Jeep Cherokee 1988 Quick Start Manual
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Jeep Cherokee 1988 Quick Start Manual

2.5l cec system
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1988 Computerized Engine Controls
JEEP 4-CYLINDER 2.5L TBI COMPUTERIZED EMISSION CONTROL
Cherokee, Comanche
DESCRIPTION
The computerized engine control system, used on 2.5L models
with throttle body fuel injection, is built around an electronic
control unit (ECU). The ECU is a microprocessor-based computer.
The major function of the system is to reduce emissions. It
accomplishes this through a series of 13 sensors or switches that
constantly monitor several engine conditions. See Fig. 16.
Fig. 1:
Vacuum Diagram for Jeep 2.5L CEC System
The computer processes input information from the sensors to
get an accurate picture of engine operation. It then provides output
control signals to regulate air/fuel ratio, ignition, idle speed and
2.5L CEC SYSTEM
1988 Jeep Cherokee

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  • Page 1 2.5L CEC SYSTEM 1988 Jeep Cherokee 1988 Computerized Engine Controls JEEP 4-CYLINDER 2.5L TBI COMPUTERIZED EMISSION CONTROL Cherokee, Comanche DESCRIPTION The computerized engine control system, used on 2.5L models with throttle body fuel injection, is built around an electronic control unit (ECU). The ECU is a microprocessor-based computer.

  • Page 2: Operation

    emission control devices. This permits optimum engine performance with minimum emissions. OPERATION The engine control system is divided into 6 sub-systems: electronic control unit (also called the ECU or computer), sensors and switches, fuel control, emission control, idle speed control, and ignition advance control.

  • Page 3: Fuel Control

    with manifold pressure information. Knock Sensor The knock (detonation) sensor, located in the cylinder head, provides an input signal to the ECU whenever detonation occurs. The ECU then retards ignition advance to eliminate the detonation at the applicable cylinders. Speed Sensor The speed sensor (or crankshaft position sensor) is mounted at the flywheel/drive plate housing.
  • Page 4 An electric in-tank fuel pump supplies fuel through the fuel filter located under the right rear floor pan to the throttle body, maintaining a constant operating pressure. Fuel enters the fuel bowl reservoir of the throttle body through the injector and overflow type fuel pressure regulator.
  • Page 5 fuel return line. The regulator is not controlled by the ECU. The regulator’s spring chamber is vented to the same pressure as the tip of the injector. Fig. 3: Cross Section View of Fuel Pressure Regulator Since fuel pressure at the injector is kept constant, the volume of fuel injected is dependent only on the length of time that the injector is energized.

  • Page 6: Emission Control

    input sensors. During engine start-up, the injector delivers an extra amount of fuel to aid in starting. EMISSION CONTROL Both EGR and canister purge operation are regulated by the ECU. Regulation of these 2 systems is accomplished through the use of an electrically-operated vacuum solenoid.
  • Page 7 Fig. 4: Location of ECU-Controlled Relays Load Swap Relay The Load Swap Relay is used on models with A/C and power steering. The relay works in conjunction with the power steering pressure switch to disengage the A/C compressor clutch. If the compressor clutch is engaged when the power steering pressure switch contacts close, the input signal from the switch to the ECU also activates the load swap relay.
  • Page 8 The ECU controls the A/C compressor clutch by means of the A/C clutch relay. See Fig. 15. UP-SHIFT INDICATOR LAMP Manual transaxle vehicles are equipped with an up-shift indicator lamp. The lamp is normally turned on when the ignition switch is turned "ON", and is turned off when the engine starts. The lamp will again light during engine operation, according to engine speed and load conditions.

  • Page 9: Wide Open Throttle Mode

    MAT, MAP, EGO, speed and knock sensors. It also is informed of throttle, gear (automatic transaxle models) and A/C control position. When the ECU receives deceleration input from the closed throttle (idle) switch, it grounds the EGR valve/canister purge solenoid. This interrupts vacuum to EGR valve and canister purge function.

