Fuel Injection System - Multi-Point Jeep Wagoneer XJ

COMPONENT TESTING

Note. Specific component testing information not available. Refer to any tests performed during REMOVAL & INSTALLATION or OVERHAUL procedures

DESCRIPTION

The AMC Multi-Point Electronic Fuel Injection system is an electronically controlled system which combines electronic fuel injection and electronic spark advance systems. The main sub-systems consist of: Air Induction, Fuel Delivery, Fuel Control, Emission Control, Electronic Control Unit (ECU) and Data Sensors.

The Air Induction system includes air cleaner, throttle body, Throttle Position Sensor (TPS) and the Idle Speed Stepper (ISS) motor.

The Fuel Delivery system provides fuel from the fuel pump to the fuel control system. It also returns excess fuel to the fuel tank. The system is composed of an in-tank electric fuel pump, fuel filter and return line. Power is provided to operate the fuel pump through a fuel pump relay located on the right inner fender panel.

The Fuel Control system handles the actual delivery of fuel into the engine. The fuel pressure regulator maintains a constant fuel pressure of 31-39 psi (2.1-2.7 kg/cm 2 ). In addition to the regulator, the system consists of the fuel rail and 4 fuel injectors.

Emission systems controls, although directly operated by the ECU, are not unique to the Multi-Point EFI engine. Used on the MPI system is an ECU controlled EGR solenoid.

The ECU is a digital microprocessor computer. The ECU receives input signals from various switches and sensors. It then computes the fuel injector pulse width ("on" time), spark advance, ignition module dwell, idle speed, canister purge cycles, EGR flow and feedback control from this information.

AIR INDUCTION

Air is drawn into the combustion chamber through the air cleaner and intake manifold. The amount of air entering the engine is controlled by the position of the throttle body valve. The throttle body houses the TPS and ISS motor. The TPS is an electrical resistor which is connected to the throttle valve. The TPS transmits a signal to the ECU in relation to throttle valve angle. From this signal, the ECU calculates fuel injector "on" time to provide adequate air/fuel mixture.

The ECU controls the idle speed by providing the appropriate voltage outputs to move the ISS motor pin inward or outward to maintain a predetermined idle speed. The ECU continuously monitors the TPS and ISS motor and issues change commands to the injectors to increase or decrease the amount of fuel injected.

FUEL DELIVERY

Power to the fuel pump relay is supplied from the ignition switch when in the "ON" or "START" position, at which time the ECU supplies a ground for the fuel pump relay. When the relay contacts are closed, power is applied to the fuel pump.

Fuel is drawn through one end of a roller-type electric fuel pump, compressed and forced out the opposite end. Pump capacity is greater than the maximum engine consumption so that the pressure in the fuel system is always maintained.

FUEL CONTROL

The fuel control system handles the actual delivery of fuel into the engine. (Scheme 35) Fuel from the pump enters the fuel rail, injectors and pressure regulator. Based upon a manifold vacuum signal, the pressure regulator maintains a constant fuel pressure in the system of approximately 31-39 psi (2.1-2.7 kg/cm 2 ) by allowing excess fuel to return to the fuel tank.

Fuel injectors are electrically operated solenoid valves which are powered by the ECU. The ECU determines injector pulse width ("on" time) based upon input from the various sensors.

Scheme 35

EMISSION CONTROL

The ECU controls the EGR valve operation. By energizing the EGR solenoid, vacuum is shut off, making this system non-operative. When the engine reaches normal operating temperatures, the ECU de-energizes the solenoid. When de-energized, the solenoid allows vacuum to flow to the EGR valve. The ECU will energize the solenoid whenever EGR action is undesirable, during idle, cold engine operation, wide open throttle and rapid acceleration or deceleration.

ECU

The ECU is a digital microprocessor computer. Data sensors provide the ECU with engine operating information in the form of varying electrical signals. The computer analyzes this information and corrects air/fuel ratio, ignition timing, and emission control as needed to maintain efficient engine operation. Other ECU output signals control the upshift indicator light (manual transmission only), ignition module dwell and A/C clutch operation.

