by Michael Brinsden

Introduction

The Electronic Lean-Burn system (ELB) was introduced to the Australian Chrysler range in mid 1978 and first appeared on the last production run of the 5.2 litre V8 equipped CL model Chryslers. When the CM model range was introduced in September 1978 the ELB package was extended to the 4.0 litre and 4.3 litre Hemi engines. To the end of CM production, in August 1981, approximately 19,000 vehicles were equipped with ELB engines.

In my experience these engines have proved to be a reliable economical unit, and with due care and maintenance, they have operated in cars driven daily for many years before extensive rework was required. One of the key factors for economy and longevity was engine tuning. The transducers that feed signals to the engine computer had a tendency to drift out of specification, or cease working altogether, with the consequent result that engine performance deteriorated and fuel economy reduced considerably. This article discusses the influence of the transducers on engine performance and the maintenance of the whole ELB system to ensure continuing engine reliability and economy.

ELB System Description

The ELB system consists of a “Spark Control Computer”, the various Engine Sensors (Transducers) and a specially calibrated Carburetor.

Spark Control Computer

This element is the heart of the entire system as it controls the infinitely variable spark advance curves for different modes of engine operation. It consists of a hermetically sealed printed circuit board that receives the signals from the transducers and, within milliseconds, computes the optimum advancement or retardation of the ignition timing. The computer then signals the ignition coil to produce the spark at the calculated optimum time. The Spark Control Computer assembly for all engines is mounted on the Air Cleaner Housing.

Engine Sensors

There are five sensors on the engine that provide the computer with the necessary information to calculate ignition timing. These sensors are-:

1. Pick Up Coil

Located in the Distributor it provides the basic engine timing signal to the computer. Except during engine cranking this signal tells the computer to provide the maximum timing advance applicable to any engine speed. The computer uses this signal to determine the engine speed.

1. Coolant Temperature Sensor

Located on the engine, near the heater hose outlet on V8 engines (Fig 1), the sensor provides a signal to the computer that indicates when the engine coolant temperature is below 65 degrees Centigrade.

Fig 1

1. Throttle Position Transducer

Mounted on the Carburetor (Fig 2) it signals to the computer the position, and the rate of change, of the throttle plates. Incremental spark advance will be generated by the computer when the throttle plates start to open, and in every position to full throttle.

Fig 2

1. Carburetor Switch Sensor

Located on the side of the carburetor (Fig 3) it signals the computer if the engine is at idle or otherwise.

Fig 3

1. Vacuum Transducer

Located physically within the Spark Control Computer it signals the computer the value of engine manifold vacuum. Generally the higher the vacuum the more advance is applied and with lower vacuum the less advance is applied. For any 25mm of vacuum the Carburetor Switch Sensor must remain open for a specified time and when this happens the computer causes the advance to build up slowly to the value specified for that vacuum. If the Carburetor Switch closes before the predetermined time the computer cancels the advance buildup, puts the calculated advance value into memory, and returns the incremental advance to zero. If the carburetor switch is reopened before the advance returns to zero the computer resumes the advance at the value stored in memory. If the carburetor switch reopens after the advance is returned to zero the cycle starts again.

Carburetor

The Carter Carburetor fitted to the ELB System was a specially modified version with adjustments to the Throttle Valves, the Intake Butterfly Valve and the Needle Jets and Jet Seats. In combination with the rest of the ELB system Chrysler Australia claimed a considerable improvement in fuel economy without a consequent loss of power.

The maintenance and servicing of this element of the system will be the subject of a future article.

ELB System Operation

The Spark Control Computer has two functional modes - “Start” and “Run”. The Start mode functions during engine cranking and starting and the Run mode functions after the engine starts and during normal engine operation.

Start During cranking and starting the Pick Up Coil sends its signal to the Computer which is locked in Start mode, with the Run mode bypassed. The fixed amount of advance built into the system is that of the Distributor at this stage.

Run After the engine starts the computer switches to Run mode and the amount of advance applied by the computer is now determined by the signals it receives from all of the sensors. The computer creates additional advance and maintains it for approximately one minute but gradually decreases the advance value. If the coolant temperature is below 65 degrees the Coolant Temperature Sensor signals the computer and prevents any additional vacuum advance being applied. When the engine reaches normal temperature additional advance from the Vacuum Transducer is then applied. The computer then determines, from all of the sensor inputs, the amount of advance required by the engine.

