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 engine-A/T-ECU
DIAGNOSIS PROCEDURE
STEP 1. M.U.T.-III data list
Item 8: Relay voltage (Refer to data list reference table ).
Q.
Is the check result normal?
Intermittent malfunction (Refer to GROUP 00 - How to Cope with Intermittent Malfunction ).
Go to Step 2.
STEP 2. Check the A/T control relay
Refer to .
Q.
Is the check result normal?
Go to Step 3.
Replace the A/T control relay.
STEP 3. Connector check: D-110 A/T control relay connector
Check for the contact with terminals.
Q.
Is the check result normal?
Go to Step 4.
Repair the defective connector.
STEP 4. Measure the voltage at A/T control relay connector D-110.
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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Intermittent malfunction (Refer to GROUP 00 - How to Cope with Intermittent Malfunction 
Go to Step 2.