Jeep CJ/Scrambler 1971-1986 Repair Guide

C4 System

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OPERATION



The open loop mode of operation occurs when:

  1. Starting engine, engine is cold or air cleaner air is cold.
  2.  
  3. Engine is at idle speed, accelerating to partial throttle or decelerating from partial throttle to idle speed.
  4.  
  5. Carburetor is either at or near wide open throttle (WOT).
  6.  

When any of these conditions occur, the mixture control (MC) solenoid provides a predetermined air/fuel mixture ratio for each condition. Because the air/fuel ratios are predetermined and no feedback relative to the results is accepted, this type of operation is referred to as open loop operation. All open loop operations are characterized by predetermined air/fuel mixture ratios. Each operation (except closed loop) has a specific air/fuel ratio and because more than one of the engine operational selection conditions can be present at one time, the MCU is programmed with a priority ranking for the operations. It complies with the conditions that pertain to the operation having the highest priority. The priorities are as described below.

Cold Weather Engine Start-Up and Operation

If the air cleaner air temperature is below the calibrated value (55°F or 13°C) of the thermal electric switch (TES), the air/fuel mixture is at a "rich'' ratio. Lean air/fuel mixtures are not permitted for a preset period following a cold weather start-up.

At or Near Wide Open Throttle (WOT) Operation (Cold Engine)

This open loop operation occurs whenever the coolant temperature is below the calibrated switching value (95°F or 35°C) of the open loop coolant temperature switch and the WOT vacuum switch (cold) has been closed because of the decrease in manifold vacuum (less than 5 in. Hg or 17 kPa). When this open loop condition occurs the MC solenoid provides a rich air/fuel mixture for cold engine operation at wide open throttle.

Temperature and switching vacuum levels are nominal values. The actual switching temperature or vacuum level will vary slightly from switch to switch.

At or Near Wide Open Throttle (WOT) Operation (Warm Engine)

This open loop operation occurs whenever the coolant temperature is above the calibrated switching temperature (135°F or 57°C) of the enrichment coolant temperature switch and the WOT vacuum switch (warm) has been opened because of the decrease in manifold vacuum (less than 3 in. Hg or 10 kPa). When this open loop condition occurs the MC solenoid provides a rich air/fuel mixture for warm engine operation at wide open throttle.

Adaptive Mode of Operation

This open loop operation occurs when the engine is either at idle speed, accelerating from idle speed or decelerating to idle speed. If the engine rpm (tach) voltage is less than the calibrated value and manifold vacuum is above the calibrated switching level for the adaptive vacuum switch (i.e., switch closed), an engine idle condition is assumed to exist. If the engine rpm (tach) voltage is greater than the calibrated value and manifold vacuum is above the calibrated switching level of the adaptive vacuum switch (i.e., switch closed), an engine-deceleration-to-idle speed condition is assumed to exist. During the adaptive mode of operation the MC solenoid provides a predetermined air/fuel mixture.

Closed Loop Operation

Closed loop operation occurs whenever none of the open loop engine operating conditions exist. The MCU causes the MC solenoid to vary the air/fuel mixture in reaction to the voltage input from the oxygen sensor located in the exhaust manifold. The oxygen sensor voltage varies in reaction to changes in oxygen content present in the exhaust gas. Because the content of oxygen in the exhaust gas indicates the completeness of the combustion process, it is a reliable indicator of the air/fuel mixture that is entering the combustion chamber.

Because the oxygen sensor only reacts to oxygen, manifold air leak or malfunction between the carburetor and sensor may cause the sensor to provide an erroneous voltage output. The engine operation characteristics never quite permit the MCU to compute a single air/fuel mixture ratio that constantly provides the optimum air/fuel mixture. Therefore, closed loop operation is characterized by constant variation of the air/fuel mixture because the MCU is forced constantly to make small corrections in an attempt to create an optimum air/fuel mixture ratio.

DIAGNOSIS AND TESTING



The C4 System should be considered as a possible source of trouble for engine performance, fuel economy and exhaust emission complaints only after normal tests and inspections that would apply to an automobile without the system have been performed. The steps in each test will provide a systematic evaluation of each component that could cause an operational malfunction.

To determine if fault exists with the system, a system operational test is necessary. This test should be performed when the C4 System is suspected because no other reason can be determined for a specific complaint. A dwell meter, digital volt-ohmmeter, tachometer, vacuum gauge and jumper wires are required to diagnose system problems. Although most dwell meters should be acceptable, if one causes a change in engine operation when it is connected to the mixture control (MC) solenoid dwell pigtail wire test connector, it should not be used.

The dwell meter, set for the six-cylinder engine scale and connected to a pigtail wire test connector leading from the mixture control (MC) solenoid, is used to determine the air/fuel mixture dwell. When the dwell meter is connected, do not allow the connector terminal to contact any engine component that is connected to engine ground. This includes hoses because they may be electrically conductive. With a normally operating engine, the dwell at both idle speed and partial throttle will be between 10 degrees and 50 degrees and will be varying. Varying means the pointer continually moves back and forth across the scale. The amount it varies is not important, only the fact that is does vary. This indicates closed loop operation, indicating the mixture is being varied according to the input voltage to the MCU from the oxygen sensor. With wide open throttle (WOT) and/or cold engine operation, the air/fuel mixture ratio will be predetermined and the pointer will only vary slightly. This is open loop operation, indicating the oxygen sensor output has no effect on the air/fuel mixture. If there is a question whether or not the system is in closed loop operation, enrich or leaning the air/fuel mixture will cause the dwell to vary more if the system is in closed loop operation.

Test Equipment

The equipment required to perform the checks and tests includes a tachometer, a hand vacuum pump and a digital volt-ohmmeter (DVOM) with a minimum ohms per volt of 10 mega-ohms.


