GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2005 🢒 Allure 🢒 Engine 🢒 Engine Controls - 3.8L 🢒 DTC P0685

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 powertrain control module (PCM) supplies the ground path to the relay coil control circuit via an internal integrated circuit, called an output driver module (ODM). One of the ODM output circuits is configured to operate as a low side driver for the main relay. The low side driver also incorporates a fault detection circuit, which is continuously monitored by the PCM. When the PCM commands the relay ON, the expected voltage on the relay coil control circuit should be a low voltage condition. If the fault detection circuit measures a high voltage condition, DTC P0685 will set. DTC P0685 indicates a short to battery positive voltage, or ignition 1 voltage on the main relay coil control circuit.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0685 Engine Controls Ignition Relay Control Circuit

Conditions for Running the DTC
    • The battery voltage is between 11 and 18 volts
    •  The ignition is ON.
    • The diagnostic runs continuously when the above conditions are met.

Conditions for Setting the DTC

The PCM detects more than 4.6 volts on the relay coil control circuit for more than 2 seconds.

Action Taken When the DTC Sets
    • The PCM will not illuminate the malfunction indicator lamp (MIL) if this diagnostic runs and fails.
    • The engine will crank, but will not run.
    • The Class II communications will be available with the use of a scan tool.

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 DTC P0685 diagnostic table assumes that the vehicle battery is fully charged. Refer to Battery Inspection/Test in Engine Electrical.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics and Power Distribution Schematics in Wiring Systems

Connector End Views: Powertrain Control Module Connector End Views and Electrical Center Identification Views

1

Did you perform the Diagnostic System Check - Vehicle

--

Go to Step 2

Go to Diagnostic System Check - Vehicle in Vehicle DTC Information

2

Attempt to start the engine.

Does the engine start and run?

--

Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems

Go to Step 3

3
  1. Turn OFF the ignition.
  2. Remove the powertrain relay from the underhood fuse block with the J 43244 Relay Puller Pliers. Refer to Relay Replacement in Wiring Systems.

    3. Notice: Refer to Test Probe Notice in the Preface section.

  3. Probe terminal cavity A6 at the underhood fuse block with a test lamp that is connected to a good ground. Refer to Troubleshooting with a Test Lamp in Wiring Systems.

Does the test lamp illuminate?

--

Go to Step 7

Go to Step 4

4
  1. Turn ON the ignition, with the engine OFF.
  2. Probe the terminal cavity A6 at the underhood fuse block with a test lamp that is connected to a good ground.

Doe the test lamp illuminate?

--

Go to Step 5

Go to Step 9

5
  1. Turn OFF the ignition.
  2. Disconnect the powertrain control module (PCM) electrical connector that contains the relay coil control circuit.
  3. Turn ON the ignition, with the engine OFF.
  4. Probe terminal cavity A6 at the underhood fuse block with a test lamp that is connected to a good ground.

Does the test lamp illuminate?

--

Go to Step 6

Go to Step 12

6
  1. Turn OFF the ignition.
  2. Disconnect the negative battery cable at the battery.
  3. Disconnect the electrical connector that contains the powertrain relay coil control circuit from the underhood fuse block.
  4. Disconnect the powertrain relay coil control circuit terminal from the underhood fuse block electrical connector.
  5. Connect the electrical connector that contains the powertrain relay coil control circuit to the underhood fuse block.
  6. Connect the negative battery cable at the battery.
  7. Turn ON the ignition, with the engine OFF.
  8. Probe the powertrain relay coil control circuit terminal at the PCM connector with a test lamp that is connected to a good ground.

Does the test lamp illuminate?

--

Go to Step 13

Go to Step 15

7
  1. Turn OFF the ignition.
  2. Disconnect the PCM electrical connector that contains the powertrain relay coil control circuit.
  3. Probe the terminal cavity A6 at the underhood fuse block with a test lamp that is connected to a good ground.

Does the test lamp illuminate?

--

Go to Step 8

Go to Step 12

8
  1. Turn OFF the ignition.
  2. Disconnect the negative battery cable at the battery.
  3. Disconnect the electrical connector that contains the powertrain relay coil control circuit from the underhood fuse block.
  4. Connect the negative battery cable at the battery.
  5. Probe the powertrain relay coil control circuit wire terminal at the underhood fuse block electrical connector with a test lamp that is connected to a good ground.

Does the test lamp illuminate?

--

Go to Step 13

Go to Step 15

9

Measure the resistance from terminal 30 of the relay to terminal 85 with a DMM.

Does the DMM display the specified value?

∞ohms

Go to Step 10

Go to Step 14

10

Measure the resistance from terminal 85 of the relay to terminal 87 with a DMM.

Does the DMM display the specified value?

∞ohms

Go to Step 11

Go to Step 14

11

Measure the resistance from terminal 85 of the relay to terminal 86 with a DMM.

Does the DMM display within the specified range?

70-110 ohms

Go to Step 17

Go to Step 14

12

Test for shorted terminals and poor connections at the PCM.

Did you find and correct the condition?

--

Go to Step 17

Go to Step 16

13

Repair the short to voltage in the relay coil control circuit.

Did you complete the repair?

--

Go to Step 17

--

14

Replace the powertrain relay. Refer to Relay Replacement in Wiring Systems.

Did you complete the replacement?

--

Go to Step 17

--

15

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 17

--

16

Replace the PCM. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming.

Did you complete the replacement?

--

Go to Step 17

--

17
  1. Reassemble the vehicle as necessary.
  2. Replace any open fuses.
  3. Turn OFF the ignition for 30 seconds.
  4. Attempt to start the engine.

Does the engine start and run?

--

Go to Step 18

Go to Engine Cranks but Does Not Run

18
  1. Clear the DTCs with a scan tool.
  2. Operate the vehicle for 5 minutes.

Does a DTC set during this ignition cycle?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information

System OK