Jeep CJ/Scrambler 1971-1986 Repair Guide

General Information


See Figure 1

There are 2 electronically controlled feedback emission systems used. The Computerized Emission Control system (CEC) is used on all but 4 cylinder engines for sale in California. The C4 system is used on all 4 cylinder engines offered for sale in California.

The Computerized Emission Control System (CEC) is used on all gasoline engines. There are two primary modes of operation for the CEC feedback system, open loop and closed loop. The system will be in the open loop mode of operation (or a variation of it) whenever the engine operating conditions do not meet the programmed criteria for closed loop operation. During open loop operation, the air/fuel mixture is maintained at a programmed ratio that is dependent on the type of engine operation involved. The oxygen sensor data is not accepted by the system during this mode of operation. The following conditions involve open loop operation.

Engine start-up
Coolant temperature too low
Oxygen sensor temperature too low
Engine idling
Wide open throttle (WOT)
Battery voltage too low

When all input data meets the programmed criteria for closed loop operation, the exhaust gas oxygen content signal from the oxygen sensor is accepted by the computer. This results in an air/fuel mixture that will be optimum for the engine operating condition and also will correct any pre-existing mixture condition which is too lean or too rich.

A high oxygen content in the exhaust gas indicates a lean air/fuel mixture. A low oxygen content indicates a rich air/fuel mixture. The optimum air/fuel mixture ratio is 14.7:1.


Micro Computer Unit (MCU)

The micro computer unit, or MCU, is the heart of the CEC system. The MCU receives signals from various engine sensors to constantly monitor the engine operating conditions, then it uses this information to make adjustments in order to achieve the optimum performance and economy with a minimum of engine emissions. The MCU monitors the oxygen sensor voltage and, based upon the mode of operation, generates an output control signal for the carburetor stepper motor or mixture control solenoid. If the system is in the closed loop mode of operation, the air/fuel mixture will vary according to the oxygen content in the exhaust gas and engine operating conditions. If the system is in the open loop mode of operation, the air/fuel mixture will be based on a predetermined ratio that is dependent on engine rpm. In addition, the MCU generates output signals to control ignition timing and engine idle speed, PCV flow and Pulse Air System operation.

Click image to see an enlarged view

Fig. Fig. 1: CEC system component and wiring diagram

Mixture Control Solenoid

On engines with the Carter YFA or Rochester E2SE carburetors, a mixture control (MC) solenoid is used to regulate the air/fuel mixture. During open loop operation, the MC solenoid supplies a preprogrammed amount of air to the carburetor idle circuit and main metering circuit where it mixes with the fuel. During closed loop operation, the MCU operates the MC solenoid to provide additional or less air to the fuel mixture, depending on the engine operating conditions as monitored by the various engine sensors.

Idle Relay and Solenoid

The idle relay is energized by the MCU to control the vacuum actuator portion of the Sole-Vac throttle positioner by providing a ground for the idle relay. The relay energizes the idle solenoid, which allows vacuum to operate the Sole-Vac vacuum actuator. This, in turn, opens the throttle and increases engine speed. The idle solenoid is located on a bracket on the left front inner fender panel and can be identified by the red connecting wires.

Sole-Vac Throttle Positioner

The Sole-Vac throttle positioner is attached to the carburetor. The unit consists of a closed throttle switch, a holding solenoid and a vacuum actuator. The holding solenoid maintains the throttle position, while the vacuum actuator provides additional engine idle speed when accessories such as the air conditioner or rear window defogger are in use. The vacuum actuator is also activated during deceleration and if the steering wheel is turned to the full stop position on vehicles equipped with power steering.

Upstream and Downstream Air Switch Solenoids

The upstream and downstream solenoids of the pulse air system distribute air to the exhaust pipe and catalytic converter. Both solenoids are energized by the MCU to route air into the the exhaust pipe at a point after the oxygen sensor. When energized, the downstream solenoid routes air into the second bed of the dual-bed catalytic converter. This additional air reacts with the exhaust gases to reduce engine emissions.

The solenoids are located on a bracket attached to the left inner front fender panel. The idle solenoid is also located on this same bracket.

