Fuel Injection System - Tbi 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 Throttle Body Injection (TBI) system is a single injector system that introduces fuel into the throttle body from above the throttle plate. The injector, located within the throttle body, is controlled by the Electronic Control Unit (ECU).

The ECU is a sealed microprocessor that receives input signals from several sensors and other related engine components. Based on these inputs, the ECU generates output signals that control and adjust air/fuel mixture and ignition timing as necessary to optimize engine performance.

The ECU also controls engine idle speed, emission control systems, the upshift indicator light (manual transmission only), and the A/C compressor clutch.

Scheme 1

ELECTRONIC CONTROL UNIT (ECU)

On Cherokee, Comanche and Wagoneer, the ECU is located under instrument panel, above accelerator pedal. On Wrangler, the ECU is located behind glove box. Input information from various engine sensors to ECU is used to determine engine operating conditions and needs. Battery voltage input is used to ensure that correct output voltage is supplied by ECU during fluctuations in battery voltage.

FUEL INJECTOR

Fuel injector is mounted in throttle body so that fuel is injected into incoming airflow. When voltage is supplied to injector solenoid, armature and plunger move upward against spring. Check ball above injector nozzle moves off seat and opens small orifice at end of injector.

Fuel supplied to injector is forced around ball and through orifice, resulting in fine spray of fuel. Volume of fuel injected is dependent only on length of time that injector is energized by ECU, as fuel pressure is constant at injector. During cold engine starts, extra fuel is supplied so richer mixture will aid in starting.

FUEL PRESSURE REGULATOR

Fuel pressure regulator is integral part of throttle body. Pressure regulator has spring chamber that is vented to same pressure as tip of injector. Because differential pressure between injector nozzle and spring chamber is same, only the length of time that injector is energized controls volume of fuel injected.

Fuel pump delivers more fuel than is required by engine. Excess fuel goes to fuel tank from pressure regulator via fuel return hose. Fuel pressure regulator function is mechanical and ECU does not control it.

FUEL PUMP

Electric roller type fuel pump is located in fuel tank. Integral check valve is used to maintain pressure in fuel delivery system after pump stops running. Fuel pump operation is controlled by ECU.

IDLE SPEED ACTUATOR (ISA) MOTOR

ISA motor acts as movable idle stop to change throttle stop angle. Both engine idle speed and deceleration throttle stop are set by ISA. ECU sends varying voltage outputs to control ISA motor, depending upon engine operating condition.

OXYGEN (O2) SENSOR

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

Oxygen sensor is equipped with a heating element that keeps sensor at proper operating temperature at all times. Oxygen sensor is located in exhaust pipe.

The ECU receives sensor voltage signal which varies with oxygen content in exhaust gas. Signal is used by ECU as reference for setting air/fuel mixture ratio. ECU varies voltage to injector both to compensate for battery voltage fluctuations and to change duration of injector opening for control of air/fuel mixture.

MANIFOLD AIR/FUEL TEMPERATURE (MAT) SENSOR

The MAT sensor provides a signal to ECU that changes depending upon temperature of air/fuel mixture in intake manifold. During high temperature conditions, ECU will compensate for changes in density of air.

MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR

MAP sensor measures absolute pressure in intake manifold. Both mixture density and ambient barometric pressure are supplied to ECU by MAP sensor. Sensor is mounted in middle of firewall in engine compartment. Sensor receives manifold pressure information through vacuum line from throttle body. (Scheme 2)

Scheme 2

COOLANT TEMPERATURE SENSOR (CTS)

Coolant temperature sensor is installed in intake manifold water jacket to provide coolant temperature input signal for ECU. During cold engine operation, ECU will make mixture richer, make up for fuel condensation in cold intake manifold, increase idle speed during warm-up period, increase ignition advance and keep EGR system inoperative until engine warms up.

THROTTLE POSITION SENSOR (TPS)

Throttle position sensor provides ECU with input signal, up to about 5 volts, to indicate throttle position. This allows ECU to control air/fuel mixture according to throttle position. TPS is mounted on throttle body assembly.

CLOSED THROTTLE (IDLE) SWITCH

Idle switch is integral with ISA motor and provides voltage signal to ECU. ECU will signal ISA motor to change throttle stop angle in response to engine operating conditions.

UPSHIFT INDICATOR LIGHT

ECU controls upshift indicator light, which is found only in vehicles with M/T. Indicator light glows as bulb check when ignition is on with engine not running. Light turns off when engine starts.

Indicator light glows when engine load and speed indicates need for upshift to ECU. If gears are not shifted, ECU will turn light off after 3 to 5 seconds. Switch on transmission keeps light from lighting when transmission is in highest gear.

