Ignition System - 6-cyl Dodge Diplomat I

DESCRIPTION

The Electronic Spark Control (ESC) system is governed by a Spark Control Computer, 6 engine sensors, a specially-calibrated carburetor and, on most models, a dual pick-up distributor. The ESC system is designed to burn a lean air/fuel mixture, with a minimum of emissions. (Scheme 1)

Wiring Diagram for Chrysler Motors ESC System. Scheme 1

SPARK CONTROL COMPUTER

The Spark Control Computer (SCC) is the heart of the entire system. It gives the system the capability of igniting a lean fuel mixture according to different modes of engine operation by delivering an infinite amount of variable advance curves. The computer determines the exact instant when ignition is required, then signals the ignition coil to produce the spark required to fire the spark plugs.

The computer consists of one electronic printed circuit board which receives signals from all the sensors and, within milliseconds, computes them so that proper advance or retard is immediately achieved.

SENSORS

The electronic spark control computer, mounted on the air cleaner, uses 6 engine sensors to determine when to fire the spark plugs. (Scheme 2) These include the distributor pick-up coil(s), coolant temperature sensor, carburetor switch, vacuum transducer, oxygen sensor and charge temperature switch. Their functions are as follows.

ESC Computer. SCC is mounted on air cleaner. Scheme 2

Magnetic Pick-Up Assembly

The pick-up coil assembly is located in the distributor. The start pick-up coil supplies a signal to the computer, which will cause the spark plugs to fire at a fixed amount of advance during cranking only. Once the engine begins to run, the run pick-up coil takes over, supplying advance information to the computer. The computer then modifies advance information to reflect other engine operating conditions supplied by the remaining sensors.

The start pick-up coil can be identified by its 2-prong male connector. The run pick-up coil can be identified by its connector having 1 male prong and 1 female prong.

Coolant Temperature Sensor

The coolant sensor is located in the cylinder head of the engines. The coolant sensor informs the computer when the engine has reached a predetermined temperature, so that proper adjustment can be made to the air/fuel ratio. It also prevents changes until such a temperature is reached.

Vacuum Transducer

This sensor, located on the spark control computer, signals the computer to inform it of engine operating vacuum. Vacuum is one of the factors used to determine whether the computer will advance or retard ignition timing or change the air/fuel ratio.

Carburetor Switch

Located on the end of idle stop, the carburetor switch informs the computer when the engine is at idle. When carburetor switch contacts throttle lever ground, the computer will cancel spark advance and prevent air/fuel ratio from being adjusted.

Oxygen Sensor

Located in the exhaust manifold, this sensor informs the computer of the amount of oxygen present in exhaust gases. The amount is proportional to the rich and lean mixtures. The computer adjusts air/fuel ratio so that it will maintain operating efficiency of the 3-way catalyst system and the engine.

Charge Temperature Switch

This sensor is located in the intake manifold. The switch will be closed when intake charge (air/fuel mixture) temperature is below 60°F (16°C). This permits no EGR timer function, no EGR valve operation and switches air injection upstream into exhaust system. When temperature is above 60°F, the switch opens, allowing EGR timer to time out, the EGR valve to operate and air injection is switched downstream into the exhaust system.

OPERATION

The Spark Control Computer has two functional modes, "Start" and "Run". The "Start" mode operates while cranking and starting only. The "Run" mode operates after the engine has started and during normal engine operation. The two modes never operate at the same time. During cranking and starting the pick-up coil sends a signal to the computer which is in the "Start" mode, the "Run" mode is by-passed. During this time a fixed advance is used. Advance is determined by distributor position (basic timing).

After the engine starts, the pick-up coil continues to send a signal to the computer, but the computer is now in the "Run" mode and "Start" mode is by-passed. The amount of timing advance is now controlled by the computer, based upon information received from the engine sensors.

The amount of spark advance is determined by two factors, engine speed and engine vacuum. At what point it occurs depends upon computer programming. Advance from vacuum will be provided when the carburetor switch is open. The amount of advance is programmed into the computer and is proportional to the amount of vacuum and engine RPM. Advance from speed will be given by the computer when the carburetor switch is open and is programmed to engine RPM.

