DTC P1788 (P1751): A/T Control Relay System: Diagnosis

Required Special Tools: 

• MB991958: Scan Tool (M.U.T.-III Sub Assembly)
  • MB991824: V.C.I.
  • MB991827: M.U.T.-III USB Cable
  • MB991910: M.U.T.-III Main Harness A

1. STEP 1. Using scan tool MB991958, check data list item 8: A/T Control Relay Output Voltage. 
   CAUTION: To prevent damage to scan tool MB991958, always turn the ignition switch to the "LOCK" (OFF) position before connecting or disconnecting scan tool MB991958.
   1. Connect scan tool MB991958 to the data link connector.
   2. Turn the ignition switch to the "ON" position.
   3. Set scan tool MB991958 to the data reading mode.
      • Item 8: A/T Control Relay Output Voltage.
        • The voltage should equal battery positive voltage.
   4. Turn the ignition switch to the "LOCK" (OFF) position.

      Q: Does the measured voltage equal battery positive voltage? 

      1. YES:  It can be assumed that this malfunction is intermittent. Refer to HOW TO COPE WITH INTERMITTENT MALFUNCTION .
      2. NO:  Go to Step 2.
         Fig 1: Connecting Special Tool MB991910 To Data Link Connector

         Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
2. STEP 2. Check the A/T control relay. 
   1. Remove the A/T control relay from the engine component relay box connector B-15X.
      Fig 2: Locating A/T Control Relay Connector B-15X

      Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
   2. Using jumper wires, connect terminal 2 to the positive battery terminal, and terminal 3 to the negative battery terminal.
   3. Measure the resistance between terminals 1 and 4 of the A/T control relay.
      • The resistance should be measured less than 2 ohms.
      • Disconnect the jumper wires. The resistance between terminals 1 and 4 should measure over limits (open circuit).
        Fig 3: Measuring Resistance Between Terminals 1 And 4 Of A/T Control Relay

        Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.

      Q: Is the measured resistance less than 2 ohms when the relay is energized, and open circuit when the relay is de-energized? 

      1. YES:  Go to Step 3.
      2. NO:  Replace the A/T control relay.
3. STEP 3. Check A/T control relay socket B-15X in the engine compartment relay box for loose, corroded or damaged terminals, or terminals pushed back in the socket. 

   Q: Is the relay connector in good condition? 

   1. YES:  Go to Step 4.
   2. NO:  Repair or replace the damaged components. Refer to HARNESS CONNECTOR INSPECTION .
      Fig 4: Locating A/T Control Relay Connector B-15X

      Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
4. STEP 4. Measure the supply voltage at A/T control relay connector B-15X in the engine component relay box. 
   1. Disconnect the A/T control relay.
      Fig 5: Locating A/T Control Relay Connector B-15X

      Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
   2. Measure the voltage between terminal 1 and ground.
      • The measured voltage should equal battery positive voltage.

      Q: Is the measured voltage equal to battery positive voltage? 

      1. YES:  Go to Step  7.
      2. NO:  Go to Step 5.
         Fig 6: Measuring Voltage Between Terminal 1 And Ground

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5. STEP 5. Check intermediate connector A-15 for loose, corroded or damaged terminals, or terminals pushed back in the connector. 

   Q: Are the connector and terminals in good condition? 

   1. YES:  Go to Step 6.
   2. NO:  Repair or replace the damaged components. Refer to HARNESS CONNECTOR INSPECTION .
      Fig 7: Locating Intermediate Connector A-15

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6. STEP 6. Check the harness for open circuit or short circuit to ground between A/T control relay connector B-15X terminal 1 in the engine component relay box and battery. 

   Q: Is the harness wire in good condition? 

   1. YES:  Go to Step  15.
   2. NO:  Repair or replace the harness wire.
      Fig 8: Locating A/T Control Relay Connector B-15X

      Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
7. STEP 7. Measure the supply voltage at A/T control relay connector B-15X in the engine component relay box. 
   1. Disconnect the A/T control relay.
   2. Turn the ignition switch to the "ON" position.
      Fig 9: Locating A/T Control Relay Connector B-15X

      Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
   3. Measure the voltage between terminal 2 and ground.
      • The measured voltage should equal battery positive voltage.
   4. Turn the ignition switch to the "LOCK" (OFF) position.

      Q: Is the measured voltage equal to battery positive voltage? 

      1. YES:  Go to Step  10.
      2. NO:  Go to Step 8.
         Fig 10: Measuring Voltage Between Terminal 2 And Ground

         Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
8. STEP 8. Check intermediate connector A-15, C-24, joint connector (1) C-28 and junction block connector C-203 for loose, corroded or damaged terminals, or terminals pushed back in the connector. 

