Code No. P1788: 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 A/T-ECU.
DIAGNOSIS CODE SET CONDITIONS
If the A/T control voltage is less than 7 V after the ignition switch is turned on.
If code P1788 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
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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: A-16X 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 A-16X.
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Remove the A/T control relay, and measure the voltage between terminal No.4 and earth at the relay box side.
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Q.
Is the check result normal?
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Go to Step 7.
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Go to Step 5.
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STEP 5. Check the harness between A/T control relay connector A-16X terminal No.4 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 6.
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Repair the wiring harness.
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STEP 6. 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.
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STEP 7. Measure the voltage at the A/T control relay connector A-16X.
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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.2 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 10.
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Go to Step 8.
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STEP 8. Connectors check: C-128 intermediate connector, C-317 ETACS-ECU 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 9.
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Repair the defective connector.
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STEP 9. Check the wiring harness between A/T control relay connector A-16X terminal No.2 and ETACS-ECU connector C-317 terminal No.5.
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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 6.
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Repair the wiring harness.
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STEP 10. Measure the voltage at A/T-ECU connector C-34.
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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 C-34 terminal No.43, No.49 and earth.
OK: System voltage
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Q.
Is the check result normal?
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Go to Step 13.
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Go to Step 11.
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STEP 11. Connector check: C-34 A/T-ECU 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 12.
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Repair the defective connector.
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STEP 12. Check the harness between A/T control relay connector A-16X terminal No.3 and A/T-ECU connector C-34 terminal No.43, 49.
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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 6.
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Repair the wiring harness.
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STEP 13. Connector check: C-34 A/T-ECU 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 14.
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Repair the defective connector.
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STEP 14. Check the harness between A/T control relay connector A-16X terminal No.1 and A/T-ECU connector C-34 terminal No.44.
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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 6.
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Repair the wiring harness.
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