If for some reason, there is a failure of the "Run" mode of the computer, the "Start" mode will come back into service and allow the vehicle to be driven in for repair. Performance and economy will be greatly reduced because of the fixed timing.

Scheme 3

1. Align one reluctor tooth with start pick-up coil. Loosen pick-up coil hold down screw. Insert a.006" (.15 mm) non-magnetic feeler gauge between reluctor tooth and start pick-up coil. Adjust air gap so that contact is made between reluctor tooth, feeler gauge, and pick-up coil. Tighten hold down screw. (Scheme 3) (Scheme 3): Checking Distributor Pick-Up Coil Air Gap
2. Remove feeler gauge. No force should be required to remove feeler gauge. Check air gap with a.008" (.20 mm) feeler gauge. The.008" (.20 mm) feeler gauge should fit into gap. DO NOT force feeler gauge into gap. If.008" (.20 mm) feeler gauge does not fit into gap, repeat adjustment procedure.
3. The run pick-up coil air gap can be adjusted using the same procedure as for the start pick-up coil. Adjust the run pick-up coil air gap using a.012" (.30 mm) non-magnetic feeler gauge and check gap with a.014" (.35 mm) feeler gauge. On single pick-up distributors, adjust using.006" (.15 mm) feeler gauge and check using.008" (.20 mm) feeler gauge.

TROUBLE SHOOTING

Note. See the TROUBLE SHOOTING - BASIC PROCEDURES article in the GENERAL TROUBLE SHOOTING section.

IGNITION SYSTEM STARTING

1. Turn ignition switch "ON". Remove ignition coil wire from distributor cap. Hold end of wire 1/4" from a good engine ground. Intermittently jump coil negative terminal to ground, while watching for spark at ignition coil wire. If there is a spark, it must be constant and bright blue.
2. If spark is good, continue to intermittently jumper coil negative terminal to ground, while slowly moving coil wire away from ground. Check for arcing at coil tower. If arcing occurs, replace ignition coil.
3. If spark is weak or not constant, or if there is no spark, proceed to "Failure to Start Test".
4. If spark is good and there is no arcing at ignition coil tower, ignition system is producing necessary high secondary voltage. Make sure this spark is getting to plugs by checking distributor rotor, cap, spark plugs, and plug wires.
5. If all this checks out okay, but engine still will not start, the ignition system is not the problem. It will be necessary to check fuel system and mechanical engine components.

FAILURE TO START

Note. Perform Ignition System Starting Test first. Failure to do so may result in lost diagnostic time or incorrect test results.

