Hino P0686 Fault Code: ECM/PCM Power Relay Control Circuit High
15/01/2025
TECHNICAL DESCRIPTION
The main relay supplies power from the battery to the ECU
DESCRIPTION OF MALFUNCTION
The main relay cannot be turned OFF
DTC SET CONDITION
DTC Detection Condition:
- Battery voltage is in the range of 10V to 16V
Judgment Criteria:
- With the main relay in OFF position, 8 V or higher power supply voltage remains for 60 seconds
RESET CONDITION
- After normal operation is restored and the starter switch is on the “LOCK” position
INDICATION, WARNING OR SYSTEM CONTROL REGULATION WHEN THE DTC IS SET
- MIL: ON
- SVS light: OFF
SYMPTONS ON THE VEHICLE WHEN THE DTC IS SET
Symptoms on the Vehicle Due to Malfunction:
- Battery is depleted
PRE-INSPECTION WORK
Check that the battery voltage is in the normal range
AFTER-INSPECTION WORK
- Clear all past DTC.
- Check that no DTC is detected after test drive
POSSIBLE CAUSES
- Harness disconnection or short-circuit
- Looseness or poor contact of connector
- Main relay failure.
- Engine ECU failure
INSPECTION PROCEDURE
TROUBLESHOOTING STEPS
Step 1: Inspect the main relay
Check the connection of the main relay (fuse and inside relay block B), (Looseness or poorcontact)
Were any failures found?
| Yes | Connect securely, repair if needed. Clear all past DTCs. Check that no DTC is detected after test drive |
| No | Go to Step 2 |
Step 2: Inspect the main relay unit
- Set the starter switch to the “LOCK” position
- Disconnect the main relay
- Use the electrical tester to measure the resistance between the terminals of the main relay
| TESTER CONNECTIONS | STANDARD VALUES |
| Main relay terminals | 1 in the figure: 167 Ω
2 in the figure: ∞ Ω |
Do the measurements meet the standard value?
| Yes | Go to Step 3 |
| No | Repair or replace replay. Clear all past DTCs. Check that no DTC is detected after test drive. |
Step 3: Inspect the main relay power supply circuit
- Set the starter switch to the “ON” position
- Use the electrical tester to measure the voltage between each terminal in the main relay vehicle side connector (fuse and inside relay block B) and ground
| MEASUREMENT CONDITIONS | TESTER CONNECTIONS | STANDARD VALUES |
| Starter switch: ON | Main relay vehicle-side connector
B – Ground S+ – Ground |
10V or more |
Do the measurements meet the standard value?
| Yes | Go to Step 4 |
| No | Repair or replace the main relay power supply circuit. Clear all past DTCs. Check that no DTC is detected after test drive. |
Step 4: Inspect the main relay power supply
- Set the starter switch to the “LOCK” position
- Connect the main relay
- Connect the signal check harness to the engine ECU vehicle-side harness. (Do not connect the harness to the ECU)
- Set the starter switch to the “ON” position
- Use the electrical tester to measure the voltage between the engine ECU (signal check harness) terminals and ground
| MEASUREMENT CONDITIONS | TESTER CONNECTIONS | STANDARD VALUES |
| Starter switch: ON | Engine ECU (signal check harness)
MRL1(V3) – Ground MRL2(V23) – Ground |
10V or more |
Do the measurements meet the standard value?
| Yes | Replace the engine ECU. Clear all past DTCs. Check that no DTC is detected after test drive |
| No | Repair or replace the harness. Clear all past DTCs. Check that no DTC is detected after test drive. |
Whatsapp: +84.899.402.402
Facebook: ECUTools.vn
Youtube: ECUToolsvn
Tiktok: ECUTools
Website: ECUTools.vn
RELATED NEWS
See all