UPSHIFT INDICATOR

On vehicles equipped with a manual transmission, the ECU controls the upshift indicator light. The indicator light is normally illuminated when the ignition switch is turned on without the engine running. The light is turned off when the engine is started.

The indicator light will be illuminated during engine operation in response to engine load and speed. If the gears are not shifted, the ECU will turn the light off after 3 to 5 seconds. A switch located on the transmission prevents the light from being illuminated when the transmission is shifted to the highest gear.

Manifold Absolute Pressure (MAP) Sensor

The MAP sensor is located in the engine compartment on the firewall behind the engine. The MAP sensor monitors manifold vacuum via a vacuum line from the intake manifold to the sensor.

The sensor supplies an electrical signal which keeps the ECU informed of manifold vacuum and barometric pressure conditions. This information is combined with data supplied by other sensors to determine correct air/fuel ratio.

Oxygen Sensor

Note. For additional oxygen sensor information, see TITANIA OXYGEN SENSOR TESTING article.

The oxygen (O2) sensor is mounted in the exhaust manifold where it is exposed to exhaust gas flow. Its function is to monitor oxygen content of exhaust gases and to supply the ECU with a voltage signal directly proportional to this content.

If the oxygen content of exhaust gases is high (lean air/fuel mixture), the voltage signal to the ECU is low. As oxygen content decreases (mixture becomes richer), signal voltage increases.

In this way, the ECU is kept constantly informed of air/fuel ratio. It can then alter fuel injector "on" time, in response to these signals, to obtain the best air/fuel ratio of 14.7:1 under any given operating conditions.

The O2 sensor is equipped with a heating element that keeps the sensor at proper operating temperatures. Maintaining correct sensor temperatures at all times guarantees a more accurate signal to the ECU. By using an O2 heater, the fuel control system may also enter the "closed loop" operating mode sooner and maintain this mode, even during periods of extended idle.

Temperature Sensors

There are 2 temperature sensors used on this system. The Manifold Air Temperature (MAT) sensor, mounted in the intake manifold, measures the temperature of incoming air/fuel mixture. The other, the Coolant Temperature Sensor (CTS), located on the left side of the cylinder block just below the exhaust manifold, measures temperature of engine coolant.

Information provided by these 2 sensors to the ECU allows the ECU to demand slightly richer air/fuel mixtures and higher idle speeds during cold engine operation.

Throttle Position Sensor (TPS)

The TPS is an electric variable resistor which is regulated by the movement of the throttle shaft. It is mounted on the throttle body and senses the angle of throttle blade opening.

A voltage signal of up to 5 Volts at wide open throttle is produced by the sensor. Voltage varies with throttle angle changes. This signal is transmitted to the ECU where it is used to adjust air/fuel ratio during acceleration, deceleration, idle, and wide open throttle conditions.

A dual TPS is used on models with automatic transmissions. This dual TPS not only provides the ECU with input voltages but also supplies the automatic transmission with input signals relative to throttle position.

Knock Sensor

The knock sensor (detonation sensor) is located on the lower left side of the cylinder block just above the oil pan. The knock sensor picks up detonation vibration from the engine and converts it to an electrical signal for use by the ECU.

The ECU uses this information to determine when a change in ignition timing is required. The knock sensor allows for engine operation on either "premium" unleaded or "regular" unleaded fuel. When knock occurs, the ECU retards the ignition timing in one or more cylinders until detonation is eliminated.

Speed Sensor

The speed sensor is secured by special shouldering bolts to the flywheel/drive plate housing.

The speed sensor senses TDC and engine speed by detecting the flywheel teeth as they pass the pick-up coil during engine operation. (Scheme 36) The flywheel has a large trigger tooth and notch located 12 small teeth before each TDC position. When a small tooth and notch pass the magnetic core in the sensor, the concentration and collapse of the magnetic field created induces a small voltage spike into the sensor pick-up coil windings. These small voltage spikes are sent to the ECU, allowing the ECU to count the teeth as they pass the sensor.