   Q: Are the connectors in good condition? 

   1. YES:  Go to Step 9.
   2. NO:  Repair or replace the damaged components. Refer to HARNESS CONNECTOR INSPECTION .
      Fig 11: Locating Intermediate Connector A-15

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      Fig 12: Locating Intermediate Connector C-24

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      Fig 13: Locating Joint Connector C-28

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      Fig 14: Locating Junction Block Connector C-203

      Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
9. STEP 9. Check the harness for open circuit or short circuit to ground between A/T control relay connector B-15X terminal 2 in the engine component relay box and junction block connector C-203 terminal 12. 

   Q: Is the harness wire in good condition? 

   1. YES:  Go to Step  15.
   2. NO:  Repair or replace the harness wire.
      Fig 15: Locating A/T Control Relay Connector B-15X

      Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
      Fig 16: Locating Junction Block Connector C-203

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10. STEP 10. Measure the A/T control relay output voltage at PCM connector B-20 by using check harness special tool MB991923. 
    1. Disconnect all the connectors from the PCM.
       Fig 17: Locating PCM Connector B-20

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    2. Connect special tool MB991923 (check harness) between the PCM and the body-side harness connector.
    3. Turn the ignition switch to the "ON" position.
       Fig 18: Connecting Special Tool MB991923 Between PCM And Body-Side Harness Connector

       Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
    4. Measure the voltage between terminal 57 and ground.
       • The measured voltage should equal battery positive voltage.
         Fig 19: Measuring Voltage Between Terminal 57 And Ground

         Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
    5. Measure the voltage between terminal 70 and ground.
       • The measured voltage should equal battery positive voltage.
    6. Turn the ignition switch to the "LOCK" (OFF) position.

       Q: Is the measured voltage equal to battery positive voltage between terminal 57 and ground, and between terminal 70 and ground? 

       1. YES:  Go to Step  13.
       2. NO:  Go to Step 11.
          Fig 20: Measuring Voltage Between Terminal 70 And Ground

          Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
11. STEP 11. Check PCM connector B-20 for loose, corroded or damaged terminals, or terminals pushed back in the connector. 

    Q: Are the connector and terminals in good condition? 

    1. YES:  Go to Step 12.
    2. NO:  Repair or replace the damaged components. Refer to HARNESS CONNECTOR INSPECTION .
       Fig 21: Locating PCM Connector B-20

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12. STEP 12. Check harness for open circuit or short circuit to ground between A/T control relay connector B-15X (terminal 4) in the engine component relay box and PCM connector B-20 (terminals 57 and 70). 

    Q: Is the harness wire in good condition? 

    1. YES:  Go to Step  15.
    2. NO:  Repair or replace the harness wire.
       Fig 22: Locating A/T Control Relay Connector B-15X

       Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
       Fig 23: Locating PCM Connector B-20

       Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
13. STEP 13. Check PCM connector B-20 for loose, corroded or damaged terminals, or terminals pushed back in the connector. 

    Q: Are the connector and terminals in good condition? 

    1. YES:  Go to Step 14.
    2. NO:  Repair or replace the damaged components. Refer to HARNESS CONNECTOR INSPECTION .
       Fig 24: Locating PCM Connector B-20

       Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
14. STEP 14. Check harness for open circuit or short circuit to ground between A/T control relay connector B-15X terminal 3 in the engine component relay box and PCM connector B-20 terminal 82. 

    Q: Is the harness wire in good condition? 

    1. YES:  Go to Step 15.
    2. NO:  Repair or replace the harness wire.
       Fig 25: Locating A/T Control Relay Connector B-15X

       Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
       Fig 26: Locating PCM Connector B-20

       Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
15. STEP 15. Using scan tool MB991958, check data list item 8: A/T control relay output voltage. 
    CAUTION: To prevent damage to scan tool MB991958, always turn the ignition switch to the "LOCK" (OFF) position before connecting or disconnecting scan tool MB991958.
    1. Connect scan tool MB991958 to the data link connector.
    2. Turn the ignition switch to the "ON" position.
    3. Set scan tool MB991958 to the data reading mode.
       • Item 8: A/T Control Relay Output Voltage.
         • The voltage should equal battery positive voltage.
    4. Turn the ignition switch to the "LOCK" (OFF) position.

       Q: Is the measured voltage equal battery positive voltage? 

       1. YES:  It can be that this malfunction is intermittent. Refer to HOW TO COPE WITH INTERMITTENT MALFUNCTION .
       2. NO:  Replace the PCM. When the PCM is replaced, register the encrypted code. Refer to ENCRYPTED CODE REGISTRATION CRITERIA TABLE .
          Fig 27: Connecting Special Tool MB991910 To Data Link Connector

          Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.