Computer Failure If the Computer fails the system implements an emergency operation mode. This enables the engine to continue running but results in poor performance and lousy fuel economy. If the Pick Up Coil, or the Start Mode of the computer, fails the engine will not start or run.

Failure to Start

If the engine cranks but fails to start conduct the following tests-:

1) Remove the coil high tension lead from the distributor cap and hold the end (make sure your fingers are about 30mm from the end of the lead otherwise you may receive a nasty electrical shock) about 6mm from a good engine earth point. Use a friend to crank the engine while you observe the spark. The spark must be constant and bright blue in colour. Slowly withdraw the end of the lead away from the earth and look for any arcing at the coil tower. If arcing occurs replace the coil and reconduct the test. If the spark is weak or there is no spark proceed to the next test.

2) a. With the Battery isolated from the vehicle wiring measure the battery voltage with a voltmeter. Note the value for future reference.

b. Disconnect the wiring connector from the Coolant Temperature Sensor (Fig 1) and place a thin piece of cardboard between the idle adjustment screw and the Carburetor Switch (Fig 3).

c. Connect the negative lead of a voltmeter to an engine earth. Turn the ignition switch to the “Run” position and measure the voltage at the Carburetor Switch Sensor terminal (Fig 3). The voltage measured should be between 5 and 10 volts. If the voltage is outside this range then check the following-:

• Voltage less than 5 – turn ignition switch “Off” and disconnect the connector from the bottom of the Spark Control Computer. Turn the ignition switch to the “Run” position and measure the voltage at terminal 2 of the computer connector (Fig 4). The voltage should be within 1 volt of the previously noted battery voltage. If the voltage is too low, or nonexistent, check the wiring between terminal 2 and the ignition switch for continuity.

• Voltage greater than 10 volts – check for continuity between terminal 10 of the computer connector (Fig 5) and ground.

Fig 4 Fig 5 Fig 6

d. If the voltage is within 1 volt of the battery voltage turn the ignition switch to the “Off” position and measure the continuity of the wiring between terminal 7 of the connector (Fig 6) and the Carburetor Switch terminal. If there is no, or poor, continuity check the wiring for open circuits, poor connection or shorts and then repair.

3) If the engine then still fails to start turn the ignition switch to the “Run” position and measure the voltage between terminal 1 of the computer connector and the engine earth. This voltage should be within 1 volt of the battery voltage. If it is then turn the ignition switch to the “Off” position and measure, with an ohmmeter, the resistance between terminals 5 and 9 on the computer connector (Fig 7). The resistance measured should be between 150 and 900 ohms. If the resistance measured is outside this range then disconnect the pick up coil lead of the distributor from the engine wiring loom and measure the resistance between the two pick up coil leads (Fig 8). If the resistance is now between 150 and 900 ohms then there is an open circuit, short or poor connection in the wiring between the pick up coil and terminals 5 and 9 of the computer connector. If resistance between the pick up coil leads is outside the range specified then the pick up coil will require replacement. Measure continuity between each lead of the pick up coil and engine earth – if there is no continuity then the pick up coil is OK. If there is continuity then replace the pick up coil (or the whole distributor if you wish).

Fig 7 Fig 8

Fig 9

4) Remove the distributor cap and check the air gap between the pick up coil and the reluctor (Fig 9). If the gap is outside specifications (0.12mm to 0.2mm), adjust and then replace the distributor cap.

5) Reconnect all wiring and the spark control computer connector and engage the ignition switch. If the engine still fails to start then unfortunately you will need to replace the spark control computer unit.

Testing for Poor Performace

The following test procedures are necessary from time to time to keep your ELB engine performing well and in good tune.

Start Advance

Connect an adjustable timing light, start the engine and with the engine in neutral immediately snap the throttle open and closed. Observe the timing mark on the crankshaft after the engine has started and note the amount of “startup advance”. This should be close to the specification value of 7 to 11 degrees above the Basic Timing values of 5 to 8 degrees for the 4 and 4.3 litre engines and 10 degrees for the 5.2 litre engine. Note that the reading will begin to change after about 10 seconds but continue to observe the timing. After about 90 seconds the timing advance should have reduced to the Basic Timing Value. If the timing advance did not increase, and then decrease, the Spark Control Computer will require replacement.

Speed Advance

Before proceeding with this test, make sure that the basic timing advance and the hot engine idle speed (approx. 750 rpm) are at specification levels. Connect a jumper wire between the Carburetor Switch terminal and an engine earth. Disconnect the wiring harness from the Throttle Position Transducer. Start and idle the engine for approximately three minutes. Raise the engine speed to a steady 2000 rpm and note the timing advance obtained. The difference between the result and the Basic Timing is the Speed Advance timing and it should be between 8 and 12 degrees for 4 and 4.3 litre engines and between 2 and 5 degrees for 5.3 litre engines. If the results obtained are outside these ranges replace the Spark Control Computer.

Throttle Position Advance

Before proceeding with this test ensure that the Throttle Position Transducer is in good order and adjusted properly. Set the ignition switch to “Off” and disconnect the connector from the Spark Control Computer. Measure the resistance between terminals 8 and 9 on the computer connector. The resistance should be between 60 and 90 ohms. If the measurement is outside these values disconnect the connector from the Throttle Position Transducer and measure the resistance between the transducer terminals. If the resistance measured is now correct there is a wiring fault between the transducer connector and terminals 8 and 9 of the computer connector.

Note: The specified resistance range for the Throttle Position Transducer may be too low at the high end as I have measured as high as 160 ohms on successfully working transducers.

Reconnect all wiring and turn the ignition switch to “Run”. With a voltmeter measure the voltage on either terminal of the Throttle Position Transducer whilst opening the throttle all the way and then closing. The voltage variation during this process should measure between 3 and 6 volts.

Note: This specified range may not be wide enough as I have measured changes as little as 1.5 volts on successfully working transducers.

Park the throttle on the fast idle cam, connect a jumper wire from the Carburetor Switch to an engine earth and disconnect the connector from the Throttle Position Transducer. Connect a known good transducer to this connector and push in the transducer core wire so that it is bottomed out. Start the engine, wait at least 90 seconds and then move the core out 25mm. Observe the timing on the crankshaft with the timing light and determine the additional advance due to the throttle position. This should be between 3 and 5 degrees for all engine types. Move the core wire back to the bottomed out position and timing should return to the basic setting. If the engine timing did not advance, or return, the Spark Control Computer will require replacement.

Note: ELB engines will still operate satisfactorily if the Throttle Position Transducer is disabled; however running economy and engine reliability will be degraded.

Poor Fuel Economy and High Idle Speed

Using an Ohmmeter check for continuity between the terminal of the Coolant Temperature Sensor and an engine earth. For a cold engine there should be good continuity. For an engine with coolant at 65 degrees or higher there should be no continuity. Replace the coolant switch if this is not true.

Vacuum Advance

Connect the timing light, start the engine and allow it to reach operating temperature. Disconnect the Throttle Position Transducer and place a thin piece of cardboard between the Carburetor Switch and the idle adjustment screw. After about 9 minutes note the timing reading. The vacuum advance above basic timing should be between 4 and 8 degrees for all engine types. If the vacuum advance is not within these values then the Spark Control Computer requires replacement.

If the Vacuum advance is within specification remove the insulating cardboard from the Carburetor Switch. The timing should return to the basic setting. If the timing does not return check that there is a good electrical connection between the idle adjustment screw and the Carburetor Switch.

Turn the engine off and check for good continuity between terminal 7 on the Spark Control Computer connector (Fig 6) and the Carburetor Switch. If this checks out satisfactorily repeat the whole test and if timing will still not return replace the Spark Control Computer.

Adjustment of Throttle Position Transducer

Start the engine and wait at least 90 seconds for the start sequence to complete. Engine must be at normal operating temperature. Connect a jumper wire between the Carburetor Switch and an engine earth and disconnect the Throttle Position Transducer connector. Check that the idle speed and the basic timing are at specification levels. Adjust if necessary.

Reconnect the Throttle Position connector and recheck timing at the crankshaft. If timing is greater than specified loosen the transducer locknut and turn the transducer clockwise until timing returns to the specified limits. Turn an additional half turn clockwise and tighten the locknut.

Engine earth

On 6 cylinder ELB models an earth wire is connected to the throttle cable bracket. The length of this wire is critical to the satisfactory operation of the system and it should remain connected and should never be lengthened. Disconnect the earth, scrape the terminals clean and reconnect the earth.

Nearly every ELB equipped engine I have seen over the years has at least one of the sensors inoperative, or running outside specification. Age and vehicle use degrades the sensors with the consequence that the vehicle runs at less than optimum performance and economy. It is a good idea to check the sensor operation every 15,000Km or so in order to keep your ELB system operating at peak efficiency.

Article based on the Chrysler Australia M Series Service Manual – Author