  • Page 10: Coolant Temperature Sensor

    sensor. With engine warm, resistance should be less than 1000 ohms. 2) Connect one ohmmeter lead to sensor connector terminal. Connect other lead, in turn, to ECU harness connector terminals 32 and 14. Repair wiring harness if resistance is greater than 1 ohm. COOLANT TEMPERATURE SENSOR 1) Disconnect wiring harness from CTS sensor.
  • Page 11 NOTE: Voltage should drop 0.5-1.5 volts with hot engine, at idle. Fig. 6: MAP Sensor Terminal Identification If markings on MAP sensor vary from illustration, use markings on sensor. 2) Test voltage at pin 33 of ECU connector for 4.0-5.0 volts to verify wiring harness condition.
  • Page 12 1) If all components have been checked and/or repaired, but a system failure or problem still exists, the ECU may be at fault. However, the ECU is a very reliable unit and must always be the final component replaced if a doubt exists concerning the cause of a system failure.
  • Page 13 Fig. 7: ECU Connector Terminal Identification...
  • Page 14 Fig. 8: Chart 1 - Ignition Switch Off...
  • Page 15 Fig. 9: Chart 2-A - Ignition Switch On...
  • Page 16 Fig. 10: Chart 2-B - Ignition Switch On (Cont.)
  • Page 17 Fig. 11: Chart 2-C - Ignition Switch On (Cont.)
  • Page 18 Fig. 12: Chart 3 - Throttle Position Sensor Test...
  • Page 19 Fig. 13: Chart 4-A - System Operational Test...
  • Page 20 Fig. 14: Chart 4-B - System Operational Test (Cont.)
  • Page 21 Fig. 15: Chart 4-C - System Operational Test (Cont.)
  • Page 22 Fig. 16: Chart 4-D - System Operational Test (Cont.)
  • Page 23 Fig. 17: Chart 4-E - System Operational Test (Cont.)
  • Page 24 Fig. 18: Chart 4-F - System Operational Test (Cont.)
  • Page 25 Fig. 19: Chart 4-G - System Operational Test (Cont.)
  • Page 26 Fig. 20: Chart 4-H - System Operational Test (Cont.)
  • Page 27 Fig. 21: Chart 4-I - System Operational Test (Cont.)
  • Page 28 Fig. 22: Chart 4-J - System Operational Test (Cont.)
  • Page 29 Fig. 23: Chart 4-K - System Operational Test (Cont.) Fig. 24: Chart 5 - Basic Engine Test...
  • Page 30 Fig. 25: Chart 6-A - Man. Trans. Up-Shift Test...

  • Page 31: Removal And Installation

    Fig. 26: Chart 6-B - Man. Trans. Up-Shift Test (Cont.) REMOVAL & INSTALLATION...

  • Page 32: Throttle Body Assembly

    THROTTLE BODY ASSEMBLY Removal 1) Remove throttle return spring, throttle cable and cruise control cable, if equipped. Disconnect wiring harness connector from injector, WOT switch, and ISA motor. Remove fuel supply and return pipes from throttle body. 2) Identify and tag vacuum hoses for installation later. Disconnect vacuum hoses from throttle body assembly.
  • Page 33 using a backup wrench on stud nuts. ISA motor internal components may be dislodged if studs disengage and motor cap comes off. Fig. 27: ISA Motor and WOT Switch Removal Bracket assembly is mounted on throttle body unit. 3) To remove ISA motor from bracket, place backup 8 mm open end wrench on ISA motor stud nuts to prevent studs from turning.

  • Page 34: Electronic Control Unit

    nuts. Install motor and WOT switch bracket assembly on throttle body. 2) Connect wiring harness connector to ISA motor and WOT switch. Connect the throttle return spring, throttle cable and cruise control cable. Adjust ISA motor and WOT switch. Install air cleaner assembly.
  • Page 35 assembly. To install, reverse removal procedure. ADJUSTMENTS NOTE: The following adjustment procedures should not be necessary during normal vehicle operation or maintenance. Adjustment of the listed components should only be required when a faulty component is replaced with a new one. IDLE SPEED ACTUATOR (ISA) MOTOR 1) With air cleaner removed, air conditioner off (if equipped) and engine at normal operating temperature, connect a...
  • Page 36 Fig. 28: Jeep 2.5L TBI System Wiring Diagram FUEL PRESSURE REGULATOR 1) Remove air cleaner assembly. Connect a tachometer to terminals 1 and 3 of small diagnostic connector D1. Remove screw plug and install special Fuel Pressure Test Fitting (8983 501 572).
  • Page 37 2) Connect accurate fuel pressure gauge to fuel pressure test fitting. Start engine and accelerate to 2000 RPM. Turn torx head adjustment screw on bottom of fuel regulator to obtain 17.3 psi (1.2 kg/cm(C)) of fuel pressure. NOTE: Turning screw inward increases pressure; turning screw outward decreases it.

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