ENGINE SPEED SENSOR

Engine speed sensor is attached to bellhousing. It senses and counts teeth on flywheel gear ring as they pass during engine operation. Signal from speed sensor provides ECU with engine speed and crankshaft angle. On flywheel gear ring, large trigger tooth and notch is located 90 degrees before each TDC point. Each trigger tooth is followed by 12 smaller teeth and notches before TDC point is reached.

As each of 12 small teeth and notches pass magnet core in speed sensor, concentration and collapse of magnetic field induces slight voltage (spike) in sensor pick-up coil winding. (Scheme 3) Larger trigger teeth and notches induce higher voltage (spike) in sensor pick-up coil winding. These voltage spikes enable ECU to count teeth as they pass speed sensor.

Higher voltage spike (from larger tooth and notch) indicates to ECU that piston will be at TDC position after 12 smaller voltage spikes have been counted. ECU will then either advance or retard ignition timing depending upon remaining sensor inputs.

Scheme 3

A/C CONTROLS

ECU receives inputs from A/C when both A/C switch is in "ON" position and compressor clutch must engage for lower temperature. ECU changes engine idle speed depending upon A/C compressor operation.

POWER STEERING PRESSURE SWITCH

ECU receives input from pressure switch during periods of high pump load and low engine RPM. ECU raises engine idle speed immediately after receiving input from pressure switch.

Starter Motor Relay

Engine starter motor relay signals ECU whenever starter motor is being operated.

System Power Relay

System power relay, located on right fender inner panel, is energized when engine is started. It remains energized for 3 to 5 seconds after ignition is off. This allows ECU to extend ISA for next start before ECU shuts down.

Fuel Pump Control Relay

Fuel pump control relay is located on right fender inner panel. Battery voltage is supplied to relay from ignition switch. When ground is provided by ECU, relay becomes energized and provides voltage to fuel pump.

A/C Compressor Clutch Relay

ECU controls A/C compressor clutch through this relay.

EGR Valve/Canister Purge Solenoid

Vacuum to both EGR valve and vapor canister is controlled by this solenoid. When solenoid is energized, neither EGR valve nor vapor canister receive vacuum.

Solenoid is energized during closed (idle) and wide open throttle operations, engine warm-up and rapid acceleration or deceleration. If solenoid wiring is disconnected, both EGR valve and vapor canister will receive vacuum at all times.

Load Swap Relay

The load swap relay works in conjunction with power steering switch to disengage A/C compressor clutch. If compressor clutch is engaged when power steering pressure switch contacts close, input signal from switch to ECU also activates load swap relay. Relay then cuts off current to A/C compressor clutch.

The A/C compressor clutch remains disengaged until pressure switch contacts reopen and engine idle returns to normal. The load swap relay contains a timer that delays engaging the compressor clutch for 0.5 seconds to ensure smooth engagement.

ADJUSTMENTS

IDLE SPEED ACTUATOR (ISA) MOTOR ADJUSTMENT

1. Adjust ISA motor plunger to establish initial position of plunger only if motor has been removed and installed or replaced. Remove air filter elbow and start engine. Run engine until engine reaches normal operating temperature. Turn A/C off (if equipped).
2. Connect tachometer leads to diagnostic connector D1, attaching negative lead to terminal D1-3 and positive lead to terminal D1-3. Turn ignition off. ISA motor plunger should move to fully extended position. When ISA motor plunger is fully extended, disconnect ISA motor wiring connector and start engine. Engine speed should be 3300-3700 RPM. If incorrect, turn hex head screw at end of plunger to provide engine speed of 3500 RPM.
3. Retract ISA motor by holding closed throttle (idle) switch plunger inward as throttle is opened. Closed throttle switch plunger should not touch throttle lever in closed position. If contact is made, check linkage and/or cable for binding or damage. Repair as necessary.
4. Connect ISA motor wiring harness connector and turn ignition off for 10 seconds. ISA motor should move to fully extended position. Start engine. Engine speed should be 3500 RPM for short period of time and then decrease to normal idle speed. Turn ignition off. Disconnect tachometer. After final adjustment of ISA motor, use thread penetrating sealant (Loctite 290) on adjustment screw to prevent movement and maintain adjustment.

Note. If adjustment screw must be moved after thread sealant hardens, loosen threads by heating screw with flameless heat such as soldering gun. DO NOT use flame or torch type of heat as damage to ISA motor will result.

