Code No.P1788 (P1751) A/T control relay system
OPERATION
If a fail-safe operation is activated, the A/T
control relay shuts off the power supply to the solenoid valve in accordance
with the signal from the ECU.
DIAGNOSIS CODE SET CONDITION
Code No.P1788 (P1751) will be set if
the A/T control voltage is less than 7 V after the ignition switch is
turned on.
If code No.P1788 (P1751) is set, the transmission will be held
in 3rd gear.
PROBABLE CAUSES
- Malfunction of A/T control relay
- Damaged harness wires and connectors
- Malfunction of the A/T-ECU <4M4>
- Malfunction of the engine-A/T-ECU <6G7>
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STEP 1. M.U.T.-III data list
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Item 8: Relay voltage (Refer to data list
reference table  ).
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Q.
Is the check result normal?
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 Intermittent malfunction (Refer to
GROUP 00 - How to Cope with Intermittent Malfunction ).
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 Go to Step 2.
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STEP 2. Check the A/T control relay
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Refer to .
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Q.
Is the check result normal?
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Go to Step 3.
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Replace the A/T control
relay.
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STEP 3. Connector check: D-110 A/T control relay
connector
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Check for the contact with terminals.
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Q.
Is the check result normal?
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Go to Step 4.
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Repair
the defective connector.
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STEP 4. Measure the voltage at A/T control relay connector
D-110.
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Disconnect the A/T control relay, and
measure the voltage between terminal No.1 and earth at the relay box
side.
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Q.
Is the check result normal?
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Go to Step 8.
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Go to Step 5.
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STEP 5. Connector check: A-03 <LHD> or D-137
<RHD> intermediate connector, D-113 J/C (6)
<4M4>
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Check for the contact with terminals.
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Q.
Is the check result normal?
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Go to Step 6.
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Repair
the defective connector.
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STEP 6. Check the harness between A/T control relay connector
D-110 terminal No.1 and battery.
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Check the power supply line for short or open
circuit.
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Q.
Is the check result normal?
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Go to Step 7.
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Repair
the wiring harness.
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STEP 7. M.U.T.-III data list
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Item 8: Relay voltage (Refer to data list
reference table ).
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Q.
Is the check result normal?
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The
trouble can be an intermittent malfunction (Refer to GROUP 00 - How
to Cope with Intermittent Malfunction ).
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Replace the A/T-ECU <4M4>
or engine-A/T-ECU <6G7>.
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STEP 8. Measure the voltage at the A/T control relay connector
D-110.
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(1)Turn the ignition switch to the ON position.
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(2)Disconnect the A/T control relay, and measure the
voltage between terminal No.4 and earth at the relay box
side.
OK: System voltage
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Q.
Is the check result normal?
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Go to Step 11.
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Go to Step 9.
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STEP 9. Connectors check: A-03 <LHD>or D-137
<RHD> intermediate connector, D-401 J/B
connector
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Check for the contact with terminals.
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Q.
Is the check result normal?
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Go to Step 10.
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Repair
the defective connector.
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STEP 10. Check the wiring harness between A/T control relay
connector D-110 terminal No.4 and J/B connector D-401 terminal
No.12.
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Check the power supply line for short or open
circuit.
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Q.
Is the check result normal?
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Go to Step 7.
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Repair
the wiring harness.
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STEP 11. Measure the voltage at A/T-ECU connector D-37
<4M4> or engine-A/T-ECU connector D-218
<6G7>.
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(1)Install the A/T control relay.
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(2)Turn the ignition switch to the ON position.
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(3)Measure the voltage between A/T-ECU connector D-37
terminal No.2, No.3 <4M4> and earth or engine-A/T-ECU connector
D-218 terminal No.123, No.124 <6G7> and earth.
OK:
System voltage
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Q.
Is the check result normal?
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Go to Step 14.
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Go to Step 12.
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STEP 12. Connector check: D-37 A/T-ECU connector <4M4> or
D-218 engine-A/T-ECU connector <6G7>, D-113 J/C (6)
<4M4>
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Check for the contact with terminals.
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Q.
Is the check result normal?
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Go to Step 13.
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Repair
the defective connector.
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STEP 13. Check the harness between A/T control relay connector
D-110 terminal No.3 and A/T-ECU connector D-37 terminal No.2, 3
<4M4> or engine-A/T-ECU connector D-218 terminal No.123, 124
<6G7>.
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Check the power supply line for short or open
circuit.
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Q.
Is the check result normal?
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Go to Step 7.
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Repair
the wiring harness.
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STEP 14. Connector check: D-37 A/T-ECU connector <4M4> or
D-218 engine-A/T-ECU connector <6G7>
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Check for the contact with terminals.
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Q.
Is the check result normal?
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Go to Step 15.
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Repair
the defective connector.
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STEP 15. Check the harness between A/T control relay connector
D-110 terminal No.2 and A/T-ECU connector D-37 terminal No.71
<4M4> or engine-A/T-ECU connector D-218 terminal No.127
<6G7>.
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Check the output line for short or open
circuit.
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Q.
Is the check result normal?
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Go to Step 7.
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Repair
the wiring harness.
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