1984 Isuzu Impulse StandardFACTORY OEM SERVICE PROCEDURE

Entering Diagnostic Mode

  1. Turn ignition "ON". "CHECK ENGINE" lamp should glow. Locate diagnostic terminal wires near MCU. Connect diagnostic terminal leads together (Brown and Black/Yellow wires) found near MCU. MCU will start to display trouble codes, in the "CHECK ENGINE" lamp.
  2. If any trouble codes are stored in MCU memory, the "CHECK ENGINE" lamp will flash 2-digit codes. Trouble codes will be displayed from lowest to highest numbered code. Each code is repeated three times before the next code sequence is displayed. The trouble codes will be repeated as long as the diagnostic terminals are connected.
MCU TROUBLE CODE IDENTIFICATION

Code Problem
12 No Ignition - Normal condition
13 Open Oxygen Sensor Circuit
14 Shorted Coolant Sensor Circuit
15 Incorrect Coolant Sensor Circuit
16 Open Coolant Sensor Circuit
21 Throttle Position Switch Circuits Connected at Same Time
22 Starter Signal Circuit
23 Output Terminal to Ignition Coil Grounded
25 Output to Vacuum Switching Solenoid Ckt Open/Grounded
33 Open or Shorted Fuel Injector Circuit
35 Ignition Power Transistor Open
41 Bad Signal From Crank Angle Sensor
43 No Idle Position Signal
44 Lean Oxygen Sensor Indication
45 Rich System Indication
51 Faulty MCU
52 Faulty MCU
53 Vacuum Switching Valve
54 Bad Ignition Transistor or Ground
55 Faulty MCU
61 Circuit to Air Flow Sensor Bad
62 Circuit to Air Flow Sensor Bad
63 No Vehicle Speed Sensor Signal
64 Fuel Injector Transistor or Ground
65 Continuous Signal From Throttle Position Switch
66 Open or Shorted Knock Sensor Circuit
Fig 1: Impulse I-TEC System Wiring Diagram
G42066
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Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.

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