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Home >> Chevrolet >> 2002 >> Blazer 4D Utility, 4WD, Part Time >> Repair and Diagnosis >> External Pages >> Different car >> Section 1865 (Engine Controls And Fuel - 3.5L (L66) - DTC P0506 To DTC P2199) >> DTC P0688 >> Test Description

Test Description

WARNING: This page is about a different car, the 2007 Saturn Vue. However, it is still accessible from the selected car via links, so may be relevant.

The numbers below refer to the step numbers in the diagnostic table.

  1. 1:  The Diagnostic System Check - Vehicle prompts the technician to complete some basic checks and store the Freeze Frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.
  2. 2:  This step determines if a condition exists. The scan tool should not communicate with the PCM with the ignition OFF.
  3. 3:  This step tests if the main relay is stuck in the ON position.
  4. 4:  This step determines if there is a condition with the control circuit of the main relay. If the control circuit of the main relay is shorted to ground, the relay will click ON and OFF when you remove and install the relay.
DTC P0688

Step Action Yes No
Schematic Reference: Engine Controls Schematics
Connector End View Reference: Powertrain Control Module Connector End Views or Engine Controls Connector End Views
1  Did you perform the Diagnostic System Check - Vehicle? Go to Step 2  Go to Diagnostic System Check - Vehicle
2 
  1. Turn OFF the ignition.
  2. Attempt to communicate with the powertrain control module (PCM) with a scan tool.
Does the scan tool communicate with the PCM?		 Go to Step 3  Go to Intermittent Conditions
3  Swap the main relay with the cooling fan relay.
Does the scan tool communicate with the PCM?		 Go to Step 4  Go to Step 9 
4 
  1. Turn OFF the ignition.
  2. Remove and install the main relay several times while listening for a clicking sound from the relay.
Does the relay click when removing and installing the relay?		 Go to Step 5  Go to Step 6 
5
  1. Turn OFF the ignition.
  2. Remove the main relay from the underhood fuse block.
  3. Test the control circuit of the main relay for a short to ground. Refer to Circuit Testing and Wiring Repairs .
Did you find and correct the condition?		 Go to Step 11  Go to Step 8 
6 Measure the voltage at the ignition 1 voltage of the main relay at the underhood fuse block with a DMM.
Does the DMM display a voltage?		 Go to Step 7  Go to Step 9 
7 Test the ignition 1 voltage supply circuit of the main relay between the ECM/CAM fuse and the PCM for a short to voltage. Refer to Circuit Testing and Wiring Repairs .
Did you find and correct the condition?		 Go to Step 11  Go to Step 8 
8 Test for shorted terminals and poor connections at the PCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .
Did you find and correct the condition?		 Go to Step 11  Go to Step 10 
9 Replace the main relay.
Did you complete the replacement?		 Go to Step 11  -
10
  1. Replace the PCM. Refer to Control Module References for replacement, setup, and programming.
  2. Perform the idle learn procedure. Refer to Idle Learn .
Did you complete the replacement?		 Go to Step 11  -
11
  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.
Did the DTC fail this ignition?		 Go to Step 2  Go to Step 12 
12 Observe the Capture Info with a scan tool.
Are there any DTCs that have not been diagnosed?		 Go to Diagnostic Trouble Code (DTC) List - Vehicle System OK
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