Circuit Description
The main relay is a normally open relay. The relay armature is held in the open position by spring tension. Battery positive voltage is supplied directly to the relay coil and the armature contact at all times. The engine control module (ECM) supplies the ground path to the relay coil control circuit via an internal integrated circuit, called an output driver module (ODM). One ODM output control is configured to operate as a low side driver for the main relay. The low side driver for the main relay also incorporates a fault detection circuit, which is continuously monitored by the ECM. When the ECM commands the ODM low side driver to turn OFF, the expected voltage on the relay coil control circuit should be a high voltage condition. DTC P0686 indicates an open, high resistance, or a short to ground in the main relay coil control circuit. If the fault detection circuit measures a low voltage condition, DTC P0686 will set.
Conditions for Running the DTC
| • | The battery voltage is between 9-16 volts. |
| • | The ignition switch is turned ON. |
Conditions for Setting the DTC
| • | The ECM detects a lower than expected voltage on the relay coil control circuit. |
| • | The condition is present for more than 200 milliseconds. |
Action Taken When the DTC Sets
| • | The control module stores the DTC information into memory when the diagnostic runs and fails. |
| • | The malfunction indicator lamp (MIL) will not illuminate. |
| • | The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Failure Records. |
| • | The driver information center, if equipped, may display a message. |
Conditions for Clearing the DTC
| • | A current DTC Last Test Failed clears when the diagnostic runs and passes. |
| • | A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other non-emission related diagnostic. |
| • | Clear the DTC with a scan tool. |
Diagnostic Aids
| • | The main relay will remain powered up for 15 seconds after the ignition switch is turned OFF. |
| • | This DTC diagnostic table assumes that the vehicle battery is fully charged. Refer to Battery Inspection/Test in Engine Electrical. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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The engine will crank and run, with a short to ground on the main relay coil control circuit. This condition could result in a discharged battery.
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The engine will crank but will not run, with a high resistance or an open on the main relay coil control circuit.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics Connector End View Reference: Engine Control Module Connector End Views and Engine Controls Connector End Views | ||||
1 | Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | |
2 |
Did the DTC fail this ignition? | -- | Go to Step 3 | Go to Intermittent Conditions |
Attempt to start the engine. Does the engine start and run? | -- | Go to Step 6 | Go to Step 4 | |
4 |
Notice: Refer to Test Probe Notice in the Preface section. Does the test lamp illuminate? | -- | Go to Step 5 | Go to Step 11 |
Does the test lamp illuminate? | -- | Go to Step 12 | Go to Step 9 | |
6 |
Notice: Refer to Test Probe Notice in the Preface section. Does the test lamp illuminate? | -- | Go to Step 7 | Go to Step 12 |
7 |
Notice: Refer to Test Probe Notice in the Preface section. Does the test lamp illuminate? | -- | Go to Step 8 | Go to Step 14 |
8 |
Does the test lamp illuminate? | -- | Go to Step 15 | Go to Step 18 |
9 |
Does the resistance measure greater than the specified value? | 5 ohms | Go to Step 16 | Go to Step 10 |
10 | Test the main relay coil control bus bar circuit of the underhood fuse block for a high resistance or an open. Refer to Circuit Testing in Wiring Systems. Does the resistance measure greater than the specified value? | 5 ohms | Go to Step 18 | Go to Step 14 |
11 | Probe the mounting stud for the battery positive cable at the underhood fuse block with a test lamp that is connected to a good ground. Does the test lamp illuminate? | -- | Go to Step 18 | |
12 | Measure the resistance from terminal #85 of the main relay to terminal #86 with a DMM. Refer to Troubleshooting with a Digital Multimeter in Wiring Systems. Does the resistance measure within the specified range? | 70-110 ohms | Go to Step 13 | Go to Step 17 |
13 | Test for an intermittent and for a poor connection at the main relay location of the underhood fuse block. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. Did you find a condition? | -- | Go to Step 18 | Go to Intermittent Conditions |
14 | Test for shorted terminals and poor connections at the ECM electrical connectors. Refer to Testing for Intermittent Conditions and Poor Connections and Micro .64 Connectors in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 20 | Go to Step 19 |
15 | Repair the short to ground in the main relay coil control circuit between the underhood fuse block electrical connector and the ECM electrical connector. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 20 | -- |
16 | Repair the high resistance or an open in the main relay coil control circuit between the underhood fuse block electrical connector and the ECM electrical connector. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 20 | -- |
17 | Replace the main relay. Refer to Relay Replacement in Wiring Systems. Did you complete the replacement? | -- | Go to Step 20 | -- |
18 | Replace the underhood fuse block. Refer to Underhood Electrical Center or Junction Block Replacement in Wiring Systems. Did you complete the replacement? | -- | Go to Step 20 | -- |
19 | Replace the ECM. Refer to Engine Control Module Replacement . Did you complete the replacement? | -- | Go to Step 20 | -- |
20 |
Did the DTC fail this ignition? | -- | Go to Step 3 | Go to Step 21 |
21 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | System OK | |