FUEL PRESSURE REGULATOR ADJUSTMENT

1. Replacement fuel pressure regulator must be adjusted to establish correct pressure. Remove air filter elbow and hose. Connect tachometer leads to diagnostic connector D1, attaching negative lead to terminal D1-3 and positive lead to terminal D1-3. Remove screw plug and install fuel pressure test fitting. NOTE: Fuel pressure test fitting is not included with throttle body. Fitting (Part No. 8983 501 572) must be obtained separately.
2. Connect fuel pressure gauge to test fitting. Start engine and accelerate to speed of approximately 2000 RPM. Turn Torx head screw at bottom of regulator to set correct pressure. Turning screw inward increases pressure and turning screw outward decreases pressure.
3. All models require fuel pressure of 14.5 psi (1.0 kg/cm 2 ). Install lead sealing ball to cover regulator adjustment screw after adjusting fuel pressure. Turn ignition off. Remove measuring equipment and test fitting. Install original plug screw and air filter assembly.

Cherokee & Wagoneer

1. Turn ignition switch to the "ON" position. Check throttle position sensor input voltage. Connect voltmeter negative lead to terminal "B" (center) of sensor. Connect voltmeter positive lead to terminal "C" (bottom) of sensor. NOTE: DO NOT disconnect TPS harness connector. Insert voltmeter test leads through back of wire harness connector.
2. Move and hold throttle plate at wide open throttle position. Ensure throttle linkage contacts stop. Note voltmeter reading. Input voltage at terminals "B" and "C" should be 5 volts.
3. Return throttle plate to closed throttle position. Check sensor output voltage. To do so, disconnect voltmeter positive lead from terminal "C" and connect it to terminal "A" (top) of sensor.
4. Move and hold throttle plate in wide open throttle position. Ensure throttle linkage contacts stop. Note voltmeter reading. Output voltage should be 4.7 volts. If output voltage is incorrect, go to next step.
5. Loosen top sensor retaining screw and pivot sensor in adjustment slot for a coarse adjustment. Loosen bottom sensor retaining screw for fine adjustments. Tighten screws after adjustment.

Comanche & Wrangler

Perform THROTTLE POSITION SENSOR TEST under RESISTANCE & VOLTAGE TESTS . If output voltage of TPS is not 4.6-4.7 volts in wide open throttle position, loosen mounting screws and pivot TPS until proper voltage is shown.

Subsystem Checks

Before testing fuel injection system for cause of malfunction, check that following subsystems and components are in good operating condition

1. Battery and charging system
2. Engine state of tune
3. Emission control devices
4. Fuel system pressure and delivery volume
5. Wiring connectors at components

General Precautions

In order to prevent injury to operator or damage to system or component parts, use following techniques

1. Turn ignition off before connecting or disconnecting any component parts.
2. DO NOT apply DC voltage greater than 12 volts or any AC voltage to system.
3. Disconnect battery cables before charging.
4. Remove ECU from vehicle if ambient temperature could exceed 176°F (80°C).
5. DO NOT modify or circumvent any system functions.

MAT Sensor

1. Disconnect wiring from MAT sensor. Using high input impedance DVOM, check resistance of sensor. Resistance should be less than 1000 ohms when engine is warm. Replace sensor if it does not fall within range shown in «TEMPERATURE-TO-RESISTANCE VALUES (CTS & MAT SENSOR)»(/jeep/wagoneer/xj-1984-1991/remont/testing-diagnostics/#fuel-injection-system-tbi) table.
2. Test resistance in wiring harness between ECU connector terminal No. 32 and sensor connector terminal. Also test resistance in wiring harness between ECU harness terminal No. 14 and sensor connector terminal. Repair wiring harness if resistance is greater than one ohm.

TEMPERATURE-TO-RESISTANCE VALUES (CTS & MAT SENSOR)

1. Disconnect wiring harness from CTS. Using high input impedance DVOM, check resistance of sensor. Resistance should be less than 1000 ohms when engine is warm. Replace sensor if it does not fall within range shown in «TEMPERATURE-TO-RESISTANCE VALUES (CTS & MAT SENSOR)»(/jeep/wagoneer/xj-1984-1991/remont/testing-diagnostics/#fuel-injection-system-tbi) table.
2. Test resistance in wiring harness between ECU harness terminal No. 32 and sensor connector terminal. Also test resistance in wiring harness between ECU harness terminal No. 15 and sensor connector terminal. Repair wiring harness if open circuit is indicated.

Throttle Position Sensor (TPS) Test

Turn ignition on. Check voltage at terminal connector without disconnecting from TPS. Pin terminal "A" is output voltage, which should be 4.6-4.7 volts at wide open throttle. Terminal "B" is sensor ground. Pin terminal "C" is input voltage, which is about 5 volts.

