AMC Computerized Engine Control
Encyclopedia
The Computerized Engine Control or CEC system was an engine management system designed and used by American Motors and Jeep
from 1980-1990, on the AMC 258 engine.
CEC was unique in that almost all of its sensors and actuators were digital
; instead of the usual analog throttle position, coolant temperature, intake temperature and manifold pressure sensors, it used a set of fixed pressure- and temperature-controlled switches (as well as a wide-open throttle switch on the carburetor
) to control fuel mixture and ignition timing. The only analog sensor in the system was the oxygen sensor
. In other respects, it was a typical feedback carburetor system of the early 1980s, using a stepper motor
to control fuel mixture and a two-stage "Sole-Vac" (which used a solenoid
for one stage, and a vacuum
motor
for the other) to control idle speed. CEC also controlled ignition timing using information from the fuel-control section and an engine knock
sensor on the intake manifold.
The CEC module itself (the most common version of which is the "AMC MCU Super-D") was built for AMC by Ford Motor Company
, and worked with a Ford Duraspark
ignition system
. Despite being built by Ford, the CEC module is not related to the Ford EEC systems internally.
Because of the many vacuum-driven components and electrical connections in the system, CEC-equipped engines have a reputation of being hard to tune. The 49-state model of the CEC has no on-board diagnostic system, making it difficult to monitor the computer's operation without a breakout box
, and the Carter BBD carburetor on most CEC-equipped models has problems with its idle circuit clogging, causing rough idle and stalling. In places where emissions testing isn't required, a popular modification is to bypass the computer and disable the BBD's Idle Servo, or replace the BBD with a manually-tuned carburetor, this process is commonly known as the "Nutter Bypass" named after John Nutter, noted to be the first person to document the process. Several vendors (including Chrysler) offer retrofit kits that replace the CEC and the carburetor with fuel injection.
Jeep
Jeep is an automobile marque of Chrysler . The first Willys Jeeps were produced in 1941 with the first civilian models in 1945, making it the oldest off-road vehicle and sport utility vehicle brand. It inspired a number of other light utility vehicles, such as the Land Rover which is the second...
from 1980-1990, on the AMC 258 engine.
CEC was unique in that almost all of its sensors and actuators were digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
; instead of the usual analog throttle position, coolant temperature, intake temperature and manifold pressure sensors, it used a set of fixed pressure- and temperature-controlled switches (as well as a wide-open throttle switch on the carburetor
Carburetor
A carburetor , carburettor, or carburetter is a device that blends air and fuel for an internal combustion engine. It is sometimes shortened to carb in North America and the United Kingdom....
) to control fuel mixture and ignition timing. The only analog sensor in the system was the oxygen sensor
Oxygen sensor
An oxygen sensor, or lambda sensor, is an electronic device that measures the proportion of oxygen in the gas or liquid being analyzed. It was developed by the Robert Bosch GmbH company during the late 1960s under the supervision of Dr. Günter Bauman...
. In other respects, it was a typical feedback carburetor system of the early 1980s, using a stepper motor
Stepper motor
A stepper motor is a brushless, electric motor that can divide a full rotation into a large number of steps. The motor's position can be controlled precisely without any feedback mechanism , as long as the motor is carefully sized to the application...
to control fuel mixture and a two-stage "Sole-Vac" (which used a solenoid
Solenoid
A solenoid is a coil wound into a tightly packed helix. In physics, the term solenoid refers to a long, thin loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electric current is passed through it. Solenoids are important because they can create...
for one stage, and a vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
motor
Pneumatic motor
A pneumatic motor or compressed air engine is a type of motor which does mechanical work by expanding compressed air. Pneumatic motors generally convert the compressed air to mechanical work through either linear or rotary motion...
for the other) to control idle speed. CEC also controlled ignition timing using information from the fuel-control section and an engine knock
Engine knocking
Knocking in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front.The...
sensor on the intake manifold.
The CEC module itself (the most common version of which is the "AMC MCU Super-D") was built for AMC by Ford Motor Company
Ford Motor Company
Ford Motor Company is an American multinational automaker based in Dearborn, Michigan, a suburb of Detroit. The automaker was founded by Henry Ford and incorporated on June 16, 1903. In addition to the Ford and Lincoln brands, Ford also owns a small stake in Mazda in Japan and Aston Martin in the UK...
, and worked with a Ford Duraspark
Duraspark
The Duraspark II is a triggering device for a Ford electronic ignition system. Coupled with either the stock ignition box or an aftermarket control box, it is a tough, readily available, reliable distributor....
ignition system
Ignition system
An ignition system is a system for igniting a fuel-air mixture. Ignition systems are well known in the field of internal combustion engines such as those used in petrol engines used to power the majority of motor vehicles, but they are also used in many other applications such as in oil-fired and...
. Despite being built by Ford, the CEC module is not related to the Ford EEC systems internally.
Because of the many vacuum-driven components and electrical connections in the system, CEC-equipped engines have a reputation of being hard to tune. The 49-state model of the CEC has no on-board diagnostic system, making it difficult to monitor the computer's operation without a breakout box
Breakout box
A breakout box is an electrical device, usually housed in a box, in which a compound electrical connector is separated or "broken out" into its component connectors. Compound connectors are used where sufficient space for connections is unavailable...
, and the Carter BBD carburetor on most CEC-equipped models has problems with its idle circuit clogging, causing rough idle and stalling. In places where emissions testing isn't required, a popular modification is to bypass the computer and disable the BBD's Idle Servo, or replace the BBD with a manually-tuned carburetor, this process is commonly known as the "Nutter Bypass" named after John Nutter, noted to be the first person to document the process. Several vendors (including Chrysler) offer retrofit kits that replace the CEC and the carburetor with fuel injection.
External links
- Jeep Adventures Under the Hood A detailed guide on the CEC system and how to tune it
- The "Nutter Bypass" method of eliminating the CEC system.