CAUTION
The use of a voltmeter with less than 10 mega-ohms per volt input impedance can destroy the oxygen sensor. Since it is necessary to look inside the carburetor with the engine running, observe the following precautions.1. Shape a sheet of clear acrylic plastic at least 0.250 in. thick and 15 in. × 15 in.2. Secure the acrylic sheet with an air cleaner wing nut after the top of the air cleaner has been removed.3. Wear eye protection whenever performing checks and tests.4. When engine is operating, keep hands and arms clear of fan, drive pulleys and belts. Do not wear loose clothing. Do not stand in line with fan blades.5. Do not stand in front of running car.

C4 SELF DIAGNOSTIC SYSTEM



Late model Jeep vehicles equipped with a six cylinder engine and California emissions package have a self-diagnostic system with a CHECK ENGINE light mounted in the instrument panel. The self-diagnostic system is designed to detect problems most likely to occur within the various system components.

When a jumper wire is connected between the trouble code test terminals 6 and 7 of the 15-terminal diagnostic connector (D2), the CHECK ENGINE light will flash a trouble code or codes that indicate a problem area. For a bulb and system check, the CHECK ENGINE light will illuminate when the ignition switch is ON and the engine not started. If the test terminals are then grounded, the light will flash a code 12 that indicates the self-diagnostic system is operational. A code 12 consists of one flash, followed by a short pause, then two more flashes in quick succession. After a longer pause, the code will repeat two more times.

When the engine is started, the CHECK ENGINE light will remain ON momentarily and then be turned off. If the CHECK ENGINE light remains on, the self-diagnostic system has detected a problem. If the trouble code test terminals are then grounded, the trouble code will be flashed three times; if more than one trouble code is stored, each will be flashed three more times in numerical order, from the lowest to the highest numbered code. This trouble code series will repeat as long as the test terminals are grounded.

A trouble code indicates a problem in a particular circuit or component. Trouble code 14, for example, indicates a problem in the coolant sensor circuit. It should be noted that the self-diagnostic system doesn't pinpoint where the problem is in the coolant sensor circuit, which includes the coolant sensor, connector, wire harness and the electronic control unit itself. The following diagnostic charts contain procedures for isolating the problem to a particular point in the circuit to avoid the unnecessary replacement of working components. A diagnosis chart is provided for each trouble code.

Because the self-diagnostic system does not detect all possible problems, the absence of a trouble code does not necessarily mean the system is functioning normally. The System Performance Test should be performed when the self-diagnostic system does not indicate a problem, but the system operation is suspect.

All system connectors in the engine compartment are sealed against debris and moisture. Because the system operates on low voltage and low current, corrosion on the connectors can cause problems. Before repairing or replacing any component, disconnect the appropriate connector(s) and check for proper installation, bent, broken or dirty terminals or mating tabs. Clean, straighten or replace the connectors as required, then reconnect everything and recheck the system operation to see if the problem has been corrected. The system should be considered as a possible cause of trouble only after normal engine diagnosis for ignition timing, carburetor and idle speed adjustments has been performed. The electronic control module (ECM) is located in the passenger compartment at the right side of the steering column below the instrument panel.

TROUBLE CODE MEMORY



When a problem develops in the feedback system, the CHECK ENGINE light will illuminate and a trouble code will be stored in the on-board computer memory. If the fault is intermittent, the CHECK ENGINE light will be turned off 10 seconds after the problem disappears. The trouble code will be retained in the memory until the battery voltage to the control unit is removed. Disconnecting the battery for 10 seconds will erase all stored trouble codes.

The CHECK ENGINE light will be illuminated only if a problem exists that pertains to the conditions listed below. It takes up to 5 seconds minimum for the light to come on when a problem occurs. Code 12 is not stored in memory and any codes stored will be cleared if the problem does not reoccur within 50 engine starts. The trouble codes indicate problems as follows:



CODE 12: No distributor reference pulses to the ECM. This code is not stored in memory and will only flash while the trouble exists. This code is normal when the ignition is switched ON with the engine not running.
 
CODE 13: Oxygen sensor circuit. The engine must operate for up to 5 minutes at part throttle, under road load, before this code will be set.
 
CODE 14: Shorted coolant sensor circuit. The engine must operate for up to 5 minutes before this code will be set.
 
CODE 15: Open coolant sensor circuit. The engine must operate for up to 5 minutes before this code will be set.
 
CODE 21: Throttle position sensor circuit. The engine must operate for at least 25 seconds at curb idle speed before this code will be set.
 
CODE 23: Mixture control solenoid circuit is shorted or open.
 
CODE 34: Vacuum sensor circuit. The engine must operate for up to 5 minutes at curb idle speed before this code will be set.
 
CODE 41: No distributor reference pulses to the ECM at the specified engine manifold vacuum. This code will be stored in memory.
 
CODE 42: Electronic spark timing (EST) bypass circuit or EST circuit has short circuit to ground or an open circuit.
 
CODE 44: Lean exhaust indication. The engine must operate for up to 5 minutes, be in closed loop operation and at part throttle before this code will be set.
 
CODE 44 & 45: If these two codes appear at the same time, it indicates a problem in the oxygen sensor circuit.
 
CODE 45: Rich exhaust indication. The engine must operate for up to 5 minutes, be in closed loop and at part throttle before this code will be set.
 
CODE 51: Faulty calibration unit (PROM) or installation. It requires up to 30 seconds for this code to be set.
 
CODE 54: Mixture control (MC) solenoid circuit is shorted or the ECM is faulty.
 
CODE 55: Voltage reference has short circuit to ground (terminal 21), faulty oxygen sensor or faulty ECM.
 

 
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