Note. ALL testing of idle switch must be done with ISA motor plunger in fully extended position. If switch cannot be tested without extending plunger, it is possible that the ISA motor has failed. Refer to the IDLE SPEED ACTUATOR (ISA) MOTOR ADJUSTMENT procedures.

Closed Throttle (Idle) Switch Test

1. Turn ignition on. Check idle switch voltage at diagnostic connector D2, between terminals No. 13 and 7. At closed throttle, voltage should be near zero volts. When switch is off closed throttle position, voltage reading should be greater than 2 volts.
2. If voltage is always zero, test for short to ground in harness or switch. Also check for open circuit between switch and terminal No. 25 of ECU connector. If reading is always greater than 2 volts, check for open circuit in wiring harness between switch connector and ECU. Also check for open between ground and switch connector. Replace or repair wiring harness as necessary.

Manifold Absolute Pressure (MAP) Sensor Test

1. Check and repair vacuum hose connections at throttle body and MAP sensor. Check output voltage at MAP sensor connector terminal "B" (marked on sensor body) with ignition on but engine NOT running. Voltage reading should be 4-5 volts. If engine is hot and idling in Neutral, reading should be.5-1.5 volts lower. Check voltage at terminal No. 33 of ECU connector. Reading should be same as that at terminal "B" on MAP sensor connector. (Scheme 4)
2. With ignition on, check MAP sensor supply voltage at terminal "C". Reading should be 4.5-5.5 volts. Same voltage reading should be obtained at terminal No. 16 on ECU harness connector. If necessary, repair or replace wiring harness. Check MAP sensor ground circuit at terminal "A" and terminal No. 17 of ECU connector. Repair wiring if necessary.
3. Using ohmmeter, check MAP sensor ground circuit between terminals No. 17 and 2 of ECU connector. If circuit is incomplete, check sensor ground connection on bellhousing near starter housing. Replace ECU if ground is good. If terminal No. 17 is shorted to 12 volts, repair problem BEFORE ECU is replaced.

DIAGNOSTIC TOOLS

To properly test throttle body fuel injection system, service technician must have the following equipment available

1. Digital volt-ohmmeter (DVOM) or volt-ohmmeter with minimum input impedance of one megaohm.
2. 12-volt test light, jumper wires and probes.
3. Hand vacuum pump with gauge.
4. Ignition timing light.

Scheme 4

DIAGNOSTIC TESTS

Note. When diagnosing fuel system problems using following procedures, no specialized service equipment is needed. Following diagnostic procedures are NOT applicable if special tester M.S. 1700 is used.

Six different test flow charts are used to fully evaluate fuel injection system

Scheme 5

TEST 1: IGNITION OFF

This test checks that system power provides for ECU memory keep-alive voltage.

Test 1: Ignition Off. Scheme 6

TEST 2: IGNITION ON: POWER

This test checks system power function and fuel pump power function.

Test 2: Ignition On: Power (1 of 3). Scheme 7

Test 2: Ignition On: Power (2 of 3). Scheme 8

Test 2: Ignition On: Power (3 of 3). Scheme 9

TEST 3 & 3A: IGNITION ON: INPUT

These tests check the following components and their circuits: closed throttle (idle) switch, throttle position sensor (TPS), MAP sensor, A/T gear selector switch, coolant temperature sensor (CTS) and MAT sensor. Coolant temperature and MAT sensors are tested in cold condition. This procedure also checks all interrelated wiring circuits as well.

Test 3: Ignition On: Input (1 of 6). Scheme 10

Test 3: Ignition On: Input (2 of 6). Scheme 11

Test 3: Ignition On: Input (3 of 6). Scheme 12

Test 3: Ignition On: Input (4 of 6). Scheme 13

Test 3: Ignition On: Input (5 of 6). Scheme 14

Test 3: Ignition On: Input (6 of 6). Scheme 15

Test 3A: Ignition On: Throttle Position Sensor. Scheme 16

TEST 4 & TEST 4A: SYSTEM OPERATIONAL

These tests check engine start-up circuit, fuel injector, closed loop air/fuel mixture function, coolant temperature sensor function, MAT sensor function, detonation sensor closed loop ignition retard/advance function, EGR valve and canister purge solenoid function, idle speed control and A/C control functions.

Test 4: System Operational (1 OF 2). Scheme 17

Test 4: System Operational (2 of 2). Scheme 18

Test 4A: System Operational (1 of 9). Scheme 19

Test 4A: System Operational (2 of 9). Scheme 20

Test 4A: System Operational (3 of 9). Scheme 21

Test 4A: System Operational (4 of 9). Scheme 22

Test 4A: System Operational (5 of 9). Scheme 23

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

Test 4A: System Operational (6 of 9). Scheme 24

Test 4A: System Operational (7 of 9). Scheme 25

Test 4A: System Operational (8 of 9). Scheme 26

Test 4A: System Operational (9 of 9). Scheme 27

TEST 5: BASIC ENGINE

This test indicates failures in related engine components that are not part of fuel injection system.

Test 5: Basic Engine. Scheme 28

TEST 6: MANUAL TRANSMISISON UPSHIFT

This test checks upshift indicator light function on vehicles with manual transmissions.

Test 6: Manual Transmission Upshift (1 of 2). Scheme 29

Test 6: Manual Transmission Upshift (2 of 2). Scheme 30

Cherokee, Comanche & Wagoneer Throttle Body Fuel Injection System Wiring Diagram. Scheme 31

Wrangler Throttle Body Fuel Injection System Wiring Diagram. Scheme 32

Removal & Installation

Allow engine to cool down. Disconnect CTS wiring harness. Remove CTS from intake manifold and rapidly plug hole to prevent coolant loss. Install replacement CTS and connect CTS wiring harness.

Removal

Remove air cleaner assembly. Remove injector wiring connector and retainer clip. Using small pliers, carefully grasp center collar of injector between electrical terminals and carefully remove injector with lifting/twisting motion. Discard both "O" rings. (Scheme 33)

Installation

1. Using light oil, lubricate new upper and lower "O" rings. Install "O" rings in housing bore. Install back-up ring over upper "O" ring. Position replacement injector in fuel body.
2. Center nozzle in lower housing bore and use a pushing/twisting motion to seat injector. Align wire connectors in proper orientation. Install retainer clip and screws. Connect injector wiring. Install air cleaner.

Scheme 33

With throttle body assembly removed, remove 3 screws holding fuel pressure regulator in throttle body. Remove fuel pressure regulator assembly. Note location of components for installation. Discard gaskets. To install, reverse removal procedure. Adjust regulator after installation. See ADJUSTMENTS in this article.

1. Disconnect throttle return spring. Disconnect wiring harness connector from ISA motor. Remove ISA motor retaining nuts while using a thin backup wrench to hold ISA motor retaining nuts. Remove ISA motor from bracket.
2. To install ISA motor assembly, reverse removal procedure. Adjust ISA motor after installation. See «ADJUSTMENTS»(/jeep/wagoneer/xj-1984-1991/remont/testing-diagnostics/#fuel-injection-system-tbi__adjustments) in this article.

1. Remove air inlet duct and adapter plate. Disconnect throttle cable and return spring. Disconnect electrical leads from fuel injector, WOT switch, and ISA motor.
2. Disconnect fuel supply and return lines at throttle body. (Scheme 34) Tag and disconnect vacuum hoses. Disconnect TPS wiring. Remove throttle body assembly. If throttle body assembly is being replaced, transfer ISA motor and WOT switch bracket assembly to new unit.

Install replacement throttle body assembly on manifold using new gasket. Reconnect all hoses, wires and cable in order of disassembly. Adjust ISA motor after installation. See ADJUSTMENTS in this article.

Scheme 34

Remove throttle body assembly. Remove Torx head retaining screws and throttle position sensor. To install, reverse removal procedure. Adjust TPS after installation. See ADJUSTMENTS in this article.

Disconnect wire harness connector from MAT sensor. Remove MAT sensor from intake manifold. To install, reverse removal procedure. (Scheme 34)

Disconnect wire harness connector, vacuum hose, and retaining nuts from MAP sensor. Remove sensor from firewall. To install, reverse removal procedure.

1. On Wrangler, remove passenger assist handle and glove box assembly. Remove ECU bracket retaining nuts from engine compartment side of firewall. Disconnect ECU wiring harness. Remove ECU from bracket. To install, reverse removal procedure.
2. On all other models, remove retaining screws and bracket that support ECU above accelerator pedal. Remove ECU and disconnect wiring harness. To install, reverse removal procedure.

Disconnect vacuum hose from EGR valve. Remove bolts which hold EGR valve to intake manifold. Remove valve and discard gasket. To install valve, reverse removal procedure. Always use new gasket. (Scheme 34)

See also:
• TITANIA OXYGEN SENSOR TESTING
• RESISTANCE & VOLTAGE TESTS
• IDLE SPEED ACTUATOR (ISA) MOTOR ADJUSTMENT
• ADJUSTMENTS