Scheme 4

Scheme 5

Scheme 6

Scheme 7

Scheme 8

Scheme 9

1. Measure and record battery voltage. Measure specific gravity, which must be at least 1.220 (temperature corrected) to deliver proper voltage.
2. Turn ignition switch "OFF". Disconnect 10-wire connector from spark control computer. Repeat step 1 of Ignition System Starting Test. If spark results, replace spark control computer.
3. If no spark is obtained, check voltage at positive terminal of ignition coil. With ignition switch "ON", connect positive voltmeter lead to positive terminal of ignition coil and negative lead to a good ground. Reading should be within 1 volt of battery voltage. If not, check wiring between battery and coil positive terminal.
4. If voltage at positive terminal of ignition coil was correct, connect positive voltmeter lead to negative terminal of ignition coil and negative lead to a good ground. Again, voltage should be within 1 volt of battery voltage. If not, replace ignition coil. NOTE: Check coil primary and secondary resistance before replacing ignition coil. However, if battery voltage exists on positive side, but not on negative side of coil, ignition coil normally requires replacement.
5. If voltage was correct at negative coil terminal, but no spark resulted in step 1 of Ignition System Starting Test, replace ignition coil.
6. If spark results, but engine will not start, turn ignition switch to the "RUN" position. Connect positive voltmeter lead to terminal 1 of 10-wire connector and negative lead to a good ground. (Scheme 4) (Scheme 4): Voltmeter Hookup for Checking Terminal 1 Voltage
7. Reading should be within 1 volt of battery voltage. If not, check wire for open and repair it, repeating this step once more. Reconnect 10-wire connector to computer.
8. If battery voltage was recorded in step 6, place a thin insulator (piece of paper) between curb idle adjusting screw and carburetor switch or make sure screw does not touch the switch. (Scheme 5) (Scheme 5): Checking Voltage at Carburetor Switch
9. Connect negative lead of voltmeter to a good ground. Turn ignition switch to "RUN" position, and touch positive voltmeter lead to carburetor switch terminal. Reading should be approximately 5 volts. If so, proceed to step 15.
10. If voltage was not at least 5 volts, turn ignition switch "OFF". Disconnect 10-wire connector from computer. Turn ignition switch back to "RUN" position. Connect positive voltmeter lead to terminal 2 of 10-wire connector and negative lead to ground. (Scheme 6) (Scheme 6): Voltmeter Hookup for Checking Terminal 2 Voltage
11. Voltage reading should again be within 1 volt of battery voltage. If not correct, check wiring between terminal 2 and ignition switch for opens, shorts or poor connections.
12. If voltage at terminal 2 was correct, turn ignition switch "OFF". Using an ohmmeter, check continuity between terminal 7 of 10-wire connector and carburetor switch terminal. (Scheme 7) (Scheme 7): Ohmmeter Hookup for Checking Carburetor Switch Wiring Harness
13. Continuity should exist. If not, check wire between connections for opens, shorts or poor connections. If continuity is present, use an ohmmeter with leads attached to terminal 10 and engine ground to check continuity of ground circuit. (Scheme 8) (Scheme 8): Ohmmeter Hookup for Checking Computer Ground Circuit
14. If there is continuity, replace spark control computer. If there is no continuity, check wire from terminal 10 to ground. If engine fails to start, proceed to step 15.
15. Turn ignition switch "OFF". Attach ohmmeter leads to terminals 5 and 9 of 10-wire harness connector to check run pick-up coil resistance and to terminals 3 and 9 to check start pick-up coil resistance. (Scheme 9) Resistance should be 150-900 ohms for both pick-up coils. If so, proceed to step 17. (Scheme 9): Ohmmeter Hookup for Checking Pick-Up Coil Resistance
16. If resistance is not 150-900 ohms, disconnect distributor connectors and attach ohmmeter leads to run pick-up coil leads and then to start pick-up coil leads coming from distributor. If resistance is now okay, wiring harness is defective. If resistance is still not 150-900 ohms, replace pick-up coil(s), as necessary.
17. Connect one lead of an ohmmeter to engine ground and touch other lead to each terminal of leads coming from 2 distributor pick-up coils. There should be no continuity. If continuity is indicated, replace pick-up coil.
18. Remove distributor cap and rotor and check reluctor-to-pick-up coil(s) air gap as described previously. Install distributor cap and reinstall all wiring. If engine fails to start, replace spark control computer. If it still fails to start, install original computer and retest.

IGNITION COIL RESISTANCE

1. If ignition coil is suspected, connect ohmmeter leads to positive and negative ignition coil primary terminals. Ignition switch should be "OFF" and coil wires removed. Primary resistance should read 1.60-1.79 ohms for Prestolite coils; 1.34-1.55 ohms for Essex coils.
2. Then move ohmmeter leads to ignition coil negative terminal and ignition coil tower. Ohmmeter resistance reading should be 9,400-11,700 for Prestolite coils, 9,000-12,200 ohms for Essex coils.
3. Replace ignition coil if either specification is not obtained.

BASIC TIMING

1. Connect timing light to engine so that timing can be checked. Connect a jumper wire between carburetor switch and a good ground. See note.
2. Be sure vacuum line is disconnected to vacuum transducer on computer. Observe timing mark and adjust timing as indicated on vehicle emission control label.

SPARK ADVANCE OF COMPUTER

Note. The Spark Control Computer has various spark advance schedules incorporated into its microprocessor for operation at differing engine temperatures. Therefore, be sure engine is at normal operating temperature before testing.