PCV Shutoff Solenoid

The positive crankcase ventilation shutoff solenoid is installed in the PCV valve hose and is energized by the MCU to turn off the crankcase ventilation system when the engine is at idle speed. An anti-diesel relay system on 4 cylinder engines, consisting of an anti-diesel relay and a delay relay, prevents engine run-on when the ignition is switched off by momentarily energizing the PCV valve solenoid when the ignition is switched off to prevent air entering below the throttle plate.

Bowl Vent Solenoid

The bowl vent solenoid is located in the hose between the carburetor bowl vent and the canister. The bowl vent solenoid is closed and allows no fuel vapor to flow when the engine is operating. When the engine is not operating, the solenoid is open and allows vapor to flow to the charcoal canister to control hydrocarbon emissions from the carburetor float bowl. The bowl vent solenoid is electrically energized when the ignition is switched ON and is not controlled by the MCU.

Intake Manifold Heater Switch

The intake manifold heater switch is located in the intake manifold and is controlled by the temperature of the engine coolant. Below 160°F (71°C) the manifold heater switch activates the intake manifold heater to improve fuel vaporization. The switch is not controlled by the MCU and does not provide input information to it.

Oxygen Sensor

This component of the system provides a variable voltage (millivolts) for the micro computer unit (MCU) that is proportional to the oxygen content in the exhaust gas. In addition to the oxygen sensor, the following data senders are used to supply the MCU with engine operation data.

Knock Sensor

The knock sensor is a tuned piezoelectric crystal transducer that is located in the cylinder head. The knock sensor provides the MCU with an electrical signal that is created by vibrations that correspond to its center frequency (5550 Hz). Vibrations from engine knock (detonation) cause the crystal inside the sensor to vibrate and produce an electrical signal that is used by the MCU to selectively retard the ignition timing of any single cylinder or combination of cylinders to eliminate the knock condition.

Vacuum Switches

Two vacuum-operated electrical switches (ported and manifold) are used to detect and send throttle position data to the MCU for idle (closed), partial and wide open throttle (WOT). These switches are located together in a bracket attached to the dash panel in the engine compartment. The 4 in. Hg vacuum switch can be identified by its natural (beige) color, while the 10 in. Hg vacuum switch is green in color. The 4 in. Hg switch is controlled by ported vacuum and its electrical contact is normally in the open position when the vacuum level is less than 4 in. Hg. When the vacuum exceeds 4 in. Hg, the switch closes. The 4 in. Hg vacuum switch tells the MCU when either a closed or deep throttle condition exists.

The 10 in. Hg vacuum switch is controlled by manifold vacuum. Its electrical contact is normally closed when the vacuum level is less than 10 in. Hg; if the vacuum level exceeds 10 in. Hg, the switch opens. This switch tells the MCU that either a partial or medium throttle condition exists.

Engine RPM Voltage

This voltage is supplied from a terminal on the distributor. Until a voltage equivalent to a predetermined rpm is received by the MCU, the system remains in the open loop mode of operation. The result is a fixed rich air/fuel mixture for starting purposes.

Coolant Temperature Switch

The temperature switch supplies engine coolant temperature data to the MCU. Until the engine is sufficiently warmed (above 135°F/57°C), the system remains in the open loop mode of operation (i.e., a fixed air/fuel mixture based upon engine rpm).

Thermal Electric Switch

The thermal electric switch is located inside the air cleaner to sense the incoming air temperature and indicate a cold weather start-up condition to the MCU when the air temperature is below 50°F (10°C). Above 65°F (18°C), the switch opens to indicate a normal engine start-up condition to the MCU.

Wide Open Throttle (WOT) Switch

The wide open throttle switch is attached to the base of the carburetor by a mounting bracket. It is a mechanically operated electrical switch that is controlled by the position of the throttle. When the throttle is placed in the wide open position, a cam on the throttle shaft actuates the switch about 15° before wide open position to indicate a full-throttle demand to the MCU.

Altitude Jumper Wire

The altitude jumper wire connector is located next to the MCU. The jumper wire provides the MCU with an indication of whether the vehicle is being operated above or below a 4000 ft. elevation (high altitude operation). The connector normally has no jumper wire installed. If a vehicle is to be operated in a designated high altitude area, a jumper wire must be installed.