When a large tooth and notch pass the magnetic core in the sensor, the increased concentration and collapse of the magnetic field induces a higher voltage spike than the smaller teeth. The higher spike indicates to the ECU that a piston will soon be at TDC position, 12 teeth later.

Ignition timing for the cylinder is either advanced or retarded as necessary by the ECU based upon the inputs from all sensors.

Scheme 36

Engine Switches

Several switches provide operating information to the ECU. These include the Park/Neutral switch (automatic transmission only), air conditioning clutch, and Sync Pulse switch. When the A/C or Park/Neutral switches supply the ECU with an "on" signal, the module signals the ISS motor to change idle speed to a specific RPM.

With the A/C on and the throttle blade above a specific angle, the ECU de-energizes the A/C relay, preventing the air conditioning clutch from engaging until throttle blade angle is reduced.

The Sync Pulse switch, located within the distributor, generates a signal to the ECU, helping to properly synchronize injector opening with intake valve opening.

PRELIMINARY CHECKS

The following systems and components must be in good condition and operating properly before assuming a fuel injection system malfunction.

1. Air filter.
2. All support systems and wiring.
3. Battery connections and specific gravity.
4. Compression pressure.
5. Electrical connections on components and sensors.
6. Emission control devices.
7. Ignition system.
8. All vacuum line, fuel hose and pipe connections.

Note. The ECU is an extremely reliable part and must be the final component replaced.

Scheme 37

1. Remove the cap from the pressure test port located in the fuel rail. (Scheme 37) Connect Fuel Pressure Gauge (J-34730-1) to the pressure fitting.
2. Start vehicle. Pressure should be approximately 31 psi (2.1 kg/cm 2 ) with the vacuum hose connected to the pressure regulator and 39 psi (2.6 kg/cm 2 ) with the vacuum hose removed from the pressure regulator.
3. Check the fuel pump flow rate. A good fuel pump will deliver at least one liter of fuel per minute with the fuel return line pinched off. If the fuel pump does not pump adequately, inspect the fuel system for a plugged fuel filter or filter sock.
4. Fuel pump flow rate can be checked by connecting one end of an old A/C gauge hose to the fuel test port on the fuel rail and inserting the other end of the hose into a container of at least one liter or more capacity.
5. Run the fuel pump by installing a jumper wire into diagnostic connector terminals D1-5 and D1-6. Be sure to pinch off the fuel return line or most of the fuel will be returned to the fuel tank.

EGR Solenoid Test

1. Verify that vacuum is present at vacuum fitting "C" of the EGR solenoid. (Scheme 38) Remove vacuum connector from "A" and "B". Connect a vacuum gauge to "B".
2. Start and idle the engine. There should be no vacuum at "B". Disconnect electrical connector "D" from the solenoid. There should now be vacuum at "B".

Scheme 38

MAP Sensor Test

1. Inspect the MAP sensor hoses and connections. Repair as necessary. With the ignition on and the engine off, test the MAP sensor output voltage at the MAP sensor connector terminal "B" (marked on sensor body). (Scheme 39) The output voltage should be 4 to 5 volts.
2. To verify the wiring harness condition, test ECU terminal C-6 for the same voltage described. Test MAP sensor supply voltage at the sensor connector terminal "C" with the ignition on.
3. The voltage should be 4.5-5.5 volts. The same voltage should also be at terminal C-14 of the ECU wire harness connector. Using a Diagnostic Tester (M.S. 1170), test MAP sensor ground circuit at terminal D-3 and terminal "A" of sensor connector.
4. Using an ohmmeter, test the MAP sensor ground circuit at the ECU connector between terminal D-3 of ECU connector and terminal B-11 with an ohmmeter.
5. If the ohmmeter or diagnostic tester indicates an open circuit, inspect for a defective sensor ground connection, located on the right side of the cylinder block. If the ground connection is good the ECU may need to be replaced.

Scheme 39

O2 Sensor Test

Note. For additional oxygen sensor information, see TITANIA OXYGEN SENSOR TESTING article.

Disconnect the O2 sensor connector. Connect an ohmmeter to terminals "A" and "B" (marked on the connector) of the O2 sensor connector. Resistance should be between 5 and 7 ohms. Replace the sensor if the ohmmeter indicates an infinity reading.

CTS Test

1. Disconnect the wire harness connector from the CTS switch. Test the resistance of the sensor with a high impedance digital ohmmeter. The resistance should be less than 1000 ohms with the engine warm. See «CTS & MAT SENSOR TEMPERATURE-TO-RESISTANCE VALUES»(/jeep/wagoneer/xj-1984-1991/remont/testing-diagnostics/#fuel-injection-system-multi-point) chart.
2. Test the resistance of the wire harness between ECU terminal D-3 and the sensor connector terminal. Repeat the test at terminal C-10 of the ECU and the sensor connector terminal. Repair the wire harness if an open circuit is indicated. See «CTS & MAT SENSOR TEMPERATURE-TO-RESISTANCE VALUES»(/jeep/wagoneer/xj-1984-1991/remont/testing-diagnostics/#fuel-injection-system-multi-point) chart.

MAT Sensor Test

1. Disconnect the wire harness connector from the MAT sensor. Test the resistance of the sensor with a high impedance digital ohmmeter. The resistance should be less than 1000 ohms with the engine warm. Replace the sensor if the resistance is not within the specified range. See «CTS & MAT SENSOR TEMPERATURE-TO-RESISTANCE VALUES»(/jeep/wagoneer/xj-1984-1991/remont/testing-diagnostics/#fuel-injection-system-multi-point) chart.
2. Test the resistance of the wire harness between the ECU wire harness connector terminal D-3 and the sensor connector terminal. Repeat the test with terminal C-8 at the ECU and the sensor connector terminal. Repair the wire harness if the resistance is greater than one ohm.

TPS Test

See THROTTLE POSITION SENSOR TEST PROCEDURE chart in this article. (Scheme 40)

CTS & MAT SENSOR TEMPERATURE-TO-RESISTANCE VALUES

Knock Sensor Test

1. Start engine until engine reaches normal operating temperature. Connect Diagnostic Tester (M.S. 1700). Observe knock value on tester. Using tip of screw driver, gently tap on cylinder block next to knock sensor and observe knock sensor value on tester.
2. Knock sensor value should increase while tapping on cylinder block. If knock sensor value does not increase while tapping on cylinder block, check knock sensor for proper connection. If connection is good, replace knock sensor.

Speed Sensor Test

Disconnect the speed sensor connector from the ignition control module. Place an ohmmeter between terminals "A" and "B" (marked on connector). Reading should be 125-275 ohms with the engine hot. Replace sensor if readings are not within specifications.

Fuel Injector Test

See FUEL INJECTOR TEST PROCEDURE chart in this article. (Scheme 41)

ADJUSTMENTS

Note. Idle speed and air/fuel mixture are controlled by the ECU and are non-adjustable. On-car adjustment procedures for other components should not be necessary during normal vehicle operation or maintenance. Adjustments of components should only be required when a faulty component is replaced with a new one.

ECU CONNECTOR PIN IDENTIFICATION 24 PIN CONNECTOR MPFI

ECU CONNECTOR PIN IDENTIFICATION 32 PIN CONNECTOR MPFI

Throttle Position Sensor Test Procedure Chart. Scheme 40

Fuel Injector Test Procedure Chart. Scheme 41

WIRING DIAGRAM

For Wiring Diagram on Cherokee & Wagoneer Models, refer to WIRING DIAGRAMS article.

Comanche Multi-Point Fuel Injection Wiring Diagram. Scheme 42

See also:
• TITANIA OXYGEN SENSOR TESTING
• CTS & MAT SENSOR TEMPERATURE-TO-RESISTANCE VALUES