1. Start engine and allow it to warm to normal operating temperature. Place transmission in neutral and set parking brake.
2. Place a thin insulator (piece of paper) between curb idle adjusting screw and carburetor switch, or make sure screw is not touching switch. (Scheme 5) Remove and plug vacuum line at vacuum transducer.
3. Connect an auxiliary vacuum supply to vacuum transducer and apply 10 in. Hg vacuum for 3.7L engines or 16 in. Hg vacuum for 5.2L engines. Increase engine speed to 2000 RPM, wait one minute for specified accumulator clock up time, and then check timing. CAUTION: Use a metal exhaust tube for this test, as high temperatures and extended test times could cause rubber hose to catch fire.
4. If computer fails to obtain settings, replace computer.

Note. Grounding the carburetor switch eliminates all spark advance.

1. Turn ignition key off and disconnect 10-wire harness connector from computer. With throttle completely closed, check continuity between terminal 7 of harness connector and a good ground. If no continuity is indicated, check wire and carburetor switch.
2. Recheck basic timing. With throttle opened, check continuity between terminal 7 of harness connector and a good ground. There should be no continuity. If continuity exists, replace carburetor switch.

1. Connect ohmmeter leads to sensor terminals. With engine cold and ambient temperature below 90°F (32°C), resistance should be 500-1000 ohms. With engine hot (normal operating temperature), resistance should be more than 1300 ohms.
2. If not to specifications, replace coolant temperature sensor. The sensor will continually change its resistance with changes in engine operating temperature. This device is NOT a switch.

CHARGE TEMPERATURE & COOLANT SWITCH

1. Turn ignition switch "OFF" and disconnect wire from charge temperature switch. Connect one lead of ohmmeter to a good engine ground (or to switch's ground terminal). Connect other lead to center terminal of coolant switch. Check for continuity.
2. For a cold engine, continuity should be present with a resistance less than 100 ohms. If not, replace the switch. The charge temperature switch must be cooler than 60°F (15°C) to obtain this reading.
3. For an engine at normal operating temperature, the terminal should show no continuity. If it does, replace coolant switch or charge temperature switch.

DISASSEMBLY

1. Remove distributor cap. Using 2 screwdrivers, pry off rotor from shaft. Remove reluctor by prying up from bottom of reluctor using 2 pry bars or screwdrivers with a maximum width of 7/16". Be careful not to distort or damage reluctor teeth.
2. Remove 2 screws and lock washers attaching plate to housing, and lift out plate and pick-up coil(s) as an assembly. DO NOT attempt to remove distributor cap clamps, as they are peened in place.
3. If distributor has more than .006" (.15 mm) shaft side play, replace housing shaft and reluctor sleeve by removing shaft retaining pin and sliding retainer off end of shaft.
4. On 3.7L engines, the distributor drive gear must be replaced if it is either worn or damaged. Scribe a line on the bottom of distributor shaft in a direct line with the original distributor drive gear retaining pin hole. Remove drive gear retaining pin and drive gear.
5. Use a file to clean burrs from around pin hole in shaft and remove lower thrust washer. Push shaft up and remove shaft through top of distributor housing.

Exploded View of 3.7L ESC Distributor. Scheme 10

1. Inspect all bearing surfaces and pivot pins for roughness, binding, or looseness. Lubricate and install upper thrust washer on shaft and slide shaft into distributor housing. NOTE: Steps 2 and 3 apply to 3.7L engines.
2. Slide new drive gear onto shaft and position drive gear retaining pin hole 90° from original hole, using scribed line on distributor shaft as a guide. It will be necessary to drill a new hole in the distributor shaft.
3. Before drilling new hole, .124-.129" (3.15-3.28 mm) in diameter, be sure that clearance between drive gear and thrust washer is .007" (.18 mm). Drill new hole and install new drive gear and drive gear retaining pin.
4. Install lower plate, upper plate, and pick-up coil(s) as an assembly.
5. Position reluctor keeper pin into place on reluctor sleeve and firmly press reluctor into place. Make sure keeper pin is in place. Lubricate felt pad in top of reluctor sleeve and install rotor and cap.

See also:
• TROUBLE SHOOTING - BASIC PROCEDURES