the components work in different ways, depending on what year the vehicle was built, so be sure you are looking at the correct explanation.  For vehicles built after 1987, most of the 1987 data should apply, though I have added any further information that I have available.  If you have more information or see something that may not be correct, feel free to e-mail me.  Any other comments are also welcome.

Obviously, the SMEC and SBEC are not separate modules, but they are architected in a similar fashion (on separate boards in the case of the SBEC).  Thus, when this site refers to the logic and power modules, just remember that they are in the same module for 1988 and later models.  The 1984 model year was unusual in that the logic module had far less control over the operation of the engine than the following years did.  Much of this was handled internally by the 1984 power module.  This makes much of the following information less accurate for 1984.

Logic Module Input Components

Below is a list of the sensors and switches the logic module receives data from.  Each is linked to a detailed explanation that follows.  Some components may not be found on some vehicles, depending on year, model, and options.  Those inputs that have switch tests associated with them are marked by an asterisk (*).  To test the input, enter diagnostic mode (view the fault codes), and activate each input.  The power loss light will come on if the input is working properly.

Brake Pedal Switch *
Clutch Pedal Switch *
Speed/Distance Sensor
A/C Damped Pressure Switch *
Speed Control Switch
Heated Rear Window Switch *
Neutral Safety Switch *
Voltage Sense
Battery Temperature Sensor
Heated Oxygen Sensor
Coolant Temperature Sensor
Air Charge Temperature Sensor
Throttle Body Temperature Sensor
Detonation Sensor
Fuel Injector Sync Pickup Sensor
Ignition Reference Sensor
Throttle Position Sensor (TPS)
Manifold Absolute Pressure (MAP) Sensor

Logic Module Controlled Components

Below is a list of components that are controlled by the logic module.  Each is linked to a detailed explanation that follows.  Some components may not found on some vehicles, depending on year, model, and options.

Power Loss (or Power Limited or Check Engine) Lamp
A/C Cutout (WOT) Relay
Radiator Fan Relay
Automatic Shutdown (ASD) Relay
Automatic Idle Speed (AIS) Motor
Purge Canister (and EGR) Solenoid
Exhaust Gas Return (EGR) Solenoid
Wastegate Control Solenoid
Barometric Pressure Read Solenoid
Speed Control (Cruise Control) Servo
Diagnostic Read Out Connector

Power Module Controlled Components

Below is a list of engine components that are controlled by the power module.  Each is linked to a detailed explanation that follows.  Some components may not be found on some vehicles, depending on year, model, and options.

In-Tank Fuel Pump
Fuel Injectors
Ignition Coil
Alternator Field Coil

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Updated 12/07/2004.

Copyright © 1996-2004 